©Copyright 2012 California Academy of Sciences

Before learning about earthquakes, let’s look at the inside of our planet.

What happens on Earth’s surface is directly related to its interior. About 4.6 billion years ago, Earth formed from a hot cloud of dust orbiting a blazing sun. As the planet cooled, dense elements became concentrated in the core of the planet, while lighter elements formed the mantle. A thin, rigid crust formed at the surface. A constant heating and cooling cycle in the mantle drives plate movement on Earth’s surface. Heat working its way out from the core of the planet fractured the crust into irregular tectonic plates that are constantly in motion.

  • Inner Core: The innermost part of Earth is the core and is about 1500 miles (2414 km) thick. Both the inner and outer cores consist primarily of iron and nickel. They're extremely hot, with temperatures ranging from 7200–9000℉ (4000–5000℃). The inner core is under intense pressure, which keeps it solid despite high temperatures.
  • Outer Core: The outer core, which is liquid, is about 1300 miles (2092 km) thick. Both the inner and outer cores consist primarily of iron and nickel and are extremely hot with temperatures ranging from 7200–9000℉ (4000–5000℃).
  • Mantle: Most of Earth's volume is in the mantle. This layer is about 1800 miles (2880 km) thick. It's composed of dark, dense rock, similar to oceanic basalt. The deeper you go inside the Earth, the hotter it gets. Mantle material near the cold outer crust is about 1300℉ (700℃) while rock near the Earth’s core heats up to about 7200℉ (4000℃).
  • Crust: Two types of crust make up Earth’s outermost layer: continental and oceanic. Continental crust is composed of silica-rich rocks and is an average of 44 miles (70 km) thick. Ocean crust is made of dark, silica-poor rocks like basalt. It is thinner and more flexible than the continents, only about 3 miles (5 km) thick.

This post is part of Exploring Earthquakes, a rich collection of resources co-presented by the California Academy of Sciences and KQED. This material is also available as a free iBooks textbook and iTunes U course.

Don’t miss Earthquake, an interactive exhibit at the Academy exploring the seismic forces that impact us today and featuring the Shake House, an earthquake simulator.

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Exploring Earthquakes

Get a new perspective on earthquakes with this rich collection of multimedia resources. You'll learn why earthquakes happen, how they've shaped the Bay Area, and what you can do to prepare for the next one.

Teacher Guide: Earthquakes

Ideas for using videos, articles, and infographics about earthquakes in the classroom.