Surface Morphology of Columnar Joints and Its Significance to Mechanics and Direction of Joint Growth

Surface morphology of columnar joints and its significance to mechanics and direction of joint growth

JAMES M. DEGRAFF | Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907 ATILLA AYDIN

ABSTRACT

Columnar joints in basaltic lava flows display conspicuous bands oriented normal to column axes. New observations show that each band contains a single plumose structure and thus represents an individual crack, or joint segment, formed during a discrete growth event. Analysis of plumose structure and intersections of cracks leads to a new kinematic model of columnar jointing, and provides the first direct proof that columnar joints grow incrementally from exterior to interior regions of solidifying magma bodies. Columnar joints form by nucleation and growth of new cracks on the edges of older cracks. Each new crack begins at a point and propagates mostly normal to column axes and along the leading edge of a developing column face, where thermal stress is concentrated. Inward propagation of cracks toward hotter regions is limited by a decrease of thermal stress and by the brittle-ductile transition of lava; outward and lateral propagation is limited by mechanical interaction with previous cracks and by low thermal stress in already fractured lava. Cracks often diverge slightly from the planes of previous cracks, probably because of spatial and temporal changes in directions of local principal stresses. Mechanical interaction causes a diverging crack to overlap, curve toward, and usually intersect the previous crack behind its edge, leaving a blind tip that points in the overall growth direction of the columnar joints. This and other directional criteria are applied to determine joint-growth patterns in several lava flows of the western United States. In two- tiered and multi-tiered flows, downward-growing columnar joints usually meet upward-growing joints well below the middle of the flows, which indicates very rapid cooling of upper portions relative to lower portions. This supports the idea that convection of water in columnar joints connected to the surface may be an important mechanism for cooling the upper portions of these flows, whereas conduc- tion is probably the dominant cooling mechanism at the bases.

INTRODUCTION

Columnar joints are networks of interconnected tension fractures that divide solids into prisms, or columns, with locally parallel axes and polygonal cross sections (Fig. 1). Joint-bounded columns occur in a wide variety of materials and range in diameter from millimetres to a few hundred metres. Contraction of cooling solidified magma promotes forma- Figure 1. Basic geometry of columnar joints, a. Joint-bounded tion of columns with large aspect ratios (length/diameter) in volcanic and columns with vertical axes in a Snake River Basalt flow (site 6 of subvolcanic rock bodies of varied composition and form, ranging from Fig. 3). Cliff height corresponds to the entire flow thickness, b. Ideal- rhyolitic, through andesitic and basaltic, to ultramafic flows, sills, dikes, ized vertical columns with horizontal bands on joint surfaces. Symbols and plugs (Boyd, 1961; Huber and Rinehart, 1967; Tomkeieff, 1940; represent column length ((), column diameter (d), column-face width Barnes and others, 1982; Wentworth and Jones, 1940; Robinson, 1956). (Wf), band width (Wb), aspect ratio ((/d).

Geological Society of America, v. 99, p. 605-617, 16 figs.,Novembe r 1987.

605

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Welded portions of rhyolitic ash-flow tuffs also commonly exhibit long, narrow column:» (Gilbert, 1938; Boyd, 1961). Columns with small aspect ratios develop in many sedimentary materials, for example, in mud and sandstone as a result of desiccation (Longwell, 1928; Neal and others, 1968; Netoff, 1971), and in frozen ground as a result of freeze-crack-thaw cycles (Lachenbruch, 1962). The combined effects of metamorphism and cooling have produced columns of large aspect ratio in granodiorite, sandstone, and coal (Knopf, 1938; Spry and Solomon, 1964; Crelling and Dutcher, 1968). Even nongeologic materials may be jointed into slender columns, which result from desiccation in the case of starch, and from cooling in the cases of optical crown glass and smelter slag (French, 1922; Jackson, 1970, p. 270). This paper focuses on the formation of columnar joints in volcanic rocks, but we believe that the conclusions also apply to other natural and man-made columnar joints. Field studies and experiments strongly suggest that thermally induced joints begin at :he boundaries of magma bodies and grow inward as magma cools and solidifies. Thermal models of magma bodies and field measurements in Hawaiian lava lakes show that cooling, solidification, and build-up of "hernial stress spread gradually inward from the boundaries (Jaeger, 1963; Peck and others, 1977; Peck, 1978). Thermally induced joints divide upper crusts of lava lakes into irregular polygonal areas when the crusts are less than a few hours old and a few inches thick (Peck and Minakami, 1968). These joints may extend downward, and new joints may form as the crust thickens and tensile stress builds up in the fractured and the newly solidified lava. Opening of joints in the crust of the 1959 Figure 2. Column faces showing bands oriented normal to verti- Kilauea Iki lava lake is correlated with thousands of high-frequency mi- cal column axes (site 6). croseisms (Peck and Minakami, 1968). Persistence of microseismic activ- ity during solidification of the lava lake and scarcity of new fracturing at Joint-surface morphology is a record of the kinematics of joint the surface indicate that the fracture process migrated from the surface to growth, and it provides useful information about fracture conditions. The the interior (Ryan and Sammis, 1978). Hawaiian lava lakes, exposed in purpose of this paper is to present new observations of the surface mor- section, display an upper and lower set of columnar joints, which meet just phology of columnar joints, and to interpret these in terms of kinematics below a lake's middle and whose axes are approximately perpendicular to and conditions of joint development. The paper focuses on incremental a lake's boundaries (Peck and Minakami, 1968). Continuity of many joint growth and on interaction between developing portions of joints and joints from surfaces to interiors of the lava lakes suggests that downward propagation of surface joints produces the upper set of columnar joints. An analogous situation occurs when a precracked edge of a glass plate is immersed in a cooling bath. As cooling and generation of tensile stress Figure 3. Sites of proceed, cracks propagate normal to isotherms away from maximum columnar joints examined tensile stress (maximum cooling) at the edge and toward minimum tensile in this study. 1, Columbia stress (minimum cooling) in the plate's interior (Geyer and Nemat-Nasser, River Basalt (CRB) flows 1982). along the Tieton River Incremental growth of columnar joints is inferred from the existence (WA); 2, CRB flows at of discrete microseisms in Hawaiian lava lakes and from joint-surface Vantage (WA); 3, CRB morphology. Column faces exhibit parallel, discontinuous bands defined flows along the Columbia by textural and geometric variations, and arranged normal to column axes River at Sentinel Gap (Figs, lb, 2). Earlier workers proposed that these bands represent succes- (WA); 4, CRB flows at sive growth increments of the joints (James, 1920; Tomkeieff, 1940; Spry, The Dalles (OR); 5, CRB 1962). Ryan and Sammis (1978, 1981) compared bands (their flows along the Columbia "striations") on columnar joints to similar bands on fatigue fractures, River at Wallula Gap produced by applying cyclic tensile loads to various test materials. They (WA, OR); 6, Snake argued convincingly that columnar joints probably result from cycles of River Basalt flow along tensile stress build-up during cooling, followed by stress release when the Boise River at Lucky fracture occurs to form a new band. They also observed that bands consist Peak Dam (ID); 7, Over- of smooth portions and rough portions, attributed, respectively, to crack hanging Cliff basalt flow initiation in relatively cool, brittle lava and to crack arrest in hotter, more at Tower Falls, Yellowstone National Park (WY); 8, phonolite por- ductile lava. Bankwitz (1978) also interpreted nearly horizontal bands, phyry plug at Devil's Tower National Monument (WY); 9, basalt whose edges he called "rings" (Ringe) and "kinks" (Knicke), as the prod- necks of the Mount Taylor volcanic field, west of Albuquerque uct of incremental growth of columnar joints (see his Figs. 6, 7). He (NM); 10, Quaternary basalt flows along North Creek near the town inferred that the edges of bands are temporary arrest lines of upward- of Virgin (UT); 11, andesitic flows at Devil's Postpile National Monu- growing joints which start at the flow base. ment (CA); 12, Boiling Pots basalt flow at Hilo (HA).

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already formed portions. Our results are based on field observations of tensile stress is greatest. If the curves are equidistant on the plume axis, well-exposed and well-preserved columnar joints in basaltic lava flows in variations of distance between them and along hackle indicate relative the western United States and Hawaii (Fig. 3). To interpret the observa- differences in velocity of crack propagation. The velocity of crack propa- tions, we use concepts and experimental results in structural geology, gation decreases from a maximum at the center to a minimum at the thermal stress analysis, fracture mechanics, and fractography of engineer- surfaces, and the local propagation direction changes from being parallel to ing materials. The result is a new kinematic model of columnar jointing, the plate at its center to being oblique or perpendicular to the plate at its with much improved resolution of the fracture process. Our observations surface. Symmetric plumose structure develops when tensile stress values also provide the first direct proof that columnar joints grow incrementally are symmetrically distributed about a maximum at a plate's center. from exterior to interior regions of solidifying magma bodies. Criteria for Fracture of a plate by bending also demonstrates the principle that a determining joint-growth directions are identified and are applied to sev- crack leads where tensile stress is greatest (Fig. 4c; Andersen and Dahle, eral lava flows to obtain joint-growth patterns, which have bearing on the 1966; McClintock and Argon, 1966, p. 502-504). A crack propagating cooling history of the flows. along the bend axis leads on the convex surface where tensile stress is

SURFACE MORPHOLOGY OF COLUMNAR JOINTS

Columnar joints consist of dilatant, or mode-I, cracks that form under the influence of thermally induced tensile stress. An idealized circular crack summarizes the basic features of mode-I crack nucleation and growth (Fig. 4a). In the example, remote principal stresses are homogene-

ous, with CTx equal to oy and az tensile. The material is isotropic and homogeneous except for randomly distributed flaws that locally concen-

trate stress. An increase of az eventually causes crack nucleation at the flaw most favorable in terms of size, shape, and orientation (Lawn and Wil- shaw, 1975, p. 16-18). The flaw at the crack origin is an inhomogeneity, such as a void or a large grain, and is usually marked by an irregularity of the crack surface, such as a hole or an inflection (Rice, 1974; Kulander and others, 1979). Linear topographic breaks in the crack surface, which col- lectively are called "hackle," are radial to the origin and perpendicular to the crack front (Woodworth, 1896; Kies and others, 1950; Frechette, 1972). Hackle are parallel to the local direction of crack propagation, because they result when a propagating crack front splits into unaligned partial fronts. Splitting of a crack front in rock may occur at an inhomogeneity, such as a void, which locally modifies the stress field such that a crack front warps or splits to remain perpendicular to maximum tension (Rice, 1974; Kulander and others, 1979). When a propagating mode-I crack encounters rotated principal stresses that produce a shear component parallel to its front (mode-Ill), the front may break up into steplike partial fronts that rotate to remain perpendicular to maximum tension (see Fig. 13, inset; Bankwitz, 1966; Sommer, 1969; Pollard and others, 1982). Hackle often join during propagation to produce larger composite hackle, as in river patterns, accounting in part for increased surface roughness in the propagation direction (Kies and others, 1950; Frechette, 1972). Two partial cracks link at a hackle by secondary cross fracture of the opening or the shear type, or by lateral propagation toward each other (Fig. 4a, inset; Gilman, 1956; Frechette, 1972; Swain and others, 1974). In the last mechanism, which is common in rock (Woodworth, 1896; Bankwitz, 1965, 1966; Kulander and others, 1979; Pollard and others, 1982), a partial Figure 4. Surface morphology of mode-I cracks, a. Idealized crack overlaps its neighbor by growing oblique to the propagation direc- mode-I crack propagating with equal velocity in all directions from its tion of the main crack front, and then curves to link with its neighbor at an origin at center of disk. Arrows on crack front show local direction of acute angle. Partial cracks in rock are often large enough to show second- propagation. Lineations radiating from the origin mark breaks in the ary hackle, the orientation of which confirms this mechanism of linkage crack plane, known as hackle. Inset shows three possible linkages at (Woodworth, 1896; Bankwitz, 1965; Kulander and others, 1979). large hackle that separate four partial cracks: left, shear fracture; Plumose structure consists of hackle that fan away from an axis center, opening fracture; right, lateral propagation of one partial crack containing a crack origin (Fig. 4b). It commonly forms when a crack to overlap, curve toward, and intersect the other. Solid lines drawn propagates in a plate of material that is loaded in tension but initially has normal to hackle mark past positions of the crack front, b. Symmetric residual compressive stress in the surface regions, as in tempered glass plate plumose structure on crack surfaces in a plate broken in tension (Preston, 1929; Murgatroyd, 1942; McClintock and Argon, 1966, (origin at dot). Hackle are shown by curved lines that fan away from p. 502-504). Curves drawn normal to hackle define past positions of a the plume axis; past positions of crack front are shown by curved lines crack front (Kulander and others, 1979; Kulander and Dean, 1985) and drawn normal to hackle, c. Asymmetric plumose structure on crack show that a crack leads along the plume axis at the center of a plate, where surfaces in a plate broken by bending.

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maximum, and lags on the concave surface where initial compressive stress axes. Therefore, each band represents an individual crack or joint segment, is reduced and ch anged to tensile stress behind the leading point of frac- which starts at a point, propagates continuously, and then stops to form its ture. Reconstruction of past crack fronts shows that the crack propagates own outline. A column face is the net result of many discrete crack events, parallel to the plate and with maximum velocity at the convex surface, each of which produces a well-defined segment of the face. Recognition of whereas it propagates oblique or perpendicular to the plate and with plumose structure and identification of bands as individual crack surfaces minimum velocity at the concave surface. The pattern of hackle on the clearly show that columnar joints form by an incremental fracture process. crack surface forms a half plume, or completely asymmetric plume, with Curvilinear hackle divide each crack into partial cracks (Fig. 6). its axis along the convex surface. A half plumose structure also develops Many hackle develop greater relief toward the edges of cracks, which when a crack propagates in a stress gradient produced at a cooling or produces an increase of surface roughness. Partial cracks often exhibit a drying surface; the crack leads along the surface where tensile stress is consistent sense of twist relative to the parent crack, which indicates greatest (Murgatroyd, 1942; Corte and Higashi, 1964). breakup of the parent-crack front under mixed mode-I and mode-Ill loading. Hackle patterns on partial cracks show that the main direction of Identification of Bands as Individual Crack Surfaces crack propagation is parallel to large hackle separating the partial cracks (Fig. 4a, inset). Construction of past crack fronts, however, shows that The parallel bands of a column face are distinguished from each other linkage of adjacent partial cracks often occurs when one spreads oblique to by relatively abrupt changes of surface roughness, surface attitude, and the main propagation direction and then overlaps, curves toward, and orientation of smaller-scale surface features (Fig. 2). Band widths, mea- intersects the other. Linkage apparently occurs behind the main crack front sured parallel to column axes (Fig. lb), are typically 3 to 12 cm for while it is propagating. columns at bases of lava flows. Band lengths, measured normal to column axes, are about equal to column-face widths, which are 20 to 100 cm. Our Crack Origins field observations show that each band of a column face is associated with one plumose structure, whose expression is best when the face is unweath- All observed crack origins are at intersections of their cracks with ered and obliquely illuminated (Fig. 5a). It is usually possible to identify adjacent ones (Fig. 5). Furthermore, origins in any portion of a flow are for each band a point from which hackle radiate, an axis from which consistently on the same side of their cracks. On columnar joints in basal hackle fan, and an outline along which hackle terminate (Fig. 5b). Plume portions of lava flows, for example, all origins are on the lower side of their axes extend along the lengths of bands and are perpendicular to column cracks. Unless otherwise specified, the following discussion of columnar

Figure 5. Interpretation of joint-surface morphology of a column face near a flow base (site 6). Scale is graduated in inches, a. Horizontal bands of the column face. Each band has a single plumose structure and represents an individual crack that formed during a discrete crack event, b. Sketch, of the column face, showing crack origins (dots), hackle of the plumose structures (short thin lines), and crack terminations or outlines (continuous thick lines). Arrows are on horizontal plume axes and show local propagation directions. Numbers give the sequence of crack formation, c. Oblique view of cracks on cutout section A-A'-B'-B. Overhangs (with stippling) and ledges of column face are shown by parallel curved lines. Profile of cracks along the cut A-A' shows plume axes (open circles) and vertical components of crack propagation (arrows).

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joints refers only to the basal portions of lava flows. The concepts apply to the upper portions, however, if the directions are reversed. The location of origins at crack intersections indicates that new cracks start on edges, or arrested fronts, of older cracks. Given a pair of cracks, the one whose origin is adjacent to the mutual intersection must have formed after the other (Fig. 7b). If the second crack somehow had formed first, it would have spread unrestricted in all directions from its origin, forming a circular front. Instead, the crack was initially semiellipti- cal, and it propagated away from one side of its origin, as determined by constructing past positions of crack fronts. It follows that the second crack did not propagate on the other side of its origin because the first crack already existed there. This establishes a criterion to determine the formation order of adjacent cracks. Applying this criterion to basal columnar joints, we find that crack formation proceeds away from the flow base and into the flow (Fig. 5). A column face therefore forms by an incremental and sequential process, whereby new cracks nucleate on the edges of upward-growing columnar joints. Figure 6. Details of hackle dividing crack 2, the youngest numbered crack, into partial cracks (site 6). Arrows show local propaga- Crack Propagation tion direction of crack 2 along the plume axis. Lines mark traces of a few hackle. Note the downward increase of hackle relief near the As a crack propagates away from its origin, it either remains nearly lower edge of crack 2, and the consistent sense of twist of partial coplanar with or diverges from the previous crack (Figs. 5, 7). In the cracks relative to the parent crack. coplanar case, the adjacent edges of two consecutive cracks abut at a relatively smooth juncture, which is disrupted only at the scale of the surface roughness (Fig. 7a). Plumose structure on the new crack has the form of a half plume. When a crack begins at the intersection of two column faces, it usually propagates along the upper edge of one face and

Figure 7. Kinematics of crack formation. Symbols as explained in Figure 5b. Continuous thin lines, drawn normal to hackle, show past positions of crack fronts, a. Coplanar cracks (site 2). Each numbered crack is mostly in the plane of the previous crack. Their origins lie to the right of the area photographed, b. Noncoplanar cracks (site 6). The numbered cracks diverge from the planes of the previous cracks.

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FACE A •] FACE B diverges from the previous crack, and overlaps this crack's upper edge (Figs. 5,7b). This may occur in both lateral directions from an origin near the middle of a column face, such that the origin is a point where the plane of the new crack crosses over the plane of the old crack. As the separation between new and old cracks increases, the overlap between them also increases (Fig. 8). Viewed in profile, a new crack usually curves to intersect obliquely the previous crack below the upper edge (Fig. 5c). A new crack, however, sometimes curves again and parallels the previous one for a short distance without intersection (Fig. 9). In either case, the old crack is not curved in the overlap region, suggesting independently that it formed before the new crack. Tte relative position of curved and straight edges of noncoplanar cracks is constant in a given portion of a flow. In basal columnar joints, for example, the lower edges are always curved and the upper edges are always straight, giving crack profiles the shape of an open, upright j (Fig. 5c). Plumose structure on noncoplanar cracks is two-sided and asymmetric with respect to the plume axis (Fig. 7b). Hackle curve away from parallelism with a plume axis and become oblique or nearly perpendicular to both sides of a crack. Therefore, noncoplanar cracks locally propagate Figure 8. Intersection of two column faces (at center) providing nearly opposite to the over-all growth direction of columnar joints as they an oblique view of the overlap of noncoplanar cracks (site 6). White overlap previous cracks. The plume axis of a noncoplanar crack is nearly arrows show local propagation directions at plume axes. As the new level with the upper edge of the previous crack, and it is asymmetrically crack (2) propagates to the left, it gradually diverges from and increas- situated toward the lower side of its crack (Figs. 5, 8). Whether a crack is ingly overlaps the older crack (1). The upper edge of crack 1 is pre- coplanar or not, therefore, its plumose structure is always asymmetric such served as a blind tip, which is seen as a vertical narrow shadow that that the larger portion is on the side of the plume axis toward the next- terminates upwiird near the tip of the black arrow. Note that crack 1 formed crack. propagated smoothly from column face B to face A. Many cracks are divided into cracklets by large breaks, many of which are periodically spaced (Figs. 10a, 10b). Large breaks between cracklets are similar to hackle in curvature and orientation. The cracklets produces a single half plume (Fig. 5, crack 2). A crack that begins in the of a crack are rotated with the same sense relative to the previous crack; middle of a column face usually propagates in both lateral directions from they form a steplike array when viewed parallel to column axes. The the origin and generates two half plumes. rotational sense of cracklets, however, may change from one crack to the Half-plume axes on basal columnar joints, like crack origins, always next. lie on the lower i;dges of their cracks (Fig. 7a). Hackle curve away from Observations of origins and plumose structures indicate that each parallelism with a half-plume axis and become oblique or nearly perpen- cracklet starts on and gradually diverges from the upper edge of the dicular to the upper edge of a crack. A coplanar crack thus has local previous crack (Figs. 10b, 10c). Each large break forms when a crack front propagation directions that vary from being perpendicular to column axes jumps from the diverged end of one cracklet to a point that is again on the at the lower edge to being upward and almost parallel to column axes at upper edge of the previous crack. This point is the origin of a new cracklet, the upper edge. Contrasts of hackle orientation and surface roughness which propagates mostly laterally and away from the end of the previous distinguish any pair of coplanar cracks at their mutual intersection. cracklet. Repetition of this restart-propagation cycle produces a regular In noncopknar cases, a new crack propagates away from its origin, series of cracklets. It also results in a zigzag intersection between new and

Figure 9. Upward- formed sequence of non- intersecting, noncoplanar cracks (site 6). The cracks are linked by later cross fractures (not shown). Profile view of cracks is to the right along the column face. Symbols fol- low those of Figure 5c.

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C d Figure 10. Formation, reproduction, and termination of cracklets. a. Cracks consisting of cracklets (site 2). Arrows show local propagation directions of three such cracks. Curved large breaks between cracklets have greater relief at the lower edges of cracks, where they begin, b. Idealized formation of three cracklets by interrupted propagation of the main crack (2). Crack 2 propagates to the left, as shown by stylized plumose structure of each cracklet. Top view shows origins of cracklets (dots), horizontal components of crack propagation (arrows), and counterclockwise rotation of cracklets (solid) with respect to older crack 1 (dashed), c. Very large cracklets (site 6). Crack 2 consists of three cracklets, which propagated to the right as shown by the white arrows. Dots mark origins of cracklets. Black arrows mark large breaks described in the text. d. Reproduction of new large breaks by older breaks, many of which begin in crack 1 (site 2). Black lines mark edges of crack 1 and the next three cracks. Leftward propagation of the cracks causes a leftward shift of new breaks relative to old breaks.

old cracks, because overlap increases gradually with greater separation and growth direction of columnar joints (Fig. 10c). Some large breaks localize decreases abruptly when a new cracklet starts on the old crack's upper the formation of large breaks in subsequent cracks, because a new crack edge. Adjacent cracklets develop simultaneously at a large break, and front may jump from one arrested partial front to the next as it passes an usually link by mutual overlap and intersection in a manner identical to existing break (Fig. lOd). By repetition of this process, some large breaks the linkage of hackle (Fig. 10b). Large breaks, however, differ from hackle are reproduced over many cycles of crack formation. Large breaks of a in the sense that the former result from out-of-plane jumps of a crack front new crack are shifted relative to previous large breaks in the direction of and restarting of a crack, whereas the latter result from splitting of a crack lateral propagation because hackle fan away from the plume axis in this front. direction. Some large breaks disappear gradually in the over-all growth direction of columnar joints as cracklets merge to form an integrated crack (Fig. Crack Termination 10a). Large breaks may also extend completely across a crack and may influence development of the subsequent crack. For example, a new crack A crack terminates vertically and laterally after it propagates away often propagates smoothly across a break in the previous crack (Fig. 10c). from the origin. Vertical terminations provide information about the over- If adjacent cracklets do not intersect, a new crack may link them by all growth direction of columnar joints. Lateral terminations define the propagating locally along their overlap region and opposite to the over-all formation sequence of intersecting cracks at the same level.

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Vertical Termination. Soon after leaving its origin, a crack reaches a are linked at triple junctions (Fig. lb). In the third dimension, a column point that marks its final vertical extent, but it continues propagating triple junction extends along the length of three abutting columns, and mostly laterally (Fig. 7b). The upper and lower terminations of the crack comprises many crack intersections. Using the principles based on origin are, respectively, a blind crack front and an intersection between new and location, plumose structure, and crack terminations, we have determined old cracks. The upper termination is preserved as a blind front when the that some cracks propagate toward and terminate at triple junctions, next crack is noncoplanar (Figs. 5c, 8); otherwise, it becomes the intersec- whereas others start at and propagate away from triple junctions (Figs. 5, tion between two coplanar cracks. 12). Still, other cracks propagate toward, through, and away from triple For any pair of noncoplanar cracks, the one that starts on the upper junctions, usually with a smooth change in direction at the junction (Figs. edge of the other generally terminates against the other in a way that 8,11a, 12). preserves the old blind tip (Fig. 5). Applying the criterion that a crack In one type of triple junction, a crack propagates toward a triple terminating against another is the younger of the two, we infer an order of junction and terminates against a through crack, indicating clearly that the crack formation that agrees with the order obtained using origin locations truncated crack forms second (Fig. 12a). In another type, a crack propa- and plume asymmetry. Blind tips are remarkably straight in profile view gates away from its origin at the triple junction, but does not cross the and point in the over-all growth direction of columnar joints (Fig. 5c). Use plane of the through crack, and so again the truncated crack forms second of this criterion is slightly hindered when a noncoplanar crack also termi- (Fig. 12b). These two types of triple junctions, which form by the intersec- nates blindly at its lower edge, without intersecting the previous crack (Fig. tion of two cracks at a T, make up about half of the junctions examined. 9). This type of blind termination, however, typically occurs near the The other half form by the intersection of three cracks, and differ mainly in curved part of a crack viewed in profile, thus distinguishing itself from the sequence and lateral propagation direction of the cracks (Figs. 12c, 12d). more common tyjie of blind termination. Careful inspection shows that triple junctions composed of three cracks are Lateral Termination within Column Faces. A column face at a tight groups of two or three, T or partial T intersections. We thus found given level may consist of more than one crack. Lateral termination of a that all examined cracks arrived at or departed from the triple junctions in crack within a column face is marked by a curved arrest line, which is a a definite order; no two cracks formed simultaneously at the triple continuation of the upper arrested edge (Fig. 11). Plumose hackle are junctions. everywhere normal to this curved line. The column face at this level is completed when £. new crack starts on or near the lateral termination, and Criteria for Determining Over-sill Growth Direction then propagates mostly away from the older collateral crack (Fig. 1 la). A of Columnar Joints new crack also may start away from and may propagate toward the older collateral crack (Fig. lib). Because collateral cracks are usually not co- The foregoing analysis of crack formation revealed many relation- planar, the propagating crack overlaps and often curves to intersect the ships that provide criteria for determining the over-all growth direction of older one. The region of overlap between collateral cracks can affect the columnar joints. The most useful of these criteria are summarized as propagation of subsequent cracks in ways described earlier for large breaks follows. between cracklets 1. Straight blind terminations of cracks, viewed in profile, point in Lateral Termination at Column Triple Junctions. Viewed parallel the direction of younger cracks. to column axes, columnar joints form a polygonal network of joints that 2. Origins of new cracks are on edges of older cracks. 3. The larger part of plumose structure is on the side of the plume f*-FACE A FACE B axis in the direction of younger cracks.

Figure 11. Lateral termination of cracks within column faces (site 6). Arrows show local propagation directions at plume axes. a. Crack 1 propagated smoothly through the intersection of column faces A and B and terminated along a curved line (convex to right) within face B. Crack 2 propagated away from an origin (dot) on the lateral termination of crack 1. b. Crack 1 terminated along a curved line (convex to left) within the column face. Crack 2 propagated toward and intersected crack 1 near the curved arrest line.

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coplanar to an older one, we infer that the new direction of ay is the same as the old direction (Fig. 13, cracks 1 and 2). When a new crack propa-

gates at an angle, a, to an older crack, we infer that the new direction of ay is rotated by a about a vertical axis with respect to the old direction (Fig. 13, cracks 2 and 3). For a similar situation, Pollard and others (1982) argued that echelon dilatant cracks form by breakup of a parent crack

when CTy at the parent-crack tip rotates and induces microcracks rotated equally with respect to the parent-crack plane. In a system of interacting columnar joints, thermal stress would concentrate in a complex way near

the uppermost edges of cracks, such that ay generally would not be perpendicular to the cracks. Also, formation of some cracks in advance of

others would modify the direction of ay along adjacent cracks (Ryan and Sammis, 1978). A new crack, therefore, usually propagates at a small angle to the previous one. A new crack never propagates far from the plane of the previous crack, however, because driving stress steeply decreases away from the upper edge of the previous crack (Pollard and others, 1982). Upward growth of a crack is limited by a decrease of driving stress and an increase of resistance to fracture in hot, ductile lava. The plume axis of a new crack is about level with the upper edge of the previous crack

INSET

Figure 12. Some observed relationships between cracks at column triple junctions (site 6). Upper and lower edges of collateral cracks are generally mismatched, not as shown in the simplified sketches. Dots show origins of cracks; stylized plumose structure and arrows indicate local propagation directions; numbers give the forma- PROFILE tion sequence, a. Crack 2 terminates against crack 1. b. Crack 2 VIEW propagates away from an origin on or near crack 1, but without crossing crack 1. c. Crack 2 propagates away from an origin on the lateral termination of crack 1, and crack 2' terminates against crack 1. The relative order of cracks 2 and 2' is ambiguous, d. Cracks 2 and 3 terminate against crack 1.

DISCUSSION 0 Mechanical Significance of Results

Cracks start at points on the arrested edges of older cracks and propagate along these edges (Fig. 13). This is expected because the edge of ò a crack, subjected to a load, concentrates stress relative to points away from the edge (Lawn and Wilshaw, 1975, p. 2-5; Broek, 1983, p. 6-11). Figure 13. Basic mechanics of the formation of cracks and partial With increased cooling, stresses near the edge of a crack eventually reach cracks. Numbers and symbols as explained above. Broken lines with magnitudes sufficient to nucleate a new crack, whereas stresses away from arrows show local propagation direction of cracks at plume axes the edge remain too low to cause nucleation. The new origin occurs at a (thin) and over-all growth direction of column face (thick). Ellipses point on the old edge where stress concentration probably is further en- represent disks parallel to the X-Y plane. Local principal tensile stress,

hanced by an inhomogeneity or a distortion of the edge. The new crack oy, does not change during formation of cracks 1 and 2, and so these

propagates along the zone of maximum stresses at the edge of the older are coplanar. Before crack 3 forms, ay changes direction to ay (tem- crack, where strain energy required for crack development is greatest. The poral change) by rotating clockwise through the horizontal angle a'. edge of the older crack guides the new crack in the same way that a score Crack 3 is rotated equally relative to crack 2, and so these cracks are mark on glass plate guides the subsequent break. This guiding of new noncoplanar. Inset. Between the lower edge of crack 3 and the upper

cracks by older ones maintains the general continuity of column surfaces edge, ay changes direction by rotating counterclockwise to ay" (spa- and the shape of column cross sections from level to level in a flow. tial change), which is arbitrarily taken parallel to CTy.Thi s produces Mode-I cracks form normal to the direction of local maximum tensile mixed mode-I and mode-Ill loading on the upward-propagating front

stress, ffy (Lawn and Wilshaw, 1975, p. 66-68). When a new crack is of crack 3, which splits into partial cracks rotated to be normal to ay".

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(Fig. 13). By analogy with experiments, tensile stress is maximum at the onal varieties of crack intersections postulated by Lachenbruch (1962), plume axis of a new crack and minimum at the upper edge when the crack and the idea of sequential crack formation at triple junctions, suggested by is forming. Theory also predicts that tensile stress is maximum at the upper Ryan and Sammis (1978) and Bankwitz (1978). It follows that crack edge of the previous crack, where cooling and stress concentration are formation at a given level in a flow is a sequential process involving lateral significant, and is negligible at the solidus isotherm above the edge. In propagation of cracks. This is very similar to sequential cracking that addition, an upwaid-growing crack encounters increasingly hotter lava, progressively divides a cooling or desiccating surface layer (Peck and which is more likely to deform ductilely than to fracture (Ryan and Minakami, 1968; Corte and Higashi, 1964). A difference is that surface Sammis, 1981). cracks are relatively unrestricted in terms of origin location and propaga- Downward growth of a crack is limited by interaction with older tion direction, whereas interior cracks are promoted and guided by the cracks, and by stress release due to these. In a coplanar case, the lower edges of previous cracks. edge of a new crack simply joins the upper edge of the previous crack (Fig. 13). In a noncoplanar case, mechanical interaction causes a new crack to Over-all Growth Directions of Columnar Joints in Lava Flows overlap, curve toward, and intersect the previous crack below the upper edge, in a manner si milar to linkage of partial cracks (Figs. 5 and 13, inset) Criteria for determining the over-all growth direction of columnar and other dilatant cracks (Pollard and others, 1982; Pollard and Aydin, joints were used to ascertain patterns of joint growth in three types of lava 1984). The planar upper part of a crack indicates that it forms without flows. We now show that these patterns have important implications for significantly interacting with other cracks. A new crack that curves and the cooling history of flows. The flow types considered are one-tiered parallels the previous one without intersection probably stops because it flows, which are generally less than 30 m thick, and thicker two-tiered and enters a zone of stress relief adjacent to the previous crack. Plumose multi-tiered flows (modified from Long, 1978; Long and Wood, 1986). structure shows that lateral growth of a new crack lags in the region below The term "tier" refers to a set of regularly spaced, generally vertical to the upper edge of the previous crack, suggesting that ay there decreases steeply inclined, columnar joints that occur between two relatively hori- downward as predicted by theory. zontal levels in a flow (Fig. 14). Our results are consistent with the idea of cyclic growth of columnar MULTI-TIERED joints, proposed by Ryan and Sammis (1978). They suggested that (1) cooling induces build-up of tensile stress at the tip of a columnar joint; (2) the joint eventually propagates upward for a short distance; and (3) it then stops upon encountering hot, ductile material that relaxes stress in the joint-tip region. This cycle produces one segment of the developing column face. Bankwitz (1978) stated that some columnar joint surfaces comprise segments, which indicate intermittent joint growth; however, he apparently misinterpreted joint-segment formation as the result of simultaneous propagatio n of the entire upper edge (temporary arrest line) of the previous segment (compare Fig. 6 of Bankwitz, 1978 with Figs. 5 and 6 of Bankwitz, 1965). He found evidence that vertically propagating portions of a column face are interrupted by laterally propagating portions, but he did not describe how these portions interrelate. Our new observations and kinematic model of columnar jointing show conclusively that each joint segment represent! an individual crack formed during a discrete event. Each crack is thus an increment of columnar joint growth. Surface morphology and intersections of cracks allow the vertical and lateral sequence of crack formation to be determined unambiguously and independently of other clues, such as proximity to the flow base, or assumptions, however reasonable they may be. Plumose structure shows that each crack propagates mostly laterally along the edge of the previous crack. A new crack may diverge horizontally from the previous crack (Fig. 13, crack 3), or it may alternately diverge from and restart on the previous edge (Fig. 10b) in response to mixed mode-I and mode-Ill loading of the edge. Mixed mode loading also causes a crack to break up into many partial cracks separated by hackle (Fig. 13, inset). Large breaks and hackle have been termed Figure 14. Configuration and over-all growth directions of co- "chisel marks" by some workers (James, 1920; Tomkeieff, 1940); crack- lumnar joints in three structural types of lava flows. Relative spacing lets and partial cracks were termed "fracture lances" by Ryan and Sammis and regularity of lines schematically show changes of column diameter (1978). The latter authors correctly interpreted cracklets and partial cracks and regularity of column faces from tier to tier. Solid arrows show as the result of mixed mode-I and mode-Ill loading. growth directions of columnar joints. Open arrows show probable Our observations of cracks at column triple junctions indicate that growth directions of irregular subvertical joints (not shown) in the true Y intersections, resulting from simultaneous departure or arrival of heterogeneous flow-top regions (pebbly pattern). One-tiered flow (site three cracks at a point (Billings, 1954, p. 116; Iddings, 1909, p. 322), are 10) has two sets of vertical joints, but only one set is regularly colum- either very rare cr non-existent in columnar joint sets. Forked intersec- nar. Two-tiered flow (site 7) was examined at a place where the tions, which result from a crack bifurcating into two arms that diverge flow-top region (not shown) is mostly missing. Multi-tiered flow (site smoothly away from the parent (Ernsberger, 1960), are similarly rare or 3) with an internal vesicular zone (stippled) is modified from Figure 3 non-existent. Instead, the observations confirm the existence of the orthog- of Long and Wood (1986).

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Figure 15. Downward-formed cracks in the upper portions of two flows. Numbers give the formation sequence, a. Relatively large cracks (view is about 30 cm wide) showing plume axis near the upper edge of crack 2 (site 1). Arrow shows the lateral propagation direction of crack 2 along its plume axis. Crack 2 overlaps, curves toward, and intersects the lower part of crack 1. b. Small cracks showing overlap and intersection of older cracks by younger ones (site 6).

Our observations of basaltic lava flows in the western United States directly. These flows are thus fractured by two basic joint sets, an upward- revealed the following characteristics of the three flow types. In one-tiered growing set represented by the basal tier, and a downward-growing set flows, the tier of columnar joints is adjacent to the flow base, and is represented by the flow-top region and all other tiers. These findings overlain by a vesicular to brecciated flow-top region (Fig. 14). Columns of confirm previous ideas about the growth directions of columnar joints in the single tier have relatively large diameters (40-200 cm) and often two-tiered flows (Iddings, 1886; James, 1920; Saemundsson, 1970; Long exhibit regular prismatic forms. Joints in the flow-top region generally and Wood, 1986). They also show that the principal structural difference have irregular surfaces and spacing, the latter being greater than the spac- between the three flow types is the complexity of the joint set that grows ing of basal columnar joints. Two-tiered flows generally consist of (1) a down from the surface. relatively thin tier of basal columnar joints; (2) a relatively thick, overlying The upper joint set of examined two-tiered and multi-tiered flows tier of columnar joints; and (3) a flow-top region. The first and last of these meets the lower joint set at a level well below the middle of the flows are equivalent to their correlatives in one-tiered flows. Columns of the (Fig. 14). For conductive cooling of a flow, the two solidification fronts upper tier have relatively small diameters (20-40 cm) and are often irregu- and joint fronts would meet at a depth of 5-tenths to 6-tenths of the lar in form. Multi-tiered flows consist of (1) a typical tier of basal colum- thickness, because heat loss through the surface would be only slightly nar joints; (2) a much thicker, overlying series of columnar tiers; and (3) a greater than heat loss through the base until complete solidification typical flow-top region. Each tier of the series is distinguished from adja- (Jaeger, 1961, 1968). Our observations contrast with this scenario and cent tiers by differences in column diameter and regularity of column indicate that heat loss through the surfaces of examined flows was much form. greater than that through the bases. This implies that cooling rates in the Application of the directional criteria showed expectedly that basal upper portions of these flows, represented by the upper joint set, were columnar joints of examined flows grew upward from the flow bases significantly greater than those in lower portions. Saemundsson (1970) (Fig. 14; see also Bankwitz, 1978). This growth direction matches the reached the same conclusion by assuming what we have just demonstrated inferred and predicted upward migration of a basal solidification front, about growth directions of columnar joints in two-tiered flows. He sup- associated with the beginning of tensile stress build-up. In contrast, direc- ported this conclusion with the observation that upper tiers of interglacial tional indicators throughout the upper tiers of examined flows are consist- basalt flows in Iceland are finer grained and glassier than lower tiers. Long ently opposite in sense to those of basal tiers (Fig. 15). Columnar joints in and Wood (1986) used similar observations of grain size, glass content, all tiers above the basal tier of these flows, therefore, grew downward from and igneous texture to show that the upper portions of many Columbia the flow-top region (Fig. 14). Irregular joints in vesicular flow-top regions River Basalt flows, including the multi-tiered example of Figure 14, also also must have grown downward in two-tiered and multi-tiered flows, and cooled very rapidly relative to the lower portions. These authors argued probably did so in one-tiered flows, although it is difficult to determine this that accelerated cooling of the upper portions of many flows results from

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FLOW TOP -«-MASTER JOINTS-*-

0.5 Ts

Figure 16. Geometry and formation of column fans. a. Fanlike arrangement of columns around a vertical master joint (arrow) near the upper surface of a flow (site 6). b. Inferred isotherms of cooling regimes that produce column fans. Isotherms are normalized to the solidus temperature, Ts. Early conductive regime (solid isotherms) produces subvertical master joints; later regime (dotted isotherms) caused by water convection in master joints produces columnar joints that grow radial to the master joints.

convection of water in joints connected to the surface, as occurs in the flows. Directional criteria also can be used independently, or with other 1959 Kilauea Iki lava lake (Hardee, 1980). criteria, to resolve a flow sequence into separate flows, because a plane Column-growth directions also provide insight on the origin of col- between two flows is marked by two joint sets that grew in opposite umn fans, which commonly occur in upper tiers of two-tiered and directions away from the plane. This new tool may be especially useful multi-tiered flows; (Fig. 16a; Iddings, 1886; Spry, 1962; Justus, 1978). when other criteria give inconclusive results, or where intraflow vesicular Most column fans observed by us comprise columns that radiate from the zones and multiple tiers complicate the flow sequence. Our results can thus lower extremes of prominent vertical joints or highly jointed zones, which benefit investigations of joint systems at two candidate sites for a nuclear extend downward, from the surface. Columns near the top of a fan gener- waste repository, by contributing to an understanding of columnar joints in ally make high angles with the vertical master joint and form a polygonal basaltic lava flows at Hanford, Washington (Long, 1978) and in rhyolitic pattern on the master-joint surface. Application of the directional criteria ash flows at Yucca Mountain, Nevada (Lipman and Christiansen, 1964). revealed that columns grew away from the lower extremes of master Our kinematic model may be relevant to mechanical modeling of cooling- joints, confirming the hypothesis of Justus (1978). Without referring to induced joints in newly formed oceanic crust (Lister, 1974) and for possi- master joints, Bankwitz (1978) suggested that columns grow away from ble schemes of geothermal energy extraction (Harlow and Pracht, 1972; the central part of fans by multiple bifurcation of columnar joints. Later- Smith and others, 1973). ally growing columns of small fans, less than about 10 m wide, often Our results concerning columnar joints probably apply to some non- curved down and grew parallel to other downward-growing columns in columnar joints as well; for example, the methods of determining joint- upper portions of flows. Columns in fans are thus anomalous and result growth directions should apply to thermal joints in dikes, sills, and stocks. from a thermal stress regime that is different from that which produced Recent work shows that layered sedimentary rocks fracture layer by layer, master joints. We infer that the earlier regime had nearly horizontal iso- and that joints propagate mostly parallel to layers (Bahat and Engelder, thermal and isotensile surfaces, such that vertical joints grew down from 1984; Engelder, 1985). Joints that cross layers are composed of smaller the surface (Fig. 16b). A column fan would start forming when a master- cracks formed within the layers. It may therefore be possible to use meth- joint surface is suddenly chilled by water ingress, such that isothermal and ods developed here to infer patterns of joint growth in sedimentary strata. isotensile surfaces are bowed downward near the joint and its tip. A new Such information would be useful in determining the causes and the set of joints would divide the chilled master-joint surface, and then would progress of stress build-up that leads to fracture of these materials. grow away and normal to the new isotensile surfaces to generate a column fan (see also Justus, 1978; and Long and Wood, 1986). SUMMARY AND CONCLUSIONS

Applications Column faces in volcanic rocks exhibit bands oriented normal to column axes. Each band contains a plumose structure which identifies it as Patterns of ixilumnar joint growth provide information about the an individual crack, or joint segment, that formed during a discrete event. kinematics and the mechanisms of cooling in many volcanic rocks, and By analyzing plumose structure and crack intersections, we conclude that thus should constrain models of heat transfer, stress build-up, and joint the formation of columnar joints is an incremental process whereby new formation. The ability to determine growth directions of columnar joints cracks nucleate and grow on the edges of older ones. The sequence of also should prove: useful in applications to flow stratigraphy. Directional crack formation is systematically away from cooling surfaces and toward criteria allow determination of the relative intraflow position of an iso- the interior of a flow and thus parallel to axes of developing columns. Each lated, incomplete exposure of columnar lava. This may facilitate correla- crack, however, starts at a point and propagates mostly normal to column tion of the exposure to nearby, more complete exposures of columnar axes and along the leading edge of a column face, where thermal stress is

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concentrated. Plumose structure shows that driving stress steeply decreases Engelder, T., 1985, Loading paths to joint propagation during a tectonic cycle: An example from the Appalachian Plateau, U.S. A.: Journal of Structural Geology, v. 7, p. 459-476. away from the leading edge of a column face, thus restricting each new Ernsberger, F. M., 1960, Detection of strength-impairing surface flaws in glass: Royal Society of London Proceedings, ser. A, v. 257, p. 213-223. crack within closely spaced levels. Cracks often diverge from planes of Frechette, V. D., 1972. The fractology of glass, in Pye, L. D„ Stevens, H. J., and LaCourse, W. C-, eds.. Introduction to previous cracks, or alternately diverge from and restart on the edges of glass science: New York, Plenum Press, p. 433-450. French, J. W., 1922, The fracture of homogeneous media: Geological Society of Glasgow Transactions, v. 17, part 1, these cracks, probably because of temporal and spatial changes in direc- p. 50-68. Geyer, J. F., and Nemat-Nasser, S., 1982, Experimental investigation of thermally induced interacting cracks in brittle tions of local principal stresses. Mechanical interaction causes diverging solids: International Journal of Solids and Structures, v. 18, p. 349-356. cracks to overlap, curve toward, and usually intersect previous cracks Gilbert, C. M„ 1938, Welded tuff in eastern California: Geological Society of America Bulletin, v. 49, p. 1829-1862. Gilman, J. J., 1956, Propagation of cleavage cracks in crystals: Journal of Applied Physics, v. 27, p. 1262-1269. behind the edge. This process produces a major component of surface Hardee, H. C., 1980, Solidification in Kilauea Iki lava lake: Journal of Volcanology and Geothermal Research, v. 7, p. 211-223. relief of column faces. Cracks arrive at or depart from triple junctions in a Harlow, F. H., and Pracht, W. E., 1972, A theoretical study of geothermal energy extraction: Journal of Geophysical definite order, which can be determined from observations of their inter- Research, v. 77, p. 7038-7048. Huber, N. K., and Rinehart, C. D., 1967, Cenozoic volcanic rocks of the Devil's Postpile quadrangle, eastern Sierra sections; therefore, crack formation at a given level in a flow is sequential Nevada, California: U.S. Geological Survey Professional Paper 554-D, p. D1-D21. Iddings, J. P., 1886, The columnar structure in the igneous rock on Orange Mountain, New Jersey: American Journal of and resembles the progressive fracturing of a cooling or desiccating surface Science, ser. 3, v. 31, no. 185, p. 321-331. layer. 1909, Igneous rocks; composition, texture and classification, description and occurrence. Volume 1: New York, John Wiley & Sons, Inc., 464 p. Jackson, K. C., 1970, Textbook of lithology: New York. McGraw-Hill Book Company, 552 p. This study uniquely establishes the kinematics of a developing joint Jaeger, J. C., 1961, The cooling of irregularly shaped igneous bodies: American Journal of Science, v. 259, p. 721-734. set by unifying the inferred sequence of crack formation, the local direc- 1968, Cooling and solidification of igneous rocks, in Hess, H. H., and Poldervaart, A., eds., Basalts: The Polder- vaart treatise on rocks of basaltic composition, Volume 2: New York, John Wiley & Sons, p. 503-536. tions of crack propagation, and the over-all migration direction of the James, A.V.G., 1920, Factors producing columnar structure in lavas and its occurrence near Melbourne, Australia: Journal of Geology, v. 28, p. 458-469, cracking envelope. It also presents clear field evidence that existing cracks Justus, P. S., 1978, Origin of curvi-colutnnar joints in basalt cooling units by fracture-controlled quenching [abs.]: EOS greatly influence the origin location and propagation path and direction of (American Geophysical Union Transactions), v. 59, p. 378-379. Kies, J. A., Sullivan, A. M., and Irwin, G. R., 1950, Interpretation of fracture markings: Journal of Applied Physics, v. 21, new cracks. An important result of the work is the identification of simple p. 716-720. Knopf, A., 1938, Partial fusion of granodiorite by intrusive basalt, Owens Valley, California: American Journal of Science, criteria, based on plumose structure and crack intersections, for determin- v. 35, p. 373-376. ing the over-all growth direction of columnar joints. Application of these Kulander, B. R., and Dean, S. L., 1985, Hackle plume geometry and joint propagation dynamics, in Stephansson, O., ed., Fundamentals of rock joints: International Symposium on Fundamentals of Rock Joints, Bjorkliden, Sweden, criteria shows that many two-tiered and multi-tiered lava flows cooled Proceedings, p. 85-94. Kulander, B. R,, Barton, C. C., and Dean, S. L-, 1979, Application of fractography to core and outcrop fracture more rapidly in their upper portions than in lower portions. This supports investigations: Washington, D.C., U.S. Government Printing Office, U.S. Department of Energy, Paper the idea of previous workers that convection of water in columnar joints METC/SP-79/3,174 p. Lachenbruch, A. H., 1962, Mechanics of thermal contraction cracks and ice-wedge polygons in permafrost: Geological connected to the surface greatly accelerates cooling of the upper portions Society of America Special Paper 70,69 p. Lawn, B. R., and Wilshaw, T. R., 1975, Fracture of brittle solids: Cambridge, England, Cambridge University Press, of many flows. 204 p. Lipman, P. W., and Christiansen, R. L., 1964, Zonal features of an ash-flow sheet in the Piapi Canyon Formation, southern Nevada: U.S. Geological Survey Professional Paper 501-B, p. B74-B78. ACKNOWLEDGMENTS Lister, C.R.B., 1974, On the penetration of water into hot rock: Royal Astronomical Society Geophysical Journal, v. 39, p. 465-509. Long, P. E., 1978, Characterization and recognition of intraflow structures, Grande Ronde Basalt: Richland, Washington, Rockwell Hanford Operations Report RHO-BWI-LD-IO, 74 p. We thank P. E. Long for his encouragement, for openly discussing his Long, P. E., and Wood, B. J., 1986, Structures, textures, and cooling histories of Columbia River basalt flows: Geological work on thermal controls of columnar jointing in Columbia River Basalt Society of America Bulletin, v. 97, p. 1144-1155. Longwell, C. R-, 1928, Three common types of desert mud-cracks: American Journal of Science, v. 15, no. 86, flows, and for referring us to a few excellent sites of columnar joints. We p. 136-145. McClintock, F. A., and Argon, A. S., 1966, Mechanical behavior of materials: Reading, Massachusetts, Addison-Wesley are grateful for C. C. Barton's general comments on joint surfaces, which Publishing Company, 770 p. helped us to make sense of the surface morphology of columnar joints. We Murgatroyd, J. B., 1942, The significance of surface marks on fractured glass: Journal of the Society of Glass Technology, v. 26, p. 155-171. also benefited from conversations with A. H. Lachenbruch, M. P. Ryan, Neal, J. T., Langer, A. M., and Kerr, P. F., 1968, Giant desiccation polygons of Great Basin playas: Geological Society of America Bulletin, v. 79, p. 69-90. G.P.L. Walker, T. M. Tharp, N. I. Christensen, D. W. Levandowski, R. A. Netoff, D. 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