Module 4: The Geosphere: Earthquakes and Volcanoes

scarp

a break in slope that results from movement along a fault and offsets physical and structured features such as rock layers and roads

creep

the slow movement of rock and soil downhill under the influence of gravity

stick-slip behavior

displacement along a fault plane characterized by long intervals of no movement punctuated by brief episodes of rapid movement, results from earthquakes

elastic rebound model

the scientific theory that sudden movements of rock along a fault and the earthquakes associated with them result from the release of elastic energy along the fault

hypocenter/focus

the point within Earth’s crust where an earthquake occurs

epicenter

the point of Earth’s surface directly above the hypocenter, or focus, of an earthquake

seismogram

a graphic representation of the passage of seismic waves from an earthquake

body waves

a seismic wave that travels through Earth’s interior

seismometer

an instrument that detects and records the passage of seismic waves from an earthquake

seismic waves

any of the elastic vibrations produced by earthquakes, such as primary waves, secondary waves, and surface waves

P (primary) waves

the fastest subsurface seismic wave generated by an earthquake, traveling through solid rock as a compressional wave

S (secondary) waves

the slower subsurface seismic wave generated by an earthquake that arrives after the P wave; it travels as a shearing wave but cannot travel far through water

surface waves

a seismic, slower than a secondary wave, that follows the surface of Earth

magma chamber

a reservoir of molten rock below Earth’s surface that typically lies beneath and feeds an active volcano

volatiles

dissolved gases contained within magma

viscosity

a measure of a liquid’s resistance to flow

*determines volcanic behavior

mafic magmas

low viscosity → very fluid and move upward rapidly along volcanic conduits → erupt with minimal violence

(volcanic) bombs

fragments of lava greater than 64 mm in diameter ejected during a volcanic eruption that cool to form rocks that rain out around the volcanic vent; travel through the air often results in streamlines forms

pāhoehoe

the Hawaiian name for a slow-moving, low viscosity basaltic lava flow that develops a ropey or billowy surface texture as it cools and solidifies

a’a

the Hawaiian name for a fast moving, high viscosity basaltic lava flow that develops a rough surface texture characterized by broken fragments of chilled material, called clinkers

felsic magmas

lower melting points → erupt at temperatures around 800°C; sticky and flows slowly → eruptions tend to be very explosive (higher viscosity and trapped gases)

pyroclastic debris

fragmented materials such as boulders, cinders, and ash that are ejected from a volcano (also called tephra)

ashfall deposits

layers of volcanic ash deposited downwind of an erupting volcano

pyroclastic flows

avalanches of ash, rock fragments, and hot gases erupted from a volcano that move down the volcano’s flanks under the influence of gravity; commonly produced by andesitic and felsic magmas

andesitic magmas

intermediate → between mafic and felsic in their eruptive styles and explosiveness → most of the material emitted from andesitic volcanoes consists of pyroclastic debris