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Ask A Scientist

How can the core of the Earth be so hot and the rest of the Earth not so hot? Also, how do we know what the layers of the Earth are made of?

Asked by: Melissa Bourizk
School: Choconut Valley Elementary School, Montrose Area
Grade: 6
Teacher: Christine Casey
Hobbies/Interests: Drawing
Career Interest: Teacher

Answer from Douglas W. Green, EdD

Adjunct Lecturer, Binghamton University

Research area: Leadership, Learning Theory and Social Media
Family: Daughter Lena, age 27, who is an animator for Nickelodeon in New York City
Interests/Hobbies: Playing banjo, biking, golf and reading
Web page address:

About 4.5 billion years ago, the sun and its planets formed from a big cloud of material left over after the creation of the universe. As the material in the Earth's orbit came together, the more dense material was mostly sorted to the center. The most abundant dense element at the time was iron so the Earth's core is mostly iron. Iron is also the most common element that can become magnetic so the Earth acts like a big magnet, which is why a compass points towards the magnetic North pole.

At the Earth's surface, the temperature of the ground is cause by local weather and will change as the temperature of the atmosphere changes. Once you go a few feet below the surface, the temperature is more constant and in the 50 to 60 degree farenheit range. This is why an unheated basement seems cool and is a good place to store food.

As the pressure from the material above increases, the temperature rises. Farther down the temperature is high enough to cause some rocks to flow like very thick syrup. The center of the Earth's core is about 4,000 miles down, and the temperature at 9,000 degrees farenheit is similar to the surface of the sun.  The ground underneath varies from one place to another. In some places, melted rock works its way up through cracks to the surface the surface, creating volcanoes. You can study the material ejected by volcanoes to learn more about what kind of rocks are below the surface. There are some places where the ground is hot near the surface. This causes hot springs and geysers. Some people use this heat to warm homes and make electricity. This type of energy is called geothermal energy. The country of Iceland has lots of hot rocks near the surface so energy is inexpensive there. Scientists called seismologists determine the structure of the Earth by bouncing sound waves off of the layers. When the nature of the material changes at the boundary between two layers, some sound will reflect back to the surface. From the time it takes the sounds to bounce back, you can tell how deep the layers are. The deepest wells allow for direct access to rocks and temperatures. Below that, scientists use indirect data  to estimate temperature and rock type. For more information on the layers, visit

Last Updated: 9/18/13