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The Geology of Mount Desert Island
A Visitor's Guide to the Geology of Acadia National Park
Glossary of Geologic Terms
Albite: Sodium-rich (Na) feldspar, NaAlSi3O8.
Basal till: A compact mixture of sediment, ranging in size from clay to boulders, deposited directly from the bottom of a glacier.
Basalt: A dark, fine-grained igneous rock that is erupted onto the surface as a lava flow.
Beds: Individual layers of a sedimentary rock.
Cauldron subsidence: The process in which a more or less cylindrical block of rock above a magma chamber collapses into the space left as the magma moves toward the surface.
Continental drift: The theory suggesting that rigid continental and oceanic plates move over the surface of the globe, producing many of the large scale geologic features (such as mountain ranges) we see today.
Continental glaciers: A glacial ice sheet of considerable thickness which covers a sizable portion of a continent and obscures most of the underlying terrain. The modern Antarctic ice sheet is a good example.
Country rock: The rock into which a mass of igneous rock was intruded.
Delta: A deposit of sand and gravel, often triangular and fan shaped, formed at the mouth of a river or stream where the current becomes too slow to carry the sediment.
Dike: Thin, tabular igneous rock body that cross-cuts the bedding of the rock that it intrudes.
End moraine: Ridges of till and/or sand and gravel formed at the margin of a glacier.
Exfoliation: The process by which horizontal to near-horizontal fractures cut rock into sheets, due to pressure release caused by erosion of overlying rock material. Very common in granites.
Extrusive: Term applied to igneous rocks that erupt onto the surface as flows or into the air as ash.
Foliation: A general term for the sub-parallel, planar arrangement of platy minerals such as micas. Typically best developed in metamorphic rocks such as the Ellsworth Schist.
Geologic time: The time extending from the beginning of the earth as a separate planetary body to the beginning of written history; implies extremely long duration or remoteness in the past: approximately 4.5 billion years.
Graded bedding: Sedimentary layering which displays a gradual change in particle size from coarse particles at the base of the bed to fine particles at the top. At the top of each bed there is an abrupt change back to the coarse particles of the next bed.
Igneous rock: Rock that solidified from molten rock material or magma.
Intrusive: An igneous rock that solidified from magma that was injected into older rocks below the earth's surface.
Mantle: The middle layer of the earth, lying below the crust. Denser than the crust; rigid plates of oceanic and continental crust move over a partially molten zone in the mantle. See plate tectonics.
Matrix: The finer-grained part of an igneous rock in which larger crystals (phenocrysts) occur. Also the light colored rock that encloses the dark blocks in the shatter zone.
Metamorphism: A process where rocks are recrystallized by heat and pressure in the earth's crust. Metamorphism is a process that usually accompanies orogeny.
Phenocrysts: Large, conspicuous crystals surrounded by fine-grained material in a volcanic rock or dike.
Plagioclase: Calcium (Ca) feldspar, CaAl2Si2O8.
Plucking: A glacial erosion process in which fragments or blocks of bedrock are first loosened by the expansion of freezing water in fractures and joints and then picked up and removed as the overlying glacial ice advances.
Quarrying: Same as plucking.
Roche moutonnée: A small elongate bedrock knob with a distinctive asymmetric profile, usually a few meters in height, length and breadth. The up-ice surface (which faced into the flowing ice) is gently inclined and rounded, and the down-ice side is steep and rough.
Rock flour: A glacial sediment composed of finely ground rock formed by abrasion at the base of the glacier.
Schist: A coarse-grained, mica-rich metamorphic rock characterized by aligned mica flakes.
Sills: Thin, tabular sheets of igneous rock parallel or sub-parallel to the bedding in the sedimentary rocks they intrude.
U-shaped valleys: A valley whose cross-sectional profile is similar to the letter "U". It has steep sides and a relatively flat bottom. This type of profile is typical of glacially carved valleys where the long axis of the valley is parallel to the former direction of ice flow.
Volcanic tuff: A term describing rocks composed of volcanic ejecta, such as broken pieces of volcanic glass, phenocrysts, rock fragments, etc.
Last updated on January 11, 2008
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