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Earth and Space Science Unit 1: Earth's structure and plate tectonics

Earth has layers, and its outer shell is broken into plates that move a few centimeters a year, causing earthquakes, volcanoes and mountains.

Covers: Layers of Earth, plate boundaries, earthquakes, volcanoes, mountain building, evidence for plate tectonics

Study guide

Earth is layered by composition and by physical state. The crust is the thin, solid outer layer, thicker under continents and thinner under oceans. The mantle beneath it is hot rock that flows slowly like a plastic solid. The outer core is liquid iron and nickel, and its motion creates Earth's magnetic field. The inner core is solid iron and nickel, kept solid by enormous pressure. Scientists learned about these layers by studying how earthquake waves bend and change speed; S-waves can't travel through the liquid outer core.

The lithosphere (crust plus the uppermost mantle) is broken into tectonic plates that move a few centimeters a year on the softer asthenosphere, driven by mantle convection. At divergent boundaries plates move apart, forming mid-ocean ridges and new crust (sea-floor spreading). At convergent boundaries they collide: oceanic crust subducts under continental crust, forming trenches and volcanic mountains, and two continents collide to build mountain ranges like the Himalayas. At transform boundaries plates slide past each other, causing earthquakes.

Earthquakes release built-up stress along faults. The focus is where the rupture starts and the epicenter is the point above it on the surface. Seismic waves include P-waves (fastest), S-waves and surface waves, and magnitude measures the energy released. Volcanoes form where magma reaches the surface, at subduction zones, rifts and hot spots like Hawaii.

Evidence for plate tectonics includes matching fossils and rock formations on separate continents, the jigsaw fit of the continents, magnetic stripes in sea-floor rock, the young age of crust near ridges, and the locations of earthquakes and volcanoes along plate boundaries.

Oceanic crust is mostly basalt: dark, rich in iron and magnesium, relatively dense (about 3.0 g/cm³) and only 5–10 km thick. Continental crust is mostly granite-like rock: lighter in color and density (about 2.7 g/cm³) and much thicker. That's why oceanic crust is the plate that subducts when the two collide. P-waves travel faster than S-waves, so the gap between their arrival times grows with distance from the epicenter. Each station's time gap gives its distance from the quake, and distances from three stations pinpoint the epicenter.

Hot mantle material rises beneath mid-ocean ridges and spreads outward, and cooler, denser material sinks at subduction zones. About 200 million years ago the continents were joined as the supercontinent Pangaea, which then broke apart, opening the Atlantic Ocean.

Key ideas

• Layers: crust, mantle, outer core (liquid), inner core (solid).
• Plates move on the soft upper mantle (asthenosphere), driven by mantle convection.
• Divergent boundaries pull apart; convergent push together; transform slide past.
• Evidence: matching fossils and rocks across oceans, seafloor spreading, earthquake patterns.
• Earthquake energy is measured by magnitude; each whole number is about 32 times more energy.

Formulas and rules

Common mistakes

• Saying the inner core is liquid. It's solid because of pressure.
• Thinking plates float on lava. They ride on hot, slowly flowing solid rock.

Words to know

Outer core

Earth's liquid layer, made of iron and nickel.

Mantle convection

Heat-driven flow in the mantle that moves tectonic plates.

Divergent boundary

Plates move apart, like the Mid-Atlantic Ridge.

Subduction

One plate sinking beneath another.

Continental drift

The idea that continents move; supported by matching fossils across oceans.

Earth's layers

Crust (solid), mantle (plastic solid), outer core (liquid), inner core (solid).

Crust

Earth's thin, solid outer layer.

Mantle

Hot rock that flows slowly like a plastic solid.

Inner core

Solid iron and nickel at Earth's center.

Lithosphere

The crust plus uppermost mantle, broken into plates.

Asthenosphere

The soft mantle layer plates move on.

Convergent boundary

Plates collide, forming trenches and mountains.

Transform boundary

Plates slide past each other, causing earthquakes.

Sea-floor spreading

New crust forms at mid-ocean ridges.

Fault

A break in rock where movement occurs.

Epicenter

The point on the surface above an earthquake's focus.

P-wave / S-wave

Fastest compression wave / slower wave that can't pass through liquid.

Hot spot

A plume of magma making volcanoes inside a plate, like Hawaii.

Sample practice questions

13 more questions like these, from level 1 to 5, are in the practice set.