Earth formed from comets & meteorites clumping together. Earth was so hot it was molten (liquid) & it is still cooling. How it all began (a little astronomy preview):

Denser materials like iron sank to the center, and Less dense materials like silicon, carbon, oxygen, and water rose to the surface. Because the Earth was liquid:

The less dense materials formed the crust, oceans and the atmosphere. The more dense materials sunk toward the center. But how do we know that the interior of Earth is layered and not the same throughout? As the Earth cooled, layers of different materials and densities developed.

Earthquakes have taught us about the way Earth is put together. o If Earth were made of the same materials throughout, seismic (earthquake) waves should travel through Earth in straight lines at constant speed. This isn’t the case. Seismic waves are bent as they travel through different layers of Earth that are made of different materials.

There are two ways to define the layers of Earth: By Chemical Composition Crust Mantle Core (Both Inner & Outer) By Physical Properties Lithosphere Asthenosphere Lower Mantle Outer Core Inner Core

Composition Layer: The Crust Thin, rocky outer layer of Earth Two types: oceanic & continental crust Oceanic Crust: 7 km thick (average) Made of igneous rocks basalt & gabbro Younger rocks Density of about 3.0 g/cm 3 Continental Crust: 40 km thick (average), but varies a lot Made of many different rock types Older rocks Density of about 2.7 g/cm 3

The Crust:

Composition Layer: The Mantle A solid, rocky shell Contains over 82% of the Earth’s volume Extends down to about 3000 km below the surface Density of about 3.4 g/cm 3

Composition Layer: The Core Sphere at the center of Earth Made of an iron-nickel alloy. Extreme pressure! Density of about 13 g/cm 3

Earth layers with different physical properties exist because Earth’s temperature, pressure, and density increase with depth. This is just another way to look at how the Earth changes from the surface to the center.

Physical Property Layer: The Lithosphere Outermost layer Consists of the crust and the uppermost mantle Relatively cool and rigid shell About 100 km thick on average

Physical Properties Layer: The Asthenosphere Soft, weaker layer Part of the upper mantle Temperature & pressure conditions cause a small amount of melting. Rocks are close enough to their melting temps that they are easily deformed and are weaker.

Physical Properties Layer: The Lower Mantle More rigid than the upper mantle, but still very hot and capable of gradual flow. The bottom few hundred meters of the lower mantle, which are laying on top of the hot core contain softer, flowing rock.

Physical Properties Layer: The Outer Core Liquid because of high temperatures About 2260 km thick. The flow of the metallic iron- nickel alloy in the outer core creates Earth’s magnetic field.

Physical Properties Layer: The Inner Core A sphere with radius of 1220 km. Despite having a higher temperature than the outer core, the material in the inner core is compressed into a solid by the immense pressure.

Discovering Earth’s Interior Seismic wave data isn’t the only way scientists have learned about Earth’s interior. We’ve also learned by: Drilling into the crust Examining lava that reaches the surface,

Examining meteorites that collide with Earth Meteorites are composed of the same material from with Earth was made. Metallic Meteorites resemble the core Stony meteorites resemble the mantle & crust. Discovering Earth’s Interior Continued