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Home , Basalt, Biotite, Country rock (geology), Craton, Eclogite, Feldspar

GLOSSARY OF From S.B. Shirey and J.E. Shigley, “Recent Advances in Understanding the Geology of ,” Winter 2013 Gems & , pp. 188-222. Some of the definitions here are adapted from Neuendorf et al.’s , 5th edition (American Geological Institute, 2011).

Accretion: the process by which two continental masses Friable: easily crumbled, as with a strongly weathered . collide and weld together, resulting in a larger . : the rate of increase of temperature Adiabatic melting: the melting of upward-moving with depth in the , with an average value of approxi - rocks as a result of depressurization, which to the mately 25°C per km. formation of . Geothermobarometer: a pair of whose chemical Alluvial: a sorted or semi-sorted deposited during composition is temperature and dependent, and comparatively recent geologic time by a stream or other which can be used to estimate the conditions under which moving body of , and which occasionally contains the minerals formed. concentrations of valuable minerals. Glaciofluvial: deposited by the streams or rivers flowing : the earliest of the four principal divisions of geo - from . logic time, extending from 2.5 to about 3.8 billion ago. Grade: a general term for content, in this case diamond : a part of the below the lith - abundance. osphere that is weak and capable of mobility, , Hybrid: an whose chemical composition results and melting. from assimilation of the into a . : a dark-colored, fine-grained igneous rock, composed Igneous: a rock that solidified from molten or partially mainly of and , that is formed molten magma (the term is also applied to the geologic by the solidification of magma near the earth’s surface. processes leading to or related to the formation of igneous : a coarse-grained rock of sedimentary or igneous rocks). origin that is composed of angular rock fragments held to - Indicator minerals: minerals that are geologically associ - gether by a cement or fine-grained . Brec - ated with diamonds but much more abundant, and which ciation is the process of forming a breccia or the magma can be used to explore for diamond deposits. that crystallizes such a rock. arc: a curved chain of arising from the deep- : a rock of magmatic origin. floor that is the volcanic product of . Continental : the portion of the earth’s crust that un - : a hybrid, volatile-rich potassic ultramafic ig - derlies the and continental shelves, ranging neous rock composed principally of , along with from about 35 to 60 km thick. lesser amounts of , pyroxene, ser - : a large, ancient, and geologically stable portion of pentine, , , , and . It can con - the continental that has been little deformed tain foreign rock fragments ( such as for a prolonged period of geologic time. In diamond geol - and ) and such as diamond (xenocrysts). ogy, a craton is the Archean portion of a much larger cra - Kimberlite is the chief host rock for commercial diamond tonic block, in which diamondiferous are mining. located on-craton. Non-diamondiferous kimberlites are lo - : a group of related dark-colored intrusive or ex - cated off-craton. trusive igneous rocks that are rich in and mag - Crust: the earth’s outermost layer or shell, consisting of the nesium and characterized by minerals, including leucite, thicker and thinner . phlogopite, and . Diamondiferous varieties carry : a tabular intrusion of igneous rock that cuts across dominant olivine and lack feldspar. the bedding or structure of preexisting rocks. : a group of dark-colored intrusive igneous rocks Eclogite: a granular, composed mainly of al - characterized by a high percentage of minerals (such as mandine- garnet and pyroxene that is mica, , and pyroxene) as larger crystals, set formed by the of from the oceanic in a fine-grained groundmass composed of the same minerals crust that have been subducted into the mantle. plus feldspars or feld s pathoids. Emplacement: referring to igneous rocks that intrude into Lithosphere: the outer portion of the earth, consisting a host rock or country rock, usually higher in the crust. of the crust and upper mantle, that is approximately 100 Equilibrium: a thermodynamic state where two minerals km thick. or components will not undergo further change at a given Longshore (or littoral) current: an current caused by pressure and temperature. the approach of waves to a coastline at an angle so that it Exsolved: when a mineral is crystallized directly in a solid flows parallel and near to the shore. host mineral, usually due to a decrease in pressure or tem - Macrodiamond: a rough diamond that is more than 0.5 perature or both. mm in diameter. Mafic: a dark-colored igneous rock chiefly composed of olivine, orthopyroxene, clinopyroxene, and perhaps garnet, - and -rich minerals. that is thought to be the most common and abundant rock Magma: molten material generated with the crust or upper type in the mantle. mantle from which igneous rocks are derived by solidifica - : the large, conspicuous crystals set in a fine- tion, and that is capable of intrusion at depths in the crust grained groundmass in a igneous rock. or at the surface as or a pyroclastic ash flow. Placer: a surface deposit consisting of valuable minerals that Magmatic: related to or derived from magma. have been weathered out and then mechanically concen - Mantle: the zone between the earth’s crust and core, con - trated (normally by flowing water) in alluvial . sisting of a rigid lithosphere and an underlying asthenos - Plate : a theory in which the lithosphere is di - phere of plastically flowing rock. vided into a number of thin, rigid crustal plates which Mantle keel: the downward-protruding, thickened portion move across the earth’s surface and interact with one an - of the lithospheric mantle that resides under the continen - other at their boundaries along zones of tectonic and seis - tal crust of the craton, and which has had an extended pe - mic activity. riod of attachment to the craton. Plume: an upwelling of molten rock that originates near Melt depletion: an igneous process by which melt is re - the core-mantle boundary, and then rises upward through moved, leaving a residual rock that is more refractory than the mantle. the original starting composition. Pyroclastic: an igneous rock composed of angular rock Metamorphism: the process that causes mineralogical, fragments that originate from a volcanic . chemical, or structural changes in solid rocks by exposing : the spontaneous disintegration of the them to new pressure and temperature conditions by bur - of certain nuclides into other nuclides, which may ial within the crust or mantle. be stable or undergo further decay until a stable nuclide is Metasomatic: formed by , a geologic process created. that produces new minerals in an existing rock by replace - : the fragmental and unconsolidated rock material ment. which nearly everywhere forms the surface of the land and Microdiamond: a rough diamond less than 0.5 mm in overlies the . diameter. Remobilized: a once-molten igneous rock that has been Mid-ocean ridge: a continuous chain of underwater moun - remelted. tains along the sea floor that mark the volcanic centers Rifting: a plate tectonic process that creates a zone where from which new oceanic crust is formed from magma the lithosphere has ruptured due to extension. being brought up by convection from the mantle. The so - Subduction: the process where one lithospheric plate de - lidified magma forms basalt, which spreads away along scends beneath another plate. both sides of the ridge to form new oceanic crust. Superdeep: an unusual type of diamond that originates at Mobile belt: a long, relatively narrow crustal region of for - depths well below the of the lithospheric mantle keel mer tectonic activity. from within the convecting mantle. building (or ): the processes by which Surficial: occurring at the earth’s surface. geologic structures in mountainous regions are formed. These processes include thrusting, folding, faulting, and Tectonic stability: a region of the earth that is not under - (at depth) metamorphism and igneous intrusions. going active geologic processes such as , moun - tain building, subduction, faulting, or rifting. These NAL: Na- and Al-bearing mineral that occurs in basaltic- regions have no or very few . composition rocks subjected to and temperatures equivalent to the top of the lower mantle (Harte, 2010). Thermal pulse: a wave of heat passing through the crust carried by fluids or magma from below. Nuclide: a species of characterized by certain num - ber of protons and neutrons in its nucleus. : an unsorted glacial deposit usually composed of finely ground , which may also contain dispersed, Oceanic crust: the portion of the earth’s crust that under - rounded cobbles or . lies the ocean basins, and that ranges in thickness from about 5 to 10 km. Ultramafic: an igneous rock composed mainly of mafic Peneplaned: leveled to a quite flat surface by the sum of minerals. erosional geologic processes. Uplift: a structurally high area in the crust that was pro - Peralkaline: a chemical classification of igneous rocks in duced by the raising or uplifting of rocks. which the molecular proportion of aluminum is less Xenocryst: a large in an igneous rock that is foreign than and potassium combined. to the rock in which it occurs. Peridotite: an ultramafic igneous rock composed of : an inclusion of a foreign rock in an igneous rock.