Yosemite in Depth: Geology

The glossary terms on this page can help you better understand the information contained in this site's Geology activities and exercises.

Glossary

Asthenosphere — A zone of the Earth that is made up of hot, semi-solid material where plastic movements take place. It is thought that tectonic plates of the lithosphere “float” over the asthenosphere.

Batholith — A large body of intrusive igneous rock. The Sierra Nevada batholith is a composite of many smaller bodies of igneous rock referred to as plutons.

Biotite mica — A brown to black mineral with thin, layered plates that easily flake. This mineral often appears hexagonal and gives granite a peppery appearance.

Carbon dioxide — A colorless, odorless gas.

Carbonic acid — A chemical dissolved in rain water that promotes weathering of rocks.

Classification — The systematic grouping of objects into categories based on shared characteristics. Rocks and minerals can be classified based on different characteristics and how they were formed.

Convergent plate boundary — A boundary where two tectonic plates bump into each other. This collision causes crumpling and uplift of one or both plates near the boundary.

Core — Central portion of the Earth; a partly solid mass under extreme heat and pressure, probably made up of nickel and iron. It is about twice the size of the moon. The liquid outer core is 2,200 kilometers thick (1,367 miles) and the solid inner core is 1,250 kilometers thick (777 miles).

Crust — Outer rigid layer of the Earth overlying the mantle. Oceanic crust is generally about 5 kilometers thick (3 miles); continental crust varies in thickness, but averages about 30 kilometers thick (19 miles).

Crystallization — The process through which matter becomes crystalline from a fluid, viscous, or gaseous state to form a solid mass, such as granite.

Decompose — The breakdown and decay of dead plants or animals that adds organic material to the soil.

Dike — An igneous rock mass that forms when molten rock is squeezed into long, sheet-like cracks within older rocks. Dikes often have a different chemical composition than the rock that surrounds them, so they appear as ribbons or streaks of lighter or darker color.

Earthquake — A sudden motion or trembling in the Earth caused by the abrupt release of slowly accumulated stresses from within the crust or mantle.

Erosion — A group of processes by which the surface of the Earth is loosened, dissolved, or worn away and moved from one place to another by water, ice, or wind. Erosion changes landforms.

Erratic — A boulder that has been transported some distance from its source by a glacier. If an erratic is made of different rock than the rock on which it lies, tracing it back to its source yields information on the direction of glacier movement.

Exfoliation — The breaking or peeling off of successive layers of bare rock. This surface process forms Yosemite’s granite domes.

Fault — A fracture in the Earth’s crust along which there has been movement of one side relative to the other parallel to the fracture (compare with joint).

Fault-block basin — A valley or depression bounded by faults that has dropped down relative to the bordering land. Lake Tahoe in California is an example of a fault-block basin.

Feldspar — The most common and widespread mineral group that makes up nearly 60% of the Earth’s crust. The two types of feldspar found in granitic rocks can be white to light pink to rose in color. In some granite rocks, feldspar forms block-shaped crystals.

Fracture — A general term for any break in a rock or mineral that includes cracks, joints, and faults.

Frost wedging — Mechanical separation of rock blocks caused by freezing water in fractures in the rock.

Geologic — Pertaining to the study of the Earth, its origin, structure, composition, history, and other aspects.

Geologic processes — Many dynamic activities contributing to the creation of the Earth’s landforms, including plate tectonics, uplift, subduction, earthquakes, volcanic eruptions, and erosion.

Geomorphic — Pertaining to the form of the Earth or of its surface features.

Geomorphic processes — Processes that change the face of the Earth’s landforms, usually involving water, wind, and ice. Examples in Yosemite might be rivers cutting V-shaped canyons, glaciers carving U-shaped valleys, frost wedging, or exfoliation of granite domes.

Glacial till — Unsorted sediment, including rocks and finer materials, carried and later deposited by a glacier. Moraines are made up of glacial till.

Glacial trough — The bed of a mountain valley, gouged into granite bedrock.

Glacier — A mass of ice formed by compaction and recrystallization of snow.

Granite — True granite is a granitic rock with about equal amounts of quartz and feldspar. Dark minerals such as biotite and hornblende may also be present.

Granitic rock — Igneous rock that crystallizes beneath the surface of the Earth in plutons. It contains quartz and feldspars. It also contains dark minerals, such as biotite mica and hornblende, in varying amounts. Granite and granodiorite are both granitic rocks.

Granodiorite — A granitic rock with plagioclase feldspar (and smaller amounts of potassium feldspar) and some quartz. It usually contains abundant biotite mica and hornblende, giving it a darker appearance than true granite. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals.

Hornblende — A dark green to almost black mineral. Hornblende may form rod-shaped crystals in granodiorite.

Igneous rock — One of the three main groups of rock formed by the solidification of hot molten material. It may be extruded as volcanic rock on the Earth's surface or can be intrusive plutonic rock (remaining below the surface of the Earth).

Intrusion — The processes by which magma forces itself upward into a rock body and eventually crystallize into an igneous rock.

Joint — A fracture in rock that has experienced no movement other than spreading apart of the blocks of rock (compare with fault).

Landform — One of a number of features that, put together, make up the surface of the Earth (i.e., valleys, mountains, plateaus, plains).

Lateral moraine — A low ridge-like accumulation of till left by a glacier along the sides of the glacier.

Lichen — A small, slow-growing plant that colonizes rock surfaces.

Lithosphere — The lithosphere is the solid, rigid upper skin of the Earth that includes both crust and the solid uppermost layer of the mantle. Plates made of rigid lithosphere move as single pieces and “float” about on the slowly flowing asthenosphere.

Magma — Molten rock material generated within the Earth, usually in the upper part of the mantle and the lower crust.

Mantle — Zone of the Earth below the crust and above the core, approximately 2,900 kilometers thick (1,802 miles).

Metamorphic rock — Rock that has been changed in composition or appearance by heat and/or pressure without being completely melted.

Mineral — A naturally occurring element or compound with a specific structure and chemical composition.

Moraine — A hill or ridge made of glacial till deposited by a glacier.

Moss — A small plant with fine root hairs that commonly grows on rock surfaces.

Pangaea — The name that Alfred Wegener proposed for the supercontinent making up all the landmasses of Earth.

Plate — see definition for Tectonic plate.

Pluton — A body of intrusive igneous rock (for example, granite) formed deep within the Earth’s crust.

Polish — A surface of a rock with luster or highlights polished by the scouring action of moving glaciers.

Quartz — A common, relatively hard mineral with a hexagonal shape when in well-formed crystals. In granitic rock quartz has an irregular shape and can be opaque white to clear with a grayish tint.

Rock — Any consolidated naturally formed mass of one or more minerals. Rocks are classified by the processes that formed them and the minerals they contain.

Sedimentary rock — Rock formed by the consolidation of deposits of originally loose sediment, such as sand, gravel, or mud.

Strike-slip — A type of fault movement in which the two sides of the fault slide side by side.

Subduction — Process by which one tectonic plate descends beneath another into the mantle.

Talus — An accumulation of fallen, broken rock at the base of a steep incline.

Tectonic plate — One of many large segments of the Earth’s lithosphere that moves in relation to other moving plates.

Terminal moraine — Terminal moraines mark the farthest advance of a valley glacier.

U-Shaped — Typical shape of a valley modified by glacial erosion.

Uplift — Elevation of a large section of the Earth's surface relative to another part.

V-Shaped — Typical shape of a valley cut by a stream or river.

Volcanic — Igneous rock solidified from magma that has been extruded onto the surface of the Earth, usually from a volcano.

Volcano — A vent or fissure in the Earth's crust through which molten magma, hot gases, and other fluids escape to the surface.

Weathering — A group of processes that break down and decompose rock, such as rain, ice, wind, temperature changes, plants, and bacteria.

Mechanical weathering — Caused by the expansion of freezing water in cracks in rocks or by plant roots penetrating into cracks in rocks.

Chemical Weathering — Caused by the acidic substances in rainwater.

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