Comp. by: KJayaraja Stage: Revises3 Chapter No.: Title Name: EPL_214584 Date:7/7/15 Time:08:57:15 Page Number: 1171 Lava Flow 1171 Morphometry Lava Fan Lava flows may reach tens to hundreds of kilo- ▶ Alluvial Fan meter distances on Earth (Aprodov 1982) and a hundred to a thousand kilometer on Mars. The longest flows are tube fed (Sakimoto and Baloga 1995), due to the insulating effect caused by the Lava Flow formation of the lid. See also section “Planetary Analogs Regional Variations” below. Ákos Kereszturi1, Henrik Hargitai2 and Jim Zimbelman3 1Konkoly Thege Miklos Astronomical Institute, Subtypes Research Centre for Astronomy and Earth Sciences, Budapest, Hungary Classification based on solidified surface texture 2NASA Ames Research Center / NPP, Moffett (these terms do not characterize the molten lava): Field, CA, USA (1) Subaerial crustal morphologies of lava flows, 3National Air and Space Museum, Center for described from Hawaii (Dutton 1884; Earth and Planetary Studies, Smithsonian MacDonald 1953). Institution, Washington, DC, USA (1.1) Pa¯hoehoe (smooth/ropy). Pa¯hoehoe crust results from low-viscosity (e.g., basaltic) flow. It displays a shiny Definition surface, smooth at the scale of decime- L ters to meters, typically producing thin Spatially distinct solidified surface rock unit pro- (1–3 m thick) flows. These flows duced by lava from a single continuous episode of appear typically as radar dark. Lava volcanic activity (Bates and Jackson 1987; Self channels and tubes often develop. et al. 1997). The term lava flow may describe Lavas that produce pahoehoe texture both the process and the product. are typically emplaced at low effusion rates (generally <10 m3/s in Hawaii) and cool at a slow rate (e.g., Rowland Description and Walker 1990). Pa¯hoehoe morphol- ogy can also be produced in flood Lava flows are typically lobate and made up of basalts emplaced with very high effu- several flow lobes (Self et al. 1997)thatoften sion rates (Thordarson and Self 1998), coalesce with previously emplaced flows, forming associated with the process of inflation lava flow fields or lava plains (▶ volcanic plains). (Hon et al. 1994). Flood lavas form lava sheet flows (Keszthelyi (1.1.1) P-type pahoehoe: pipe vesicle- et al. 2000). Channelized lava flows are fed by bearing lobes, typically inflated ▶ lava channels or lava tubes (where a roof (Wilmoth and Walker 1993). forms over the flow). Accumulation of thick (1.1.2) S-type pahoehoe: spongy lobes and/or high viscosity flows may result in volcanic with spherical vesicles, with constructs (▶ volcano, ▶ steep-sided dome (Io), higher concentrations toward and ▶ steep-sided dome (Venus),etc.). the center; it forms with mini- mal inflation (Walker 1989). (1.2) ‘A’a¯ (rough/rugged, often spelled plainly “aa”). ‘A’a¯ crust results from higher (than pahoehoe) viscosity lava Comp. by: KJayaraja Stage: Revises3 Chapter No.: Title Name: EPL_214584 Date:7/7/15 Time:08:57:15 Page Number: 1172 1172 Lava Flow flows. It is characterized by rubbly, sections, thick glassy rinds, and paucity highly irregular, autobrecciated, and of vesicles (Keszthelyi et al. 2010 and rough surface, with a molten core, references therein). typically occurring in thick (3–20 m) (2.2) Submarine lobate texture (Perfit and flows. These flows show low albedo Chadwick 1998). and are radar bright. Lavas that produce (2.3) Submarine sheet lava surface morphol- ‘a’a¯ texture are emplaced at high effu- ogies may be analogous to pa¯hoehoe sion rates (generally >20 m3/s) and (smooth, lineated, ropy (with lava high strain rates (Peterson and Tilling coils)) or to ‘a’a¯ (jumbled and hackly) 1980) and cool rapidly. Pa¯hoehoe sur- (Perfit and Chadwick 1998). faces may change downslope to ‘a’a¯ (because lavas’ viscosity may increase due to cooling and gas loss during Additional Lava Flow Surface Textures transport), but the opposite case cannot occur. During the course of an eruption, (1) Ropy. Typical of pahoehoe surfaces in which ‘a’a¯ flows may change into pahoehoe the glassy pahoehoe surface has been flows due to a drop in effusion rate dragged (by continued flow beneath the (Hon et al. 2003). cooled crust) into a series of folds. The final (1.3) Transitional types between pa¯hoehoe appearance is similar to what happens when a and ‘a’a¯, recognized recently from carpet is pushed (not pulled) across a smooth lava flows outside Hawaii, e.g., in surface. Iceland, the Columbia River Basalts, (2) Platy. The crust breaks in large plates. Rafted the Kerguelen Plateau, etc. (Rowland plates are commonly seen in terrestrial and in and Walker 1987; Keszthelyi 2000; Martian flows and also documented on the Keszthelyi and Thordarson 2000; Hon Moon (Carter et al. 2012)(▶ platy material). et al. 2003). (3) Ridged. (▶ Pressure ridge). Martian flows (1.3.1) Spiny pahoehoe (toothpaste). It show a unique platy-ridged-type morphology displays longitudinal striations (▶ platy material) (Keszthelyi et al. 2000). oriented parallel to flow direc- tion, caused by scraping against orifice roof irregulari- Classification Based on Radar Properties ties (Rowland and Walker 1987). (See also ▶ radar feature). Radar brightness may (1.3.2) Rubbly and slabby pahoehoe correlate with flow rheology (Byrnes and Crown composed of broken 2002; Carter et al. 2012). autobrecciated pahoehoe lobes (1) Radar-bright flow (surface has lots of irregu- or slabs of broken pahoehoe larities comparable to the scale of the inci- surfaces, respectively dent radar wavelength and scatters radar (Keszthelyi and Thordarson waves easily back). 2000; Duraiswami et al. 2008). (2) Radar-dark flow (smooth surface at wave- (1.4) Blocky. High viscosity (e.g., silicic) length scales with only little variation; radar flows display blocky surface. They typ- waves are only rarely reflected directly ically produce very thick (>20 m) toward the remote sensing satellite or strata. Blocky flows appear bright in spacecraft). radar images. (2) Subaqueous (may be subglacial). (2.1) Pillow lava. Mafic lavas emplaced in liquid water. They have oval cross Comp. by: KJayaraja Stage: Revises3 Chapter No.: Title Name: EPL_214584 Date:7/7/15 Time:08:57:15 Page Number: 1173 Lava Flow 1173 Classification Based on Flow include both compound and simple flows, Emplacement but the most extensive and far-reaching flows are simple” emplaced at high effusion (1) Lobate flows formed by medium volumetric rates. (Walker 1972). rate. (3) Submarine pillow lava flows formed by (1.1) Simple flows have single lobes or low-volume flux. Pillow lavas extrude as branch into a few main lobes and are spherical or cylindrical tubes of lava which typically characterized by ‘a’a¯ surface cool and crust over on all sides preventing the texture (Byrnes and Crown 2002; coalescence of individual lobes. Pillows Byrnes 2002). They are not divisible build steep-sided mounds as new pillows are into separate flow units and develop at added to the top of the stack (e.g., Perfit and high extrusion rates. Higher viscosity Chadwick 1998). lavas are more typically simple than compound (Walker 1972). They can develop sheet-like forms. Visually Formation simple flows may be technically com- pound, composed of a large sheet and Lava flows are produced from the outpouring of several much smaller flows, or may magma which has reached the surface. Surface be juxtaposed laterally rather than morphology (pa¯hoehoe or ‘a’a¯) is controlled by vertically (Self et al. 1998). viscosity and the rate of shear strain (Peterson and (1.2) Compound flows are emplaced in mul- Tilling 1980). ▶ Inflated lava flow (pahoehoe) tiple overlapping and interfingering sheet flow forms when a chilled crust allows flow lobes that typically display continued lava emplacement into the flow to L pahoehoe texture (Byrnes and Crown raise its upper surface. It may reach thicknesses 2002; Byrnes 2002). They are visually of tens of meters (Self et al. 1996). The insulating divisible into separate flow units and crust can maintain cooling rates of <0.1C/km develop at low extrusion rates and allow travel distances up to several hundreds (Walker 1972). They develop digitate of kilometer (Thordarson and Self 1998). Inflated forms. Stratigraphy is quite easily sheet flows have been observed in Hawaii, and figured. Pahoehoe flows are generally probably the standard way of emplacing large compound (Walker 1972). lavas is as inflated compound pahoehoe sheet (2) Flood lavas: laterally extensive low-relief flows (Hon et al. 1994; Self et al. 1998). continuous sheet lava flows. Flood lavas form by high-volume flux, where lava spreads out in a sheet because its high flux Eruption Rates inhibits the formation of a crust during emplacement (Perfit and Chadwick 1998). The calculated emplacement rate for basaltic Sheet flows have “a continuous body of liq- flows on the Earth is typically between 50 and uid lava across the entire width of the flow. In 7,100 m3/s (Keszthelyi and Self 1998), of the contrast, the pathways within a hummocky order of 104 m3/s for flood lavas (Keszthelyi flow should have many dead ends (ending in et al. 2006). On Io, at Pillan Patera, the estimated tumuli) and areas in which the lava cannot eruption rate was 18,000–59,000 m3/s based on flow (inflation pits)” (Self et al. 1998). “Flood observations in 1997 (Davies et al. 2006). Similar lavas” are defined as “a package of sheet-like high eruption rates (up to 104 m3/s) are expected lava flows that inundates a large region, pro- at pahoehoe and platy-ridged lavas on Mars ducing a smooth plain without 100 m tall (Davies et al. 2007). At Alba Patera on Mars, constructs” (Keszthelyi et al. 2000)(▶ large based on observed properties of lavas and model igneous province). “Flood basalt piles Comp. by: KJayaraja Stage: Revises3 Chapter No.: Title Name: EPL_214584 Date:7/7/15 Time:08:57:16 Page Number: 1174 1174 Lava Flow computations, the effusion rate reached the order of 103 m3/s (Cattermole 1987).