Inside the Earth - What's Under the Cover?
What is under the surface of the Earth? If you think about a volcano, you know Earth must be hot inside. The heat inside Earth moves continents, builds mountains and causes earthquakes. Where does all this heat inside Earth come from?
Earth was hot when it formed. A lot of Earth’s heat is leftover from when our planet formed, four-and-a-half billion years ago. Earth is thought to have arisen from a cloud of gas and dust in space. Solid particles, called “planetesimals” condensed out of the cloud. They’re thought to have stuck together and created the early Earth. Bombarding planetesimals heated Earth to a molten state. So Earth started out with a lot of heat.
Earth makes some of its own heat. Earth is cooling now – but very, very slowly. Earth is close to a steady temperature state. Over the past several billion years, it might have cooled a couple of hundred degrees. Earth keeps a nearly steady temperature, because it makes heat in its interior.
In other words, Earth has been losing heat since it formed, billions of years ago. But it’s producing almost as much heat as it is losing. The process by which Earth makes heat is called radioactive decay. It involves the disintegration of natural radioactive elements inside Earth – like uranium, for example. Uranium is a special kind of element because when it decays, heat is produced. It’s this heat that keeps Earth from cooling off completely.
Many of the rocks in Earth’s crust and interior undergo this process of radioactive decay. This process produces subatomic particles that zip away, and later collide with surrounding material inside the Earth. Their energy of motion is converted to heat. Without this process of radioactive decay, there would be fewer volcanoes and earthquakes – and less building of Earth’s vast mountain ranges. (1a text from http://earthsky.org/earth/what-is-the-source-of-the-heat-in-the-earths-interior)
Exploring Earth’s Interior
How do scientists know what is inside the Earth? We don't have direct evidence! Rocks yield some clues, but they only reveal information about the outer crust. In rare instances, a mineral, such as a diamond, comes to the surface from deeper down in the crust or the mantle. To learn about Earth's interior, scientists use energy to “see” the different layers of the Earth, just like doctors can use an MRI, CT scan, or x-ray to see inside our bodies.
One ingenious way scientists learn about Earth’s interior is by looking at how energy travels from the point of an earthquake. These are seismic waves (Figure below). Seismic waves travel outward in all directions from where the ground breaks at an earthquake. These waves are picked up by seismographs around the world. Two types of seismic waves are most useful for learning about Earth’s interior.
How P-waves travel through Earth’s interior.
- P-waves (primary waves) are fastest, traveling at about 6 to 7 kilometers (about 4 miles) per second, so they arrive first at the seismometer. P-waves move in a compression/expansion type motion, squeezing and unsqueezing earth materials as they travel. This produces a change in volume for the material. P-waves bend slightly when they travel from one layer into another. Seismic waves move faster through denser or more rigid material. As P-waves encounter the liquid outer core, which is less rigid than the mantle, they slow down. This makes the Pwaves arrive later and further away than would be expected. The result is a Pwave shadow zone. No P-waves are picked up at seismographs 104o to 140o from the earthquakes focus.
- S-waves (secondary waves) are about half as fast as P-waves, traveling at about 3.5 km (2 miles) per second, and arrive second at seismographs. S-waves move in an up and down motion perpendicular to the direction of wave travel. This produces a change in shape for the earth materials they move through. Only solids resist a change in shape, so S-waves are only able to propagate through solids. S-waves cannot travel through liquid.
By tracking seismic waves, scientists have learned what makes up the planet’s interior (Figure below).
Letters describe the path of an individual P-wave or S-wave. Waves traveling through the core take on
the letter K.
- P-waves slow down at the mantle core boundary, so we know the outer core is less rigid than the mantle.
- S-waves disappear at the mantle core boundary, so the outer core is liquid.
This animation shows a seismic wave shadow zone.
Other Clues about Earth’s Interior
Earth’s overall density is higher than the density of crustal rocks, so the core must be made of something dense, like metal.
Since Earth has a magnetic field, there must be metal within the planet. Iron and nickel are both magnetic.
Meteorites are the remains of the material that formed the early solar system and are thought to be similar to material in Earth’s interior (Figure below).
This meteorite contains silica minerals and iron-nickel. The material is like the boundary between Earth's core and mantle. The meteorite is 4.2 billion years old.
Source: Open Education Group Textbooks - Earth Science