Earth’s Structure, Seismic Waves, and Plate Tectonics

Twenty vocabulary flashcards covering Earth’s compositional layers, seismic concepts, and major tectonic theories.

Crust

Earth’s outermost shell composed mainly of light granitic rocks; average thickness 7–10 km under oceans and 40–70 km under continents.

Oceanic Crust

Basaltic, magnesium-rich crust forming the ocean floor; about 7–10 km thick and denser than continental crust.

Continental Crust

Granitic, aluminum-rich crust underlying the continents; 40–70 km thick and less dense than oceanic crust.

Mantle

Semisolid layer of ferro-magnesium silicate rocks between crust and core, ~2 900 km thick, making up ~80–84 % of Earth’s volume.

Outer Core

Earth’s only liquid layer, 2 270 km thick, composed of molten iron and nickel; its flow generates Earth’s magnetic field.

Inner Core

Solid, iron-rich central layer ~1 220 km thick; hottest part of Earth with temperatures up to ~6 000 °C.

Lithosphere

Brittle outer layer consisting of the crust and uppermost mantle; broken into tectonic plates that move and interact.

Asthenosphere

Weak, ductile zone beneath the lithosphere containing partially molten rock; allows plates to glide over it via convection.

Mohorovičić Discontinuity (Moho)

Boundary between crust and upper mantle discovered by Andrija Mohorovičić in 1909.

Gutenberg Discontinuity

Interface between the lower mantle and liquid outer core, identified by Beno Gutenberg in 1913.

Lehmann Discontinuity

Boundary separating the liquid outer core from the solid inner core, discovered by Inge Lehmann in 1936.

Seismometer

Instrument that detects and measures the strength of seismic waves passing through Earth.

Seismograph

Instrument that records ground motions produced by seismic waves, providing a written trace of earthquakes.

Primary Waves (P Waves)

Fastest body waves; compressional; first to arrive; travel through solids and liquids but bend in the liquid outer core.

Secondary Waves (S Waves)

Slower transverse body waves (about 60–70 % of P-wave speed); travel only through solids and stop at the liquid outer core.

Surface Waves

Seismic waves confined to Earth’s surface; major contributors to earthquake damage but give little interior information.

Shadow Zone

Area on Earth’s surface where specific seismic waves are absent (S-waves entirely; P-waves partially) due to core interactions.

Continental Drift Theory

Alfred Wegener’s 1912 idea that continents once formed the supercontinent Pangaea surrounded by Panthalassa and have since moved apart.

Seafloor Spreading Theory

Harry Hess’s 1960s proposal that new oceanic crust forms at mid-ocean ridges as magma upwells, pushing plates apart.

Plate Tectonics Theory

Unifying model combining continental drift and seafloor spreading: the lithosphere is divided into moving plates driven by mantle convection, slab pull, and ridge push.