What is density? Take two objects of the same size: a styrofoam ball and a rock. Which one weighs more? The rock, right? It’s kind of a no brainer, but why does a styrofoam ball weigh so much less if both objects are the same size?

to measure how close the atoms are to one another in an object. The closer the atoms are, the more compact the object is. Also, the more dense an object is, the heavier it will feel for its size. In other words, density is how compact an object is, not how much the object weighs.

Density has two factors, mass and volume. We can calculate density by identifying an object's mass and measuring the object’s volume.

Mathematically stated, density equals mass divided by volume, or the amount of mass per unit of space. See the equation above.

Density determines many things in the natural world. It is part of the reason each of the inner planets is made of rocky materials, while the outer planets are made up of gases. It is why the air is thick near the ground, but very thin at the top of the atmosphere. Density of building materials and boats is also a useful application of density.

Earth’s Layers

You probably heard in elementary school that the Earth is made up of layers. The outermost layer is the crust. The next layer is the mantle, while the inner layers are the outer and inner cores. Each layer has a slightly different composition than the layer above it.

At the crust are high levels of silicon and oxygen. As you go deeper into the Earth there is more and more magnesium, and iron. These materials didn’t just collect in certain places because they wanted to. It all has to do with density.

Some molecules and single atoms naturally have a lower or higher density than others. In fact on average, as you go deeper into the Earth, materials get more and more dense. It seems strange to think of this happening by accident.

When we look to other solar system bodies, the same thing is true there too. There is an emerging pattern. The outer layers are the least dense, while the inner layers are more dense.

Millions of collisions from comets and asteroids worked together to create the planet. There was heat energy in the Earth that is still left over from those violent beginnings. This heat energy doesn’t just make rock hot, it also moves it!

Before we explain exactly how this happens, think about when you have a campfire. The heat from the fire warms the air around it. This makes the air less dense and causes it to rise. You can observe flames and embers rising to the sky. Later, as the air moves away from the fire, it cools, becomes less dense, and sinks back towards the ground. Convection is the transfer of heat caused by the movement of the heated parts of a liquid or gas.

The Earth, in its early beginnings, was entirely molten, meaning in a liquid state. As convection currents carried heat from the center outward, it brought along matter with it as well. This churning motion cycling through the Earth brought dense materials (like iron) from the surface to the core and allowed less dense materials (like silicon and oxygen) to settle at the surface.

The inner core is the most dense layer of the Earth. It is a solid, mostly iron core with lesser amounts of nickel, gold, platinum, uranium, and lead. All of these materials are naturally very dense materials. This layer is also the hottest layer in the Earth.

The next layer of the Earth is the outer core. This layer is the only fully liquid layer inside of the Earth. It is very hot and dense. Just like the inner core, it is primarily made up of dense metals with the largest percentage of materials being nickel and iron.

The mantle, the layer between the core and the crust, is a very hot, moderately dense layer made more out of rocky materials than metals. This layer is primarily made up of magnesium, iron, and very little to no silicon and oxygen.

The crust, which is the outer rock layer of Earth, is made up primarily of rocks that have a much higher silicon and oxygen content. The crust is the layer that is directly in contact with the mantle, and the surface is where we walk.

Earth’s hydrosphere (rivers, lakes, and ocean) sit on top of the crust, while the least dense layer, the atmosphere, sits on top of all other layers.