# How do we know what materials the interior of the Earth is made of if the farthest we have dug is 8 miles?

We learned about the three layers of the Earth during grade school – the crust, mantle, and core. For the last five decades, science researchers have been digging into our planet’s center. The most in-depth hole humans ever dug was only 8 miles down. That 20-year excavation project barely penetrated the crust, the Earth’s thinnest and outermost layer. How to know the materials beneath the Earth?

Scientists estimate the Earth’s mass by observing the gravity relative to the objects at the surface. It turns out, the Earth’s mass is at a whopping 5.9 sextillion tonnes. However, there is no sign of a massive landform weighing that much on the surface. The verdict: most of Earth’s mass must be located at its center.

## Seismic Waves

When earthquakes occur, they radiate energy waves from the earthquake’s focus and travel through the Earth before arriving at seismometers. The speed at which these waves travel is dependent on the properties of the material they pass through. To know the material properties of the path taken by waves, scientists note the difference in time.

Seismologists, people who study Seismic Waves, use the same logic to understand the Earth’s interior. As seismic waves pass through the Earth, they refract how light rays bend when they pass through a glass full of water. There are two types of waves: the P-waves and S-waves.

P-waves, short for Primary waves, are compressional waves that apply force toward propagation. Since most of the Earth’s interior is incompressible, P-waves can easily pass through and arrive quickly. S-waves move perpendicular to the direction of propagation. This wave is less easily transmitted through the medium, thus coming slower. P-waves can travel both liquid and solid materials, but S-waves can travel only through solids.

In 1909, Croatian seismologist Andrija Mohorivic theorized a boundary between the crust and the mantle. He observed the differences in the number of seismic waves as they passed through these layers. Some waves arrived earlier than others, convincing Mohorovicic that there must be a change in the composition of rocks in different depths.

## Crust

The crust is almost 10 miles long. Most of the discoveries we unearthed came from this layer. Humans have dug more than 800 feet to mine diamonds and coppers. At over 28,000 feet deep, scientists used equipment to drill a hole.

Important discoveries in the crust include the presence of minerals, a natural gas reservoir, an underwater sinkhole, and fossil remains of prehistoric life.

In the attempt to learn more about Earth’s interior, Soviet Scientists dug the Kola Superdeep Borehole. This borehole is only nine inches in diameter but is the most bottomless hole in the planet. It is approximately eight miles deep and is deeper than the Marianas Trench.

## Mantle

The mantle makes up most of the Earth’s Interior. No scientific technology has ever dug up to this layer. Most of what we know about this part is through the indirect observations of Seismic Waves.

The mantle comprises dense magnesium-iron silicate rocks. This layer is solid as both the P and S waves can pass through. Interestingly, some parts are less dense and are akin to that of a very thick liquid. This fluid-type of rocks exists over long geological time scales in giant convection cells.

## Core

Further down lies the Earth’s core. The outer core is liquid while the inner core is solid, later learned to be made of iron. The inner core makes up 19% of the planet’s total volume and has a temperature of about 3,000 – 5,000 Kelvins. Scientists know that a boundary exists between the core because seismic wave refractions typically form a shadow zone. The outer core is made of molten iron and nickel, while the inner core is solid iron and nickel. The outer core flows around the inner core, and this motion is what creates the Earth’s magnetic field.

## What’s the purpose of knowing?

We have learned more about outer space rather than what is inside of our Earth. To precisely understand its nature, scientists probe the Earth by means of scientific drilling and seismology. The data retrieved from these projects will explain volcanic eruptions, earthquakes, continental drifts, and, importantly, the evolution of Earth and life.