Key Terms
Plate Tectonics: The scientific theory that seeks to explain how major landforms are created as a result of subterranean movement of large slabs of rock (tectonic plates).
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Tectonic Plate: a massive slab of rock composing the outermost layer of the earth (lithosphere) and the upper mantle (asthenosphere).
Mantle: the molten part of the earth between the crust and core. Technically it is mostly solid, but it behaves viscously.
Convection: the phenomenon of heat transfer through the mantle from the core to the crust (naturally, heat rises- the core is VERY hot, the crust, not so much).
This heat transfer creates currents in the mantle like currents in the ocean.
Continental Plate: A type of tectonic plate chiefly composed of lighter, less dense, silicon-rich rock.
Oceanic Plate: A type of tectonic plate chiefly composed of heavier, denser, iron-rich rock.
Subduction: A geologic process between two tectonic plates where one plate is forced underneath the other.
Plate Boundary: A three-dimensional surface or zone where two tectonic plates either collide, slide past one another, or spread apart from one another.
Convergent Plate Boundary: A plate boundary where two tectonic plates collide.
Transform Plate Boundary: A plate boundary where two tectonic plates slide past one another.
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Divergent Plate Boundary: A plate boundary where two tectonic plates spread apart from one another.
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Subduction Zone: an area of collision between two tectonic plates where one plate is forced underneath the other.
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Volcanic Arc: a belt of volcanoes formed above a subducting oceanic plate, situated right above the area of the subduction zone where the oceanic plate begins to melt.
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Accretionary Wedge: also referred to as Accretionary Prisms, they are areas of subduction zones where masses of sedimentary material from the subducting plate has been scraped off and piled up at the edge of the non-subducting plate. As such, accretionary wedges/prisms are often built into mountain ranges.
Fore-Arc Basin: basins between volcanic arcs and accretionary wedges/prisms where sediment eroded from volcanic arcs and accretionary wedges accumulate.
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Offshore Trench: also known as an Oceanic Trench or simply a Trench, these are large depressions formed at the boundary of the two plates in a subduction zone, due to the weight of the down-going plate literally dragging the surface of the earth down with it. Trenches are the deepest parts of the ocean.
Cascadia Subduction Zone: A Subduction Zone on the western margin of North America stretching roughly 620 miles from Cape Mendocino in northern California past Vancouver Island in British Columbia. The oceanic Juan De Fuca, Gorda, and Explorer Plates subduct underneath the continental North American Plate at the Cascadia Subduction Zone.
Earthquake: the shaking of the Earth’s surface in response to a sudden release of energy in the Earth’s lithosphere that creates seismic waves.
Lithosphere: the outermost, rocky layer of the Earth.
Fault: planar fractures or discontinuities between two blocks of rock where there has been displacement due to rock mass movement.
Dip-Slip Fault: a fault where the two blocks of rock on either side of the fault move up and down relative to each other.
Strike-Slip Fault: a fault where the two blocks of rock on either side of the fault move laterally past each other.
Normal Fault: a dip-slip fault in which the block above the fault has moved downward relative to the block below the fault.
Reverse Fault: a dip-slip fault in which the block above the fault has moved upward relative to the block below the fault. Also known as a thrust fault.
Magnitude: the size of an earthquake, denoted as a value between 0.0-10.0 on the Moment Magnitude Scale.
Hypocenter: also known as the focus, is the point inside the earth where fault rupture causing an earthquake begins, or in simple terms, where an earthquake occurs.
Epicenter: the geographic location directly above the hypocenter/focus.
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Moment Magnitude Scale: the gold standard for accurate seismic energy measurement. It is a logarithmic scale and it measures an earthquake’s seismic moment- taking shear modulus (ratio of shear stress to shear strain) of rocks involved in the earthquake [measured in newtons/square meter (pascals)], area of rupture along the involved fault (in square meters), and average slip of the involved fault (in meters). This scale records earthquakes from 1-10 to 1 decimal place (tenths), and magnitudes of specific earthquakes are denoted as “Mw” on this scale. Simply put, it measures the amount of energy produced in an earthquake.
Seismic Moment: a quantity used by seismologists to measure the size of an earthquake.
Shear Modulus: a measure of rigidity of a material, it is the ratio of shear stress to shear strain. In terms of earthquakes, geophysicists are interested in the shear modulus of the rock involved when an earthquake occurs. G=Txy/Yxy.
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Modified Mercalli Intensity Scale: a non-mathematical scale that measures the strength of an earthquake, with regard to shaking intensity rather than raw energy released. The MMI is utilized by estimating shaking intensity at specific locations during an earthquake, taking its effects on people, objects, and buildings into account. The MMI goes to 10, and anything above a 6 is considered “high intensity”, causing damage to buildings.
Megathrust Earthquake: the largest types of earthquakes on the planet. They occur when the reverse/thrust fault that forms the contact between the two plates in a subduction zone ruptures, and can often be larger than magnitude 9.0. All magnitude 8.5+ earthquakes that have ever occurred have been megathrust earthquakes.
Deep Intraplate Earthquake: types of earthquakes that occur deep in a subduction zone where the subducting plate experiences significant changes in temperature and pressure (possibly due to phase change of minerals in the plate), on normal faults branching from the main subduction zone.
Crustal Faulting Earthquake: shallow earthquakes that occur on thrust faults within the fore-arc basin of a subduction zone, usually of magnitude 6.5-7.5 at their largest. They can be very damaging due to their shallow depth.
Tsunami: Japanese for “harbor wave”, a tsunami is a series of extremely long waves in a large body of water caused by sudden displacement of a large volume of water. They are most commonly caused by earthquakes, but they can be caused by asteroid impacts or landslides as well.
Volcano: a rupture in the crust that allows hot lava, ash, and gasses to escape from a subterranean magma chamber.
Volcanic Eruption: the escape of hot lava, ash, gasses, and/or other volcanic material from a volcano.
Lava: molten rock that has erupted out of a volcano onto the surface of the Earth.
Magma: molten rock that has not erupted and is subterranean. Magma turns into lava when it erupts out of the Earth.
Magma Chamber: a subterranean reservoir of magma, usually found inside of volcanoes.
Mantle Plume: an anomalous plume of magma rising from the mantle into the crust. It is the main driver of hot spot volcanism.
Hot Spot: an area on Earth above a mantle plume inside of the crust where magma is hotter than other areas and often erupts. Two famous hot spots include the Hawaiian and Yellowstone Hot Spots.
Mafic Igneous Rock: rock that is derived from iron-rich, silicon-poor magma, such as basalt/gabbro. Mafic magma is considered “primitive”- it is the default geochemistry of magma without any chemical change. Oceanic Crust is chiefly composed of mafic igneous rock and is heavier and denser than felsic igneous rock.
Felsic Igneous Rock: rock that is derived from silicon-rich, iron-poor magma, such as rhyolite/granite. Felsic magma is considered “evolved”- mafic magma has to undergo chemical change in order to become felsic. Continental crust is chiefly composed of felsic igneous rock and is lighter and less dense than mafic igneous rock.
Extrusive Igneous Rock: also referred to as Volcanic Igneous Rock, this is rock derived from magma/lava that cools at or very near the surface of the earth, often erupting from a volcano or a fissure. Crystal size is small, as there was not ample time for crystals to grow since extrusive rock is derived from magma/lava that cools quickly.
Intrusive Igneous Rock: also referred to as Plutonic Igneous Rock, this is rock derived from magma that cools deep below the earth’s surface, often in the magma chambers or cores of volcanoes. Crystal size is large, and intrusive igneous rock cools slowly, allowing for crystals to grow large.
Basalt: Mafic Extrusive Igneous Rock.
Gabbro: Mafic Intrusive Igneous Rock.
Rhyolite: Felsic Extrusive Igneous Rock.
Granite: Felsic Intrusive Igneous Rock.
Evolution (Petrology): the process by which magma undergoes chemical change from its primitive mafic state, due to a plethora of factors, most notably fractional crystallization and assimilation.
Metasomatism: the chemical alteration of a rock by fluids, chiefly hydrothermal fluids.
Fractional Crystallization: chemical processes that separate crystals from a liquid, particularly particular minerals from a magma during cooling.
Assimilation (Petrology): partial melting and incorporation of existing crust while magma cools. This is especially important to the evolution of magma in a subduction zone.
Stratovolcano: also known as composite volcanoes, are tall, conical, steep volcanoes built by many layers of viscous hardened lava, tephra, and pyroclastic flows.
Cinder Cone: steep conical hills, composed of loose volcanic material such as ash, pumice, and/or scoria that has been built around a volcanic vent.
Caldera: a large depression that occurs after a volcano has erupted inwards and collapsed itself.
Shield Volcano: A type of volcano exemplified by its low profile, often resembling a shield lying on the ground. They are formed by eruptions of highly fluid lava, most notably mafic basalts with low viscosity.
Viscosity: resistance to flow. As a rule of thumb, mafic lava has low viscosity whilst felsic lava has high viscosity.
Scoria: highly gaseous volcanic rock that was ejected as a molten blob during an eruption and cooled in the air.
Volcanic Ash: distinct from regular ash from a campfire, it is composed of fine particles of glass that’s been ejected from a volcano, and as such, inhaling volcanic ash can be very dangerous.
Ash Fall: the phenomenon when volcanic ash falls from the sky immediately after an eruption.
Pyroclastic Flow: a hot, fast moving ‘cloud’ of gas, ash, and rock debris known as tephra. They tend to “hug the ground” rather than creating a plume in the air, and can travel as fast as 700 km/hr and be as hot as 1000 degrees celsius. They destroy everything in their paths, but tend to fizzle out not too far from the volcano in which they erupted from.
Tephra: any type of debris thrown in the air during a volcanic eruption.
Lahar: volcanic mudflows that contain at least 60% volcanic material (ash, tephra, lava, rock, etc), have the density of wet concrete, and travel down valleys from the volcano they erupted. Unlike pyroclastic flows, they travel much further from the volcano that they originated from due to their liquidity, but like pyroclastic flows, they destroy everything in their paths. Lahars also include a large amount of water, and in the PNW, hazards of lahars are exceedingly high due to the extensive glaciation on most Cascade Volcanoes.
Fumarole: an opening on or near a volcano where hot volcanic gas emerges.