Geology Terms

Aa - a Hawaiian word used to describe rough, jagged lava. So named because if you fall down on it you’re likely to cry out "Ah! Ah!" See Pahoehoe.

Adit - a nearly horizontal entrance to a mine.

Agate - a semi-precious stone prized by rockhounds. It is composed of bands of chalcedony, a type of cryptocrystalline quartz, which means that the crystals are too small to be seen even with a microscope. Agates are formed in the vesicles of volcanic rocks.

Alluvial - any material or process associated with transportation or deposition by a stream. "Alluvium" refers to the material (sand) and alluvial refers to the process. Alluvial is pretty much interchangeable with the word “fluvial.”

Alluvial Fan - a very large, fan-shaped deposit of sand and gravel at the base of a mountain in the arid west. The fan is formed when a stream courses down the steep mountain and then hit the plains below. The stream loses all of its energy and dumps its load of sand, silt and gravel. When there are numerous alluvial fans that blend together, the resulting deposit is called a bajada.

Alpine - the name given to any environment that resembles the Alps, a high, mountainous area above the tree line with a cold climate.

Ammonite - a beautiful, coiled mollusk that was predominant in the Mesozoic seas. The fossil shells are so abundant and diverse that the Mesozoic was subdivided into smaller pieces of time based on this one animal. A modern example of an ammonite is the chambered nautilus.

Andesite- a fine-grained volcanic rock that is in-between basalt and rhyolite in chemical composition. Andesite has more quartz and light-colored feldspars than basalt, but less than rhyolite. You can recognize it as a sort of gray, indistinct kind of rock.

Angle of Repose - the maximum angle at which material such as soil or loose rock remains stable. Often used in conjunction with descriptions of cinder cones to describe why they all have the same basic shape and steepness.

Anticline - a convex, upward-tending fold in rock, created by intense pressure.

Arête - a narrow, saw-toothed mountain ridge formed when cirques (see below) are carved out on either side of the ridge by glaciers

Badlands - an area where loose sediments with minimal vegetation erode easily by contact with water. Generally, badlands form where deposits of shale have eroded so much that there is hardly actual rock left. Because of the presence of a variety of minerals, badlands are often multi-colored and have names like Artist’s Palette. The example in the accompanying photo is Zabriskie Point in Death Valley. Dinosaur fossils have often been found in badland areas. Probably the most well-known of all the badlands is in North Dakota, around Mount Rushmore.

Bar - the name for the sand and gravel that is deposited on the inside bend of a winding river or stream.

Basalt - a fine-grained, dark-colored igneous rock containing primarily calcic plagioclase with some pyroxene and sometimes olivine.

Batholith - large (huge!) volume of igneous rock that cooled slowly underground at great depth. Whole mountain ranges are formed from a single batholith. The name comes from the Greek bathos (depth) and lithos (stone). In order to qualify as a batholith, the igneous mass must be so deep into the ground that the bottom cannot be found and have an area of surface exposure greater than 100 square kilometers (that’s about 70 square miles)!

Bedrock - a general term for the solid rock that underlies the soil and other unconsolidated material or that is exposed at the surface.

Bench - a shelf-like or ridge-like cut in rock.

Brachiopod - an extinct mollusk abundant in the Paleozoic that looks a lot like a clam with bilaterally symmetrical shells. Their muscles worked the opposite of clams, however, so that when brachiopods relaxed (died) their shells shut instead of coming open. That means that when you find a fossil brachiopod you usually find both shells together clamped shut.

Breccia - a rock made up of angular, coarse fragments of other rock.

Caldera - a large, basin-shaped volcanic depression caused by an explosion or collapse associated with an eruption.

Chevron Fold - a dramatic feature where rocks are folded so sharply that they form a zig-zag pattern, similar to a chevron shape. Chevron folds are a sign of intense deformation.

Cinder Cone - a conical elevation formed by the accumulation of ash or cinders (pyroclastic ejecta) around a volcanic vent. A cinder cone can’t form unless a main volcano is in the area.

Cirque - a bowl-shaped depression open in front, caused by glaciation, and sometimes containing a round lake, or tarn.

Cleavage - the property of a mineral that determines how it splits along planar surfaces which are determined by the particular crystalline structure of the material.

Crossbedding - sedimentary strata that cross at an angle with each other. The most dramatic cross beds form from the migration of wind-formed dunes (Checkerboard Mesa in Zion National Park, Utah). Also known as inclined bedding or current bedding. Crossbedding is useful to geologists who are trying to reconstruct depositional environments, because they offer a clue as to what direction was upstream (or upwind).

Delta - a body of alluvium, nearly flat and fan-shaped at or near the mouth of a stream where it enters a body of relatively quiet water, usually a lake. Deltas are named for their shape, because the shape is supposed to resemble the Greek letter delta. A classic delta is the Mississippi delta, of course, but Stockton, California also boasts a large delta that starts at Stockton and continues to Antioch on its way to the San Francisco Bay.

Depression - any relatively sunken part of the Earth’s surface, especially a low-lying area surrounded by higher ground. In the environmental world, depressions are good places to investigate for soil and groundwater contamination, because humans typically view depressions as overly large trash containers.

Detritis - the general term for loose sediment, sand, gravel, silt and clay. The term is particularly used for small rock fragments derived from larger rocks.

Dike - a tabular insertion of igneous rock that cuts across a preexisting rock body. Sometimes they are very obvious stripes, especially when the surrounding rock is darkly colored and the dike is aplite (mostly quartz and therefore white). Such a dike is shown in the accompanying photo.

Drift - a term applied to all detrital material (clay, sand, silt, boulders) transported by a glacier and deposited directly by or from the ice, or by running water emanating from the glacier. Generally applies to Pleistocene glacial deposits.

Dune - a low mound, ridge, bank, or hill of loose, windblown, granular material (generally sand), either bare or covered by vegetation, that is capable of movement from place to place but always maintaining its characteristic shape. This dry description does not convey how breathtakingly beautiful dunes can be, especially in the late afternoon sun with shadows in their folds. Dunes are very difficult to trudge through, however. Think of John Wayne in “Three Godfathers.” The Algodones Dunes, located in Imperial County about 20 miles west of Yuma, Arizona, are the most spectacular dunes in California and have been the site of a large number of Hollywood films.

Earthcaching - a spin-off activity from geocaching which focuses on earth science education. Sponsored by and the Geological Society of America (GSA).

Ejecta - material thrown out by a volcano, such as ash, lava bombs, and lapilli.

Erratic - a rock or rock fragment, often very large, that is distinctly out of place. The erratic is usually carried by glacial ice, or, less commonly, by floating ice, and subsequently deposited at some distance from the outcrop where it originated.

Exfoliation - breaking or peeling off of concentric sheets (like an onion) from bare rock surfaces by physical or chemical forces.

Facies - part of a rock body as differentiated from other parts by composition or appearance. Also, igneous facies is a type of igneous rock that is somewhat abnormal compared to the larger mass of which it is a part.

Fault - a fracture in a body of rock, with displacement of the rocks on either side relative to one another. Rocks on one side of the fault have either moved up, down, or sideways relative to rocks on the other side

Fault Scarp - a cliff formed by a fault. The most dramatic fault scarp in the United States is the Grand Teton Mountains in Wyoming. A good local example is the Berkeley Hills that formed on one side of the Hayward Fault.

Flowstone - deposits of calcium carbonate on cave walls caused by dripping water.

Flood plain - the nearly level plain that borders a stream and is subject to inundation under flood stage conditions unless protected artificially, such as by levees. People sometimes build on the flood plain, but, as every geologist knows, the flood plain ultimately belongs to the river.

Fluvial - of or pertaining to rivers; produced by river action (see alluvial).

Fold - a bend in strata or any planar structure, usually because of deformation. Folds show up very well in the bedded cherts around San Francisco, as well as in outcrops along highway 14 near Lancaster.

Frost Weathering or Wedging - the disentegration of rock brought about by freezing and thawing. This is a tremendously important mechanism of weathering in mountainous areas. As water gets into the small cracks and macropores in a rock, it freezes and subsequently expands, pushing out against the rock. During the day, the ice converts back to water and then at night freezes again and starts the process all over again.

Fumerole - a hole or vent in the earth’s crust through which gases and fumes emanate, usually in volcanic areas, often in late-stage volcanism. You can observe several fumeroles at Lassen National Park.

Fusilinid - tiny football shaped microfossils that have been very well studied because of their association with oil-producing rocks.

Geocaching - an activity where participants search, using a GPS-receiver, for "caches," usually containers hidden at a specified latitude and longtitude. The containers can include anything, but often include a notebook so geocachers can record their visit. Official website:

Geomorphology - the science that treats the general configuration of the earth’s surface; specifically the study of the classification, description, nature, origin, and development of landforms and their relationship to underlying structures, and of the history of geologic changes as recorded by these surface features.

Glacier - a large mass of ice formed by the compaction and recrystallization of snow that moves slowly downslope due to its own weight. Glaciers are one of the three agents of erosion (the other two being wind and water) and are responsible for the spectacular scenery in Yosemite and elsewhere in the Sierra Nevadas. Although the really large glaciers of the past ice ages have long since receded from California’s landscape, smaller glaciers still ring the summit of Mt. Shasta and the high mountains in the Sierras.

Glacial lake - a lake created by glacial ice that forms a dam, holding the water in place. Glacial lakes are usually associated with continental ice sheets, the broad, deep sheets of glacial ice that covered much of the United States during the ice age (as opposed to Valley or Alpine glaciers, the type that formed in Yosemite).

Glaciation - the formation, movement, and recession of glaciers or ice sheets, geologic processes of glacial activity.

Glaciofluvial deposits - material moved by glaciers and subsequently sorted and deposited by streams flowing from the melting ice. Materials are commonly stratified.

Granite - one of the most recognized igneous rocks, consisting primarily of quartz and potassium feldspar with crystals that are easily visible to the naked eye. Granite may be very pink or almost white, depending on the amount of pink potassium feldspar present in the matrix. Granite that cooled very slowly underground will have relatively large crystals while granite that cooled more quickly will have smaller crystals.

Grüs - an accumulation of angular, coarse-grained fragments resulting from the disintegration of granite. When you’re walking on grüs you’ll know it by the crunch underfoot.

Hanging Valley - a valley carved by a tributary glacier that flowed into a main, or "trunk" glacier. The smaller valley is higher in elevation because the main glacier had greater erosive power than the smaller, tributary glacier.

Helictite - a calcium carbonate deposit in a cave which grows at odd angles in a twig-like manner.

Hoodoo - pillar left standing by the rainwater erosion of the material around them.

Hornito - a small mound of spatter formed by lava thrown up through the ceiling of a lava tube.

Icthyosaur - an extinct marine lizard shaped something like a porpoise or a shark, but more like a whale in size, that lived in the oceans of the Middle Triassic to Upper Cretaceous periods.

Igneous - rocks solidified from a melt. The melt is usually associated with volcanic activity on the surface, but occurs abundantly underground as well.

Joint - a fracture in rock along which no appreciable movement has occurred.

Kame - a low mound, ridge, or hummock deposited by streams flowing out of glaciers. Kames are more commonly encountered in the eastern United States where large continental ice sheets formed.

Lahar - a volcanic mudflow.

Lakebed - the flat to gently undulating ground that underlies a lake or former lake. Generally composed of fine-grained sediment. Several ancient lakebeds exist in Central California; in fact, the entire Great Valley itself was once covered by a lake, or a series of lakes.

Lake plain - a nearly level surface marking the floor of an extinct lake filled with well-sorted generally fine-textured sediments that are commonly stratified.

Landform - any physical, recognizable form or feature on the earth’s surface, having a characteristic shape and range in composition, and produced by natural causes.

Lapilli - volcanic ejecta of small size.

Lateral Moraine - a ridge-like moraine carried on and deposited at the side margin of a valley glacier.

Lava - fluid rock that issues from a volcano or fissure in the earth, or the solidified rock which results after cooling. If the fluid rock is underground and not on the surface, it is referred to as magma.

Limestone - a sedimentary deposit made out of calcium carbonate. Limestone mainly forms in the ocean from the millions of marine skeletons and shells that rain down from the surface every day. Over time, the ocean bottom is buried and subject to heat and pressure, and eventually becomes limestone. The origin of limestone is readily apparent in many rocks in which the fossils are abundant and easily seen. Some limestone rock, however, was formed from a limey mud and fossils are not apparent. Limestone that was formed in fresh water (such as a lake) is called travertine.

Magma - naturally occurring mobile rock material within the earth (see lava, which is the name given to magma that occurs above the surface).

Metamorphic - type of rock that has been changed from another, preexisting rock due to heat, temperature, and chemically active fluids associated with burial underground. Limestone, for example, metamorphoses into marble, and shale into slate. The heat and pressure the rock is exposed to is enough to change the rock but not enough to melt it.

Monzonite - a granular plutonic rock containing equal amounts of orthoclase (potassium feldspar) and plagioclase. Monzonite has less quartz than granite but more than gabbro.

Moraine - a general name given to unsorted silt, sand, gravel, and cobbles deposited by glaciers. Various types include ground moraine, lateral moraine, recessional moraine, etc., all of which refer to what was going on with the glacier at the time. A lateral moraine, for example, is material that was deposited on the side of a glacier, while a recessional moraine was deposited during a pause in a glacial retreat. A ground moraine is also known as a bottom moraine, and refers to material that was carried along the bottom of a glacier. And a terminal moraine marks the farthest advance of a glacier and can be a hill of a fairly significant size. Moraines can be huge deposits if formed from large continental ice sheets. In Minnesota, a popular ski resort is built on a terminal moraine.

Moraine kame - an end moraine that contains numerous kames. A kame is a low ridge of glacial deposit that formed as a delta at the glacial front by meltwater streams.

Obsidian - volcanic glass, usually black. If you get a small enough piece you’ll see that it is actually opaque rather than black. Obsidian can be recognized by its conchoidal fracture, which is the semi-circular breakage pattern that all glass exhibits (think of a broken glass coke bottle, if you’re old enough).

Ophiolite Suite - a piece of the oceanic crust that has ended up on the continent through the action of plate tectonics. The suite consists of a base unit of gabbro that formed from the initial magma chamber, vertical sheeted dikes that formed as the magma moved upward in the crust, and a top layer of pillow basalt, which formed as the magma extruded out onto the oceanic floor and was cooled by oceanic waters.

Orogeny - the process of mountain building.

Pahoehoe - a Hawaiian word used to describe smooth, ropy lava. See (AA).

Petrified - turned to stone. Organic substances such as wood or seashells embedded in sediment are petrified by the gradual replacement of their tissues, cell by cell, with mineral grains.

Pillow basalt - basalt that was extruded initially at an oceanic trench. As the basalt emerged, the waters of the ocean cooled and rounded the magma, forming "pillows" that are seen on land today. Pillow basalts are the top sequence in an ophiolite suite.

Phenocryst - the name given to visible crystals that are larger than the surrounding crystals in an igneous rock. Pothole Dome in Yosemite has some real beauties.

Physiographic province - a region of which all parts are similar in geologic structure and climate and which has consequently had a unified geomorphic history; a region whose relief features and landforms differ significantly from that of adjacent regions.

Plate tectonics - theory of the movement of rigid plates of earth's crust around the globe, resulting in continental drift and changes to the shapes of the continents as well as ocean basins. At the meeting of the plate boundaries, considerable tectonic activity, such as earthquakes, occurs due to stress.

Playa - broad, desert plain in an undrained basin, occasionally filed with a lake. The most famous playa in the U.S. is the Great Salt Lake and surrounding area. In California, playas are most common in the southern desert regions. As the water evaporates in these basins, evaporite minerals such as salt deposits are left behind.

Pluton - a body of igneous rock that has formed beneath the surface of the earth by consolidation from magma.

Plutonic - igneous rocks formed at great depth beneath the earth’s surface.

Porphyry - rocks containing conspicuous phenocrysts in a fine-grained ground mass, representing two stages of cooling. The larger crystals formed slowly and then, as conditions changed, the remaining crystals formed quickly.

Post-glacial - refers to the Holocene (i.e., present time).

Pumice - a porous, lightweight volcanic glass pitted with gas bubble vesicles.

Radiolarians - numerous marine protozoa with complex silica-based skeletons. Layers of sea-bed deposits are often composed of such skeletons.

Rain Shadow - The dry side of a mountain range that results when air loses its precipitation as it moves across the range. The windward side of a mountain range is typically wet while the lee side is dry.

Roche Moutonnée - a resistant knob of rock left behind after a glacier carved its way around it. This is a French term meaning “sheep rock,” named for the common shape such features take. Fannette Island in Emerald Bay at Lake Tahoe is a roche moutonnée.

Roof Pendant - older rocks atop a batholith.

Rhyolite - an igneous rock with very small crystals, giving it a homogenous appearance.

Sand Dune - (see Dune)

Scarp - a line of cliffs, usually formed by faulting.

Sea Stack - an isolated pillar of rock, separated from the mainland scarp by wave erosion.

Sedimentary - type of rock formed from sediments deposited by the action of wind, water or ice. Examples of sedimentary depositional environments include lakes, streams, oceans, glaciers, or sand dunes.

Serpentinite - California’s state rock, metamorphic, consisting of serpentine minerals derived from the alteration of previously existing mantle materials olivine and pyroxene.

Shale - a sedimentary rock exhibiting bedding, usually soft and easily broken into its constituent layers.

Shield Volcano - a broad, gently sloping volcanic cone of flat, domical shape, covering several tens or hundreds of square miles, built chiefly of overlapping basaltic lava flows.

Skarn - rock composed almost entirely of lime-bearing silicates, derived from limestone or dolomite.

Slate - a fine-grained metamorphic rock.

Soil Creep - the gradual, slow, downhill movement of soil on a steep hillside. The actual movement is imperceptible to the human eye but is evidenced by trees bending in the downslope direction.

Spattercone - a particularly messy volcanic vent or fissure. These are general low with steep sides created by the spatter of a lava fountain.

Stalactite - a cylindrical cave feature formed from calcite or aragonite, hanging from the roof of a cavern.

Stalagmite - Columns or ridges of carbonate of lime rising from a limestone cave floor. Formed by water dropping from the stalactites above. When stalactites and stalagmites meet, they form a column.

Stratified - sedimentary rocks that have been formed, arranged, or laid down in layers.

Stratovolcano - a volcanic cone, generally large, characterized by alternating layers of lava and pyroclastic materials.

Stream terrace - one of a series of platforms in a stream valley, flanking and more or less parallel to the stream channel, originally formed near the level of the stream, and representing the dissected remnants of an abandoned flood plain, stream bed, or valley floor produced during a former state of erosion or deposition.

Striations - lines scratched into the bedrock upon which a glacier flowed. The scratches were created by rocks caught up and carried at the base of the glacial ice.

Strike-slip fault - a fault in which the rock mass on either side of the fault moves parallel to each other (rather than up or down).

Subduction - in plate tectonics, the process of the edge of one tectonic plate sliding beneath another.

Subduction Zone - the area along which subduction takes place. Deep oceanic trenches occur along subduction zones.

Summit - the topographically highest point on a mountain top or hill and exhibiting a nearly level surface.

Syncline - a downward-tending fold in rock created by intense pressure.

Tafoni - any rock that has weathered so that it has a honeycomb structure. The name comes from Sicily where impressive honeycomb structures have formed in the coastal granite. The tafoni pictured here is from a S.F. Bay area trail called, aptly enough, Tafoni Trail.

Talus - rock fragments, usually large and angular, that have piled up at the base of a scarp, cliff, or steep slope.

Tarn - the lake which occupies a cirque, a mountain basin carved out by a glacier. Tarns come with a guarantee of alpine beauty.

Tectonic - pertaining to the rock structure resulting from the deformation of the earth's crust.

Terrain - topography, or the character of rock in a geographic region.

Terrane - a fault-bounded grouping of rocks characterized by being stratigraphically or structurally distinct from the surrounding rocks. Generally referred to as "suspect" terrane.

Topography - the relative position and elevations of the natural or manmade features of an area that describe the configuration of its surface.

Transform Fault - a strike-slip fault characteristic of oceanic ridges and along which the ridges are offset.

Trilobite - extinct Paleozoic marine arthropod whose dorsal skeleton consisted of three distinct parts (hence the "tri" in trilobite), namely, the cephalon (head), thorax (body), and pygidium (tail). The closest living relative of the trilobite is the modern horseshoe crab.

Tsunami - a Japanese word meaning "harbor wave," a tsunami is a wave or series of waves caused by a displacement of water by an earthquake or landslide. When a tsunami encounters land, it can wreak destruction of disastrous proportions.

Tufa - also known as travertine, a type of sedimentary rock composed of calcium carbonate or of silica, deposited from solution in the water of a spring or of a lake or from percolating ground water. In other words, freshwater limestone. Dramatic tufa towers are found in and around Mono Lake and at Trona Pinnacles in Southern California.

Tuff - a rock formed of compacted volcanic fragments, generally smaller than 4 mm. in diameter. Tuff often occurs in small, distinct layers and is very useful for dating events. The hypothesis that dinosaurs became extinct because of meteor impact is based on the occurrence of a rare earth metal found in a distinct tuff layer that is almost world wide.

Ultramafic - igneous rock that contains almost no quartz or feldspar and instead is composed of mantle-type minerals such as dunite, perodite, amphibolite, and pyroxenite.

Volcano - c’mon, everyone knows what a volcano is. Okay, it’s a vent in the earth’s crust that provides a conduit for magma to reach the earth’s surface. Also used to describe a mountain which erupted in the past.

Volcanic Plug - what is left of the volcano after the outer part has eroded away leaving behind the central plug. Volcanic plugs can be recognized because they are usually a large, contorted monolithic mass of solidified igneous rock for which no other reasonable explanation exists.

Xenolith - a preexisting rock which has been incorporated into an igneous flow. The xenoliths in the Sierra Nevadas are easily seen as large rectangles of dark basalt incorporated into the lighter colored granite.

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