Picture Earth as a giant apple with its center located more than 6,000 km (3,728 mi) deep. You would be hard pressed to know what is actually inside that apple if you could hardly make it past the peel! So how do we know what is inside Earth when we have barely even scratched the surface?

Technology has allowed us to probe deep into our planet using wave monitoring devices. In fact, the very things that cause some of the destruction and devastation we see around the world–seismic or earthquake waves—are used to help better understand the structure of the planet. When seismic waves encounter material of different composition, the direction in which they travel is altered. By monitoring the path of seismic waves, scientists have been able to map out the different layers deep below the surface.

 

 

Earth's Outermost Layer

Earth's outermost layer is commonly known as the crust. We are most familiar with this part of the planet because we live on it. Like the hard shell of an egg, it is very thin and brittle compared to what lies deep beneath it. Earth's crust is thinnest along the ocean floor, averaging about 7 km (4.3 mi) in thickness. This part of the crust is called oceanic crust and is composed of dense mafic rocks, such as basalt and gabbro.

The part of Earth we live on is called the continental crust. It is generally thicker (30-100 km or 19-62 mi) and made of less dense felsic rock, such as granite. The density difference between oceanic and continental crust affects the way they interact during collisions

 

 

 

Deeper into Earth

Venturing deeper into Earth, we find the thickest section of the planet (2,900 km or 1,802 mi) called the mantle. The uppermost portion of the mantle is much different than the regions beneath it. It is about 100 km (62 mi) thick, and is hard like the crust. Combined with Earth's crust, this area is called the lithosphere. The rigid nature of the lithosphere causes it to break up into giant slabs of rock called lithospheric or tectonic plates. These plates float and move on top of the asthenosphere, the part of the mantle just below the lithosphere. The combination of temperature and pressure makes the asthenosphere gooey and ductile. Scientists describe this part of the mantle as being "weak" because it is able to compress, stretch, and flow like Silly Putty®.

As we go deeper into the mantle, temperature and pressure increases. The effect of temperature softens rocks, while the effect of pressure makes them stronger. These two contradictory tendencies interact in a complex way to form the "stronger" mesosphere, the lower portion of the mantle.

 

 

Earth's Core

Seismic waves have also helped us probe deep into Earth's core. We have learned that the core of our planet is divided into two parts, the liquid outer core and the solid inner core. Together they represent about 15 percent of Earth’s volume, and have a diameter of about 3,480 km (2,162 mi).

The intense heat from the solid inner core and the rotation of Earth causes the liquid outer core to move in convection currents. This moving liquid is thought to cause the magnetic field that surrounds Earth.

It is theorized that both parts of the core are composed mainly of iron. Some scientists have even speculated that the solid inner core is a giant iron crystal that is slightly smaller than the moon! The inner core is around 4,500°C (8,132°F). The liquid outer core is around 3,550°C (6,422°F), which is about three to four times hotter than the lava that erupts from volcanoes.