Aa - Aa (pronounced "ah-ah" - a Hawaiian term), is lava that has a rough, jagged, spiny, and generally clinkery surface. In thick aa flows, the rubbly surface of loose clinkers and blocks hides a massive, relatively dense interior.

Active volcano - A volcano that is currently erupting, or has erupted during recorded history.

Aftershock - An earthquake which follows a larger earthquake or main shock and originates in or near the rupture zone of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time.

Amplitude - The maximum height of a wave crest or depth of a trough.

Andesite - A medium-colored dark gray volcanic rock containing 53-63 percent silica with a moderate viscosity when in a molten state. Intermediate in color, composition, and eruptive character between basalt and dacite.

Aseismic - Not associated with an earthquake, as in aseismic slip. Also used to indicate an area with no record of earthquakes; an aseismic zone.

Ash (volcanic) - Fragments less than 2 millimeters (about 1/8 inch) in diameter of lava or rock blasted into the air by volcanic explosions.

Ash cloud - The fine material that is generated by a pyroclastic flow and rises above it.

Ash flow - A pyroclastic flow consisting predominantly of ash-sized (less than 4 millimeters in diameter) particles. Also called a glowing avalanche if it is of very high temperature.

- Dark-colored, low-silica (less than 53 percent SiO2), low viscosity volcanic rock that is relatively fluid when molten; eruptions of basalt are generally nonexplosive and tend to produce relatively long thin lava flows like those common in Hawaii.

Body wave - A seismic wave that can travel through the interior of the earth. P-waves and S-waves are body waves.

Bombs - Tephra is the general term now used by volcanologists for airborne volcanic ejecta of any size. Historically, however, various terms have been used to describe ejecta of different sizes. ... Fragments larger than about 2.5 inches are called blocks if they were ejected in a solid state and volcanic bombs if ejected in semi-solid, or plastic, condition. ... Volcanic bombs undergo widely varying degrees of aerodynamic shaping, depending on their fluidity, during the flight through the atmosphere. Based on their shapes after they hit the ground, bombs are variously described, in graphic terms, as "spindle or fusiform,", "ribbon", "bread-crust", or "cow-dung"

- Cinders are lava fragments about 1 centimeter (about 1/2 inch) in diameter

Cinder cone - A steep-sided volcano formed by the explosive eruption of cinders that form around a vent.

Composite volcano - A steep-sided volcano built by lava flows and tephra deposits.

Core - The innermost layers of the Earth. The inner core is solid and has a radius of about 1300 kilometers. (The radius of the Earth is about 6371 kilometers.) The outer core is fluid and is about 2300 kilometers thick. S-waves cannot travel through the outer core.

Continental Drift - The theory, first advanced by Alfred Wegener, that Earth's continents were originally one land mass. Pieces of the land mass split off and migrated to form the continents.

Crater - The circular depression containing a volcanic vent.

Crust - The thin outer layer of the Earth's surface, averaging about 10 kilometers thick under the oceans and up tp about 50 kilometers thick on the continents. This is the only layer of the Earth that humans have actually seen.

Dome -
A steep-sided mount that forms when very viscous lava is extruded from a volcanic vent.

Dormant volcano - An active volcano that is in repose (quiescence) but is expected to erupt in the future.

Earthquake -
Shaking of the Earth caused by a sudden movement of rock beneath its surface.

Epicenter - That point on the Earth's surface directly above the hypocenter of an earthquake.

Extinct volcano - A volcano that is not expected to erupt again.

Fault -
A weak point in the Earth's crust and upper mantle where the rock layers have ruptured and slipped. Faults are caused by earthquakes, and earthquakes are likely to reoccur on pre-existing faults.

First arrival - The first recorded signal attributed to seismic wave travel from a source.

Focus - That point within the Earth from which originates the first motion of an earthquake and its elastic waves.

Foreshock - A small tremor that commonly precedes a larger earthquake or main shock by seconds to weeks and that originates in or near the rupture zone of the larger earthquake.

Hazard -
A risk. An object or situation that has the possibility of injury or damage.

Hot Spot - An area in the middle of a lithospheric plate where magma rises from the mantle and erupts at the Earth's surface. Volcanoes sometimes occur above a hot spot.

Hypocenter - hat point within the Earth from which originates the first motion of an earthquake and its elastic waves; where the break in the fault actually occurred. 

Intensity -
A measure of the effects of an earthquake at a particular place on humans, structures and (or) the land itself. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the point and the local geology at that point.

Igneous rocks - Igneous rocks are formed from melted rock that has cooled and solidified. When rocks are buried deep within the Earth, they melt because of the high pressure and temperature; the molten rock (called magma) can then flow upward or even be erupted from a volcano onto the Earth's surface. When magma cools slowly, usually at depths of thousands of feet, crystals grow from the molten liquid, and a coarse-grained rock forms. When magma cools rapidly, usually at or near the Earth's surface, the crystals are extremely small, and a fine-grained rock results. A wide variety of rocks are formed by different cooling rates and different chemical compositions of the original magma. Obsidian (volcanic glass), granite, basalt, and andesite porphyry are four of the many types of igneous rock.

"Island Arc" Volcanoes - In a typical "island-arc" environment, volcanoes lie along the crest of an arcuate, crustal ridge bounded on its convex side by a deep oceanic trench.


Landslide - An abrupt movement of soil and bedrock downhill in response to gravity. Landslides can be triggered by an earthquake or other natural causes. Undersea landslides can cause tsunamis.

Lava - The term used for magma once it has erupted onto the Earth's surface.

Lava flow - Stream of molten rock that erupts relatively nonexplosively from a volcano and moves slowly downslope.

Liquefaction - The process in which a solid (soil) takes on the characteristics of a liquid as a result of an increase in pore pressure and a reduction in stress. In other words, solid ground turns to jelly.

Mafic - Term used to describe volcanic rock or magma composed chiefly of dark-colored, iron- and magnesium-rich minerals.

Magma - Molten rock containing liquids, crystals, and dissolved gases that forms within the upper part of the Earth's mantle and crust. When erupted onto the Earth's surface, it is called lava.

Magnitude - A measure of the strength of an earthquake or strain energy released by it, as determined by seismographic observations. This is a logarithmic value originally defined by Charles Richter (1935). An increase of one unit of magnitude (for example, from 4.6 to 5.6) represents a 10-fold increase in wave amplitude on a seismogram or approximately a 30-fold increase in the energy released. In other words, a magnitude 6.7 earthquake releases over 900 times (30 times 30) the energy of a 4.7 earthquake - or it takes about 900 magnitude 4.7 earthquakes to equal the energy released in a single 6.7 earthquake! There is no beginning nor end to this scale. However, rock mechanics seems to preclude earthquakes smaller than about -1 or larger than about 9.5. A magnitude -1.0 event release about 900 times less energy than a magnitude 1.0 quake. Except in special circumstances, earthquakes below magnitude 2.5 are not generally felt by humans.

Mantle - The layer of rock that lies between the crust and the outer core of the Earth. It is approximately 2900 kilometers thick and is the largest of the Earth's major layers.

Metamorphic rocks - Sometimes sedimentary and igneous rocks are subject to pressures so intense or heat so high that they are completely changed. They become metamorphic rocks, which form while buried within the Earth's crust. The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks.

Modified Mercalli scale - Mercalli intensity scale modified for North American conditions. A scale, composed of 12 increasing levels of intensity that range from imperceptible shaking to catastrophic destruction, that is designated by Roman numerals. It does not have a mathematical basis; instead it is an arbitrary ranking based on observed effects.

Mudflow - The flowing mixture of water and debris (intermediate between a volcanic avalanche and a water flood) that forms on the slopes of a volcano.

P wave -
Primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave. P waves are the fastest body waves and arrive at stations before the S waves, or secondary waves. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layers of the Earth. P waves are generally felt by humans as a bang or thump.

Pahoehoe - Pahoehoe (pronounced "pah-hoy-hoy" - a Hawaiian term), is lava that in solidified form is characterized by a smooth, billowy, or ropy surface.

Paleomagnetism - The natural magnetic traces that reveal the intensity and direction of Earth's magnetic field in the geologic past. Also, the study of these magnetic traces

Pillow Lava - Fluid lava erupted or flowing under water may form a special structure called pillow lava. Such structures form when molten lava breaks through the thin walls of underwater tubes, squeezes out like toothpaste, and quickly solidifies as irregular, tongue-like protrusions. This process is repeated countless times, and the resulting protrusions stack one upon another as the lava flow advances underwater. The term pillow comes from the observation that these stacked protrusions are sack- or pillow-shaped in cross section.

Plate - One of the huge sections which make up the Earth's crust. The plates are continuously moving.

Plate boundary - The place where two or more plates in the Earth's crust meet.

Plate Tectonics - The theory that the Earth's crust and upper mantle (the lithosphere) is broken into a number of more or less rigid, but constantly moving, segments or plates.

Pluton - Pertaining to igneous rock bodies that form at great depth.

Pumice - A light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly perceived as lumps or fragments of pea size and larger but can also occur abundantly as ash-size particles. Because of its numerous gas bubbles, pumice commonly floats on water.

Pyroclastic flow - A hot, fast-moving and high-density mixture of fine and coarse particles and gas formed during explosive eruptions or from the collapse of a lava dome.

Reflect -
To bounce back from a surface.

Refract - To bend or change direction.

Rhyolite - Volcanic rock (or lava) that characteristically is light in color, contains 69 percent silica or more, and is rich in potassium and sodium.

Richter scale - The system used to measure the strength of an earthquake. Developed by Charles Richter in 1935 as a means of categorizing local earthquakes. It is a collection of mathematical formulas; it is not a physical device.

Rupture Zone - The area of the Earth through which faulting occurred during an earthquake. For very small earthquakes, this zone could be the size of a pinhead, but in the case of a great earthquake, the rupture zone may extend several hundred kilometers in length and tens of kilometers in width.

S wave -
Shear, secondary, rotational, tangential, equivoluminal, distortional, transverse, or shake wave. These waves carry energy through the Earth in very complex patterns of transverse (crosswise) waves. These waves move more slowly than P waves, but in an earthquake they are usually bigger. S waves cannot travel through the outer core because these waves cannot exist in fluids, such as air, water or molten rock.

Scoria - Scoria forms when blobs of gas-charged lava are thrown into the air during an eruption and cool in flight, falling as dark volcanic rockcontaining cavities created by trapped gas bubbles.

Seismicity - Earthquake activity.

Seismic - Of or having to do with earthquakes.

Seismic Sea Wave - A tsunami generated by an undersea earthquake.

Seismic Zone - A region in which earthquakes are known to occur.

Seismogram - A written record of an earthquake, recorded by a seismograph.

Seismograph - An instrument that records the motions of the Earth, especially earthquakes.

Seismograph Station - A site at which one or more seismographs are set up and routinely monitored.

Seismologist - A scientist who studies earthquakes.

Shield volcano - A volcano that resembles an inverted warrior's shield. It has long gentle slopes produced by multiple eruptions of fluid lava flows.

Silica - The molecule formed of silicon and oxygen (SiO2) that is the basic building block of volcanic rocks and the most important factor controlling the fluidity of magma. The higher a magma's silica content, the greater its viscosity or "stickiness."

Spreading Center - An elongated region where two plates are being pulled away from each other. New crust is formed as molten rock is forced upward into the gap. Examples of spreading centers include the Mid-Atlantic Ridge and the East African Rift.

Subduction - The process in which one lithospheric plate collides with and is forced down under another plate and drawn back into the Earth's mantle.

Subduction zone - An elongated region along which a plate descends relative to another plate, for example, the descent of the Nazca plate beneath the South American plate along the Peru-Chile Trench.

Tidal Wave -
A term that seismologists hate. The correct word for the big waves people often call "tidal waves" is tsunami. True "tidal waves" - or waves caused by the tides - are the ordinary waves people see on the ocean.

Travel time - The time required for a wave train to travel from its source to a point of observation.

Tuff - Used loosely as a collective term for all consolidated pyroclastic rocks.

Tsunami - One or a series of huge sea waves caused by earthquakes or other large-scale disturbance of the ocean floor. (Referred to incorrectly by many as a tidal wave, but these waves have nothing to do with tides.) The word tsunami is Japanese, meaning "harbor wave."

Unconsolidated -
Loosely arranged, not cemented together, so particles separate easily.
-The opening at the Earth's surface through which volcanic materials (lava, tephra, and gases) erupt. Vents can be at a volcano's summit or on its slopes; they can be circular (craters) or linear (fissures).

Volcano - A vent (opening) in the surface of the Earth through which magma erupts; it is also the landform that is constructed by the erupted material.