Earthquakes and Igneous Geology Key Terms

Comprehensive vocabulary flashcards covering key geological terms for earthquakes, faulting, seismic hazards, and igneous rock processes.

Earthquake

Shaking of the ground caused by a sudden release of stored energy

Fault

A break in the crust where rocks on either side move past each other

Elastic rebound

Rocks bend and store energy while a fault is stuck, then snap back when it slips

Stick-slip

Faults stay stuck for a long time, then slip suddenly; this repeats

Hanging wall / footwall

The block above a sloping fault / the block below it

Normal, reverse, thrust, strike-slip

Fault types: hanging wall down; hanging wall up; low-angle reverse; sideways sliding

Focus (hypocenter)

Point underground where the fault first slips

Epicenter

Point on the surface directly above the focus

Body waves (P, S)

Waves that travel through Earth's interior

Surface waves (Love, Rayleigh)

Slow waves that travel along the surface; usually the most damaging

Seismograph / seismogram

Instrument that records ground motion / the record it makes

S-P time

Time between the P-wave and S-wave arrivals; bigger means farther away

Triangulation

Finding the epicenter where distance circles from three stations meet

Intensity (Mercalli)

What people felt and what was damaged at one place (I-XII)

Magnitude

Energy released by an earthquake; one value per earthquake; $+1 = 32\times$ energy

Amplification

Stronger, longer shaking on soft sediment than on hard rock

Resonance

Shaking grows when the ground's rhythm matches a building's sway

Liquefaction

Shaken, water-soaked sand loses strength and flows like a liquid

Soft story

A weak ground floor with few walls (open garages or shops)

Early warning

Alert sent after the P wave is detected, before strong shaking arrives

Hazard / exposure / vulnerability

Shaking expected / who and what is in harm's way / how easily they are harmed

Risk

$$\text{Hazard} \times \text{Exposure} \times \text{Vulnerability}$$

Disaster capitalism

Using a crisis to push through economic changes, often benefiting outside investors

Seismic justice

Recognizing unequal earthquake risk and working to make safety available to everyone

Magma / lava

Molten rock underground / at the surface

Igneous rock

Rock formed when magma or lava cools and solidifies

Intrusive (plutonic)

Cooled underground; usually coarse-grained

Extrusive (volcanic)

Cooled at the surface; fine-grained, glassy or fragmental

Pluton, batholith

Large intrusive body; a batholith is a very large one

Dike / sill

Sheet-shaped intrusion that cuts across layers / runs between layers

Solidus

The melting line: temperature at which rock begins to melt

Decompression melting

Melting caused by a drop in pressure as hot rock rises

Flux melting

Melting caused by adding water, which lowers the melting temperature

Heat transfer melting

Melting caused by heat from nearby magma

Geothermal gradient

How fast temperature rises with depth ($\sim 25\,^\circ\text{C}$ per km in continental crust)

Felsic / intermediate / mafic / ultramafic

Magma types from most to least silica

Viscosity

Resistance to flow; more silica = more viscous

Vesicle

A gas bubble hole frozen into volcanic rock

Porphyritic

Large crystals in a fine-grained background; two-stage cooling

Obsidian / pumice / scoria

Felsic glass / felsic glass foam that floats / vesicular mafic rock

Tuff, volcanic breccia

Rocks made of ash and fragments from explosive eruptions

Partial melting

Only part of a rock melts; the first melt is richer in silica

Assimilation

Magma melts and absorbs surrounding wall rock

Magma mixing

Two different magmas blend

Bowen's reaction series

The order minerals crystallize from cooling magma

Fractional crystallization

Early crystals separate from the melt, leaving it more felsic

Cross-cutting relationship

A rock that cuts across another is younger

Columnar (cooling) joints

Cracks formed as igneous rock cools and shrinks