Ask A Scientist

What is the earth made out of? 

Asked by: Ariana Velez
School: Johnson City Elementary School
Grade: 3
Teacher: Jill Swartz
Hobbies/Interests: Playing outside, riding her bike


Answer from Kyle Reeser

PhD Student in Biomedical Engineering

Research area: Magnetic stimulation of taste
Interests/hobbies: Traveling, language studies, cooking, music

Think of the Earth like a peach.  A thin layer of skin and fuzz covers a thick flesh, which, at its center, contains a hard, dense pit. We live on the thin outer layer of Earth, called the crust, which is somewhere between 3 and 25 miles thick. Earth’s crust contains an abundance of elements, most notably oxygen. By the way, you and I also contain more oxygen by weight than any other element, largely in the form of water. Other crust elements in great quantity include: silicon, aluminum, iron, calcium and sodium. Trace amounts of most of the contents of the periodic table can be found, if one looks hard enough. 

The thick middle layer of the Earth is called the mantle, made of mostly solid rock rich in silicon, magnesium and iron. The crust is divided into huge plates that are in constant motion above the mantle.  This layer constitutes about 84 percent of Earth’s entire volume. 

Below the mantle lies Earth’s core, divided into a molten outer layer and a solid inner layer. The core consists mainly of iron and nickel. It appears that in Earth’s infancy, heavier elements tended to settle towards the center, pushing those lighter elements towards the surface. 

Thankfully so.  

In 1990, the Voyager 1 space probe took the famous ‘Pale Blue Dot’ photograph of Earth. From a distance of more than 3 billion miles, the Earth appears as but a spec against the darkness and vastness of space. I often reflect on a quote by the great scientist and author Carl Sagan, who requested this portrait of our planet be taken: "Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives."

For me, this fact is a profound one — that everything that has ever lived and died on this planet is still here, in some form. The basic elements that make up this Earth cycle through the eons, from the living to the inanimate. 

So, what is the Earth made out of? Sooner or later, in a very small way, all of us.


Last Updated: 9/25/14