Earth's Layers, Density, and Seismic Waves

A 12-card vocabulary set covering the composition and properties of Earth's layers, density differentiation, faulting, and the behavior of P-waves and S-waves.

Crust

The thin, outermost solid chemical layer of Earth where we live, with a thickness of $5\text{--}70\,\text{km}$; it is the least dense layer and is composed mostly of oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), potassium (K), and magnesium (Mg).

Mantle

A thick layer beneath the crust measuring about $2{,}900\,\text{km}$ in thickness, made of solid rock that flows slowly over time, composed mainly of magnesium (Mg), iron (Fe), silicon (Si), and oxygen (O).

Outer Core

A liquid layer about $2{,}200\,\text{km}$ thick located beneath the mantle, made mostly of liquid iron (Fe) and nickel (Ni), which creates Earth's magnetic field.

Inner Core

A solid metal ball at Earth's center with a radius of about $1{,}220\,\text{km}$, primarily composed of solid iron (Fe) and nickel (Ni); it is the densest layer due to extreme pressure.

Density

A physical measurement defined as mass divided by volume ($\text{Density} = \frac{\text{Mass}}{\text{Volume}}$), which increases with depth inside Earth as materials are compressed by pressure.

Separation by Composition

The process during planetary formation in which lighter elements (such as oxygen and silicon) remained near the surface while heavier metals settled deeper toward the interior.

Faults

Cracks inside Earth along which rocks suddenly break or slip, releasing stored energy that generates earthquakes.

Seismic Waves

Waves of energy produced by earthquakes or explosions that move through Earth's layers, changing speed and direction depending on the materials they encounter.

Primary Waves (P-waves)

The fastest seismic waves, which compress and expand material like sound waves as they travel through solids, liquids, and gases.

Secondary Waves (S-waves)

Seismic waves that are slower than P-waves and move material side to side or up and down perpendicular to the wave direction; they can only travel through solids, not liquids or gases.

S-wave Shadow Zone

A region on Earth's surface where no secondary waves are detected because S-waves cannot travel through the liquid outer core.

P-wave Shadow Zones

Gaps on Earth's surface where direct P-waves are not detected because the waves slow down and bend (refract) when passing from the solid mantle into the liquid outer core.