Structure of the Earth

What lies beneath the surface of the Earth? How do we know? What are tectonic plates?

cutaway view of the Earth

Earth's inner structure consists of an inner core made of solid iron, an outer core made of molten iron and a mantle, surrounded by an outer crust. The inner core is about as hot as the surface of the sun, and rotates somewhat faster than the surface of the planet. The outer core of molten metal supplies the source for the magnetic field of the Earth. The mantle convects heat upward from the core to the crust.

tectonic plate map

The surface of the Earth is divided up into sections called tectonic plates. These plates move very slowly, about as fast as your fingernails grow. At the boundaries between the plates, they can be moving toward, away from, or alongside each other. If they are moving toward each other, typically one plate subducts, or slides underneath the other. Where they move apart, deep suboceanic trenches are formed. Volcanic activity is very commonly found along the boundaries of the tectonic plates.


Notice how the shape of the coast of North and South America seems to match the shape of the coast of Europe and Africa. This led geologists to believe that the continents may have once been joined together.

Convection currents, shown in this video, rise from the core of the Earth through the mantle. Heated material becomes less dense and buoyant, which causes it to rise. When it reaches the crust, it spreads out and cools, and then sinks back toward the core. Convection helps to drive the motion of tectonic plates.

volcanic plumes in the Earth's mantle

Where the hotspots from the upwelling magma is able to melt through the crust, volcanoes erupt. As the tectonic plate moves across the hotspot, new volcanoes are created in a chain.

world volcano map

The red triangles on this map signify the locations of volcanoes around the world. Notice the large population of volcanoes circling the Pacific Ocean. This group of volcanoes is known as the "Ring of Fire." It is also the location of many earthquakes, as it lies along the faultline between tectonic plates.

San Andreas fault

The San Andreas fault in southern California is an example of the collision of two tectonic plates. Two kinds of motion are actually detected here, vertical and horizontal. The sideways motion is well documented, and exhibits a stick-and-slip quality. Stress builds up as the plates move, released suddenly during an earthquake. The vertical motion of the material being uplifted has recently been detected using GPS data.

Atlantic trench

This map of the Atlantic trench shows a deep underwater crack where two plates are pulling apart. New seafloor is created as the hot magma hits the seawater and solidifies. This magma creates a timeline map of the spreading of the plates.

P and S waves

Earthquakes produce two kinds of waves, P waves and S waves. P waves are also known as primary waves. They are compression (pressure) waves and move very fast, compares to the S waves. S waves, also known as secondary waves, have an up-and-down motion, perpendicular to the direction the wave is traveling.


P waves can travel through solids or liquids. S waves can only travel through solids. The wave speeds depend on the density and pressure of the material, traveling faster through low-density material. This gives them curved paths through the mantle of the Earth.

cutaway of Earth showing P waves and S waves

When a very powerful earthquake happens, seismic stations around the Earth are able to measure the strength and timing of the waves. This, combined with the knowledge of the curved paths of the waves, allows us to map the interior of the Earth.


Since the S waves cannot travel through liquid, they cannot travel through the outer core. The shadow zones of the P and S waves allow us to measure the size of the core of the planet, as well as its density structure at various depths.

Monitoring Earth’s interior via earthquakes

  • P waves, Primary or Pressure
    • Sound waves (compression waves)
    • Longitudinal (molecules move side to side)
    • Can pass through liquid
    • Travel faster than S waves
  • S waves Secondary or Shear waves
    • Transverse waves – molecules move up and down
    • Cannot pass through liquid
      • Creates shadow zones where S waves cannot pass
      • Indicates liquid outer core
    • Travel slower than P waves
  • Waves travel faster through denser material
    • Tighter bonds between molecules
  • Scientists time the arrival of P and S waves at various points on Earth’s surface to calculate the density of the material along the paths of the waves



cutaway view of the Earth tectonic plate map volcanic plumes in the Earth's mantle world volcano map San Andreas fault Atlantic trench P and S waves cutaway of Earth showing P waves and S waves
cutaway view of the Earth tectonic plate map volcanic plumes in the Earth's mantle world volcano map San Andreas fault Atlantic trench P and S waves cutaway of Earth showing P waves and S waves