Geologic Terms
Compiled from:
Dictionary of Geological Terms, R. L. Bates and J. A. Jackson, Doubleday Press, 1984.
Volcano World <http://volcano.und.nodak.edu/vwdocs/glossary.html>
Eruptions of Hawaiian Volcanoes: Past, Present and Future, R. I. Tilling, C. C.
Heliker, T. L. Wright, U. S. Geological Survey, 1987
Geological Field Guide of Kïlauea Volcano, R. Hazlett, Hawaii Natural History
Association, 1990
- A’ä
- A very jagged, clinkery type of lava, which when melted is more viscous than pahoehoe.
At Kilauea due primarily to loss of gas and temperature as the lava is flowing.
- Active volcano
- A volcano that is erupting, or a volcano that is not presently erupting but that has
erupted within historical time and is considered likely to do so in the future. Mauna Loa
and Kilauea are active volcanoes.
- Ash
- Fine particles of pulverized rock blown from an explosion vent less than 1/ 10 inch in
diameter.
- Asthenosphere
- The layer of the earth below the lithosphere beginning at a depth of 60 miles (100 km)
and extending to a depth of about 580 miles (350 km). The asthenosphere is a weak layer
and it may be where melts are generated.
- Basalt
- Volcanic rock (or lava) that characteristically is dark in color, contains 45 to 54
percent silica, and generally is rich in iron and magnesium.
- Caldera
- The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at
least a mile in diameter. Such large depressions are typically formed by the subsidence of
volcanoes. Crater Lake occupies the best known caldera in the Cascades.
- Central vent
- A central vent is an opening at the Earth's surface of a volcanic conduit of cylindrical
or pipe-like form.
- Cinder cone
- A volcanic cone built entirely of loose fragmented material.
- Conduit
- A passage that magma follows within the volcano.
- Crater
- A steep-sided, usually circular depression formed by either explosion or collapse at a
volcanic vent.
- Curtain of fire
- A row of lava fountains along a fissure; a typical feature of a Hawaiian-type eruption.
- Density
- The mass of a substance per unit volume (kg/m 3 )
- Dike
- A long, narrow body (like a table top on its side) that cuts into cooler rocks.
- Dome
- Formed where a steep-sided mass of viscous lava is extruded from a volcanic vent. Domes
often have circular bases. Dome surfaces are often rough and blocky as a result of
fragmentation of the cooler, outer crust during growth of the dome.
- Earthquake
- A sudden motion or trembling in the earth caused by the abrupt release of slowly
accumulated strain.
- Epicenter
- The position where the energy released from an earthquake first reaches the earth’s
surface.
- Extrusive activity
- What happens when magma is erupted at the surface
- Fault
- A crack or fracture in the earth's surface. Movement along the fault can cause
earthquakes.
- Fissure
- Elongate fracture or crack on the surface of a rift zone through which an eruption can
occur.
- Fumurale
- A vent on a volcano through which gases other than just water vapor escape.
- Graben
- A valley or trough bounded on both sides by normal faults.
- Hot spot
- A volcanic center, 100 to 200 km across, continuing for at least a few tens of million
of years. Thought to be the surface expression of a persistent rising plume of hot mantle
material. The Hawaiian Islands have been built from a hot spot as the Pacific plate moves
slowly to the northwest.
- Hydrothermal vent
- Vent on the seafloor where hot water comes out from the crust. The hot water cools
abruptly when it hits the cold seawater and minerals precipitate out to build a chimney.
- Intrusion
- When magma is injected into pre-existing rock.
- Kipuka
- An older island of land, usually vegetated, that was completely surrounded by a younger
lava flow. Kipukas are often formed when a lava flows splits into two arms, flows for some
distance, and then rejoins.
- Lava
- Molten rock that has reached the surface; also hardened volcanic rock. Beneath the
surface, the molten rock is called magma.
- Lava flow
- An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified
tongue like or sheet like body formed by outpouring lava.
- Lava pond
- A lake of molten lava
- Lava tube
- A tunnel in a lava flow formed when the surface of a lava stream or channel (similar to
a river of lava) cools and freezes and the interior remains hot.
- Lithosphere
- The rigid crust and uppermost mantle of the Earth. Thickness is on the order of 60 miles
(100 km). Stronger than the underlying asthenosphere.
- Littoral cone
- Spatter cone that is built adjacent to the ocean. Often these constructional features
are built quickly, and may be destroyed equally quickly by constant ocean surf erosion.
Littoral explosions occur when sea water comes into contact with the molten inner core of
the lava flow, flashes instantly to steam, and triggers an explosion of the solidified
outer and molten inner parts of the flow. An example of a half-eroded littoral cone is the
30 m high (100 ft.) Pu’u o Mahana (also known as green sand beach) near Ka Lae (South
Point), which formed during a prehistoric Mauna Loa eruption.
- Magma
- Molten rock that has not reached the surface. Once the magma has reached the surface, it
is called lava
- Magma chamber
- A large body of magma.
- Mantle
- The zone of the earth below the crust and above the core.
- Melt
- Same as magma: molten rock beneath the earth’s surface.
- Mid-ocean ridge
- The site where tectonic plates move apart and new oceanic crust is created.
- Moho
- Also called the Mohorovicic discontinuity. The surface or discontinuity that separates
the crust from the mantle. The Moho is at a depth of 5-10 km beneath the ocean floor and
about 35 km below the continents (but down to 60 km below mountains).
- Normal fault
- A fault where the side of the fault which is lowered slips down a surface which is not
overhanging.
- Oceanic crust
- The earth’s crust where it underlies oceans.
- Olivine
- A translucent green mineral made up of iron, magnesium, and silica. It is the most
abundant phenocryst in the lavas of Kilauea.
- Paho’eho’e
- A smooth-skinned, glassy, lumpy and in places ropy form of lava. When melted, pahoehoe
is less viscous than a’a, the other principal form of lava found at Kilauea. The low
viscosity of melted pahoehoe is due primarily to high temperature (typically exceeding
1100 C) and high dissolved gas content.
- Paleomagnetization
- Properties of a rock which allow one to determine the characteristics of the
earth’s magnetic field at the time and location of the rock’s formation.
- Pele's hair
- Thin strands of golden brown volcanic glass formed as lava is spewed from the volcano;
thick mats often form downwind from lava fountains near a vent.
- Pele's tears
- small, shiny lapilli of volcanic spatter usually takes on a "tear shape".
- Perched lava pond
- Built when a lava flow moving in a channel down the side of a cone hits flat ground,
spreads out radially, and cools. Perched lava ponds are usually a few 100 m in diameter
and a few meters high.
- Phenocryst
- A crystal in lava, often apparent to the naked eye.
- Pillow lava
- Fluid lava erupted or flowing under water which squeezes out through thin walls of
submarine tubes like toothpaste and quickly solidifies forming glassy outer skin; sack or
pillow-shaped in cross section ranging up to about 1 meter in diameter. Erupted under high
pressure due to overlying water, so little or no explosive interaction between hot lava
and cold water.
- Pit crater
- Steep-sided craters, often with flat rims, that form by collapse of the ground.
- Porosity
- The percentage of open or void space in a rock relative to solid material.
- Pumice
- Light-colored, frothy volcanic rock, formed by the expansion of gas in erupting lava.
Commonly seen as lumps or fragments of pea size and larger, but can also occur abundantly
as ash sized particles.
- Relief
- The vertical difference between the top of a volcano and the adjacent valley or plain.
- Rift zone
- Elongate tapering ridges radiating from summit calderas and extend down volcano flanks
into the sea. Fissures, pit craters, cinder and spatter cones, and small volcanic shield
delineate the rift axis. Orientation of rift zones influenced by gravitational stresses
and buttressing effects of pre-existing neighboring volcanoes (e.g., Kilauea volcano is
buttressed against Mauna Loa). During rift or flank eruptions or intrusions, the summit
usually deflates as magma moves into the rift zone accompanied by earthquake swarms map
the magma movement.
- Seafloor spreading
- The mechanism by which new seafloor crust is created at oceanic ridges and slowly
spreads away as the plates separate.
- Seamount
- Underwater volcano.
- Seismic swarm
- A group of earthquakes which occur n an area during a short interval of time.
- Shield volcano
- Type of volcano formed by many fluid, non-explosive, lava eruptions that successively
pile up forming a broad, gently sloping, convex-upward landform whose profile resembles a
Roman warrior’s shield. Hawaiian shield volcanoes are the largest mountains on Earth.
Typically they have broad summit caldera with rift zones radiating from the summit; most
eruptions take place within the summit caldera or along its rift zones. Repeated rift zone
magmatic intrusions have pushed the south flank of Kilauea seaward at rates of about 10
cm/year, and intrusions and eruptions load the volcano flanks contributing to flank
instability and ultimately resulting in a large earthquake (e.g., 1972 magnitude 7.2
Kalapana earthquake which activated the Hilina Fault System and caused a 3-meter drop and
8-meter southward shift in Kilauea’s south flank).
- Skylight
- Holes in the roof of lava tubes. In active tubes you can see the red hot lava when you
look into the tube through the skylight.
- Spatter
- The droplets and clots of very fluid molten lava which fall around the base of a lava
fountain.
- Strain
- Change in the shape or volume of a body as the result of stress.
- Stress
- A concentration of force (force per unit area)
- Strike-slip fault
- A fault along which the movement is entirely horizontal (just like the San Andreas fault
in California)
- Subaerial
- Pertaining to regions above sea level.
- Submarine
- Pertaining to regions below sea level.
- Talus
- Rock debris, as from landsliding, at the base of the slope
- Tectonic
- Refers to the large scale processes by which rocks become deformed.
- Terrace
- A volcanic feature similar to a perched lava pond, but fed from a lava tube.
- Tilt
- The angular change in the slope of a volcano accompanying inflation or deflation of the
volcano surface as magma moves up into it
- Tremor
- Low amplitude, continuous earthquake activity often associated with movement of magma
into a rift zone
- Tumulus
- A small hill or knob formed when the upper solid surface of a lava flow cracks and bows
upward due to fluid pressure within the flow. Sometimes lava flows out of the crack.
- Vent
- The opening at the earth’s surface through which lava and gas issue forth.
- Viscosity
- The resistance of a fluid to flow. A highly viscous substance does not readily flow,
like thick syrup. Water is an example of a fluid with low viscosity.
- Volatiles
- Substances in magma which can be released as gases.