The Earth is made of three principal layers:
crust, mantle, and core. Each of these is
made of different materials. The crust and mantle
are both made of rocks, but the mantle is
made of rocks that are more dense than the
crust. You can think of mantle rocks as being
heavier than the crust. Our planet's core is
mostly made of iron and other metals.
Nearly all scientists are interested in the
inner layers of the Earth. We don't have the
technology to drill a hole deeper than about 7.5
miles, so we have to use other techniques to
discover what is deep inside the Earth. Many
scientists have studied the layers of the Earth.
An early hypothesis about the density of the
Earth was given by Sir Isaac Newton in
1687. Other scientists in the 1600s thought
there should be concentric spheres within the
Earth (like onion layers). Although most
scientists continued to believe there were layers
inside the Earth, no one discovered how deep they
were (or how thick they were) until the 1900s.
One important scientist was Andrija
Mohorovicic, from Croatia. In 1909, he
discovered the boundary between the crust and the
mantle using seismic data (earthquake waves). To
honor Dr. Mohorovicic's discovery, we call this
boundary the Mohorovicic discontinuity.
Usually, we refer to this boundary as the
"Moho", which is about 22 miles below the
continents (where we live at the Earth's surface).
Another important seismologist was Inge
Lehmann, from Denmark. She was the first to
hypothesize that the Earth's core is made of two
layers: a liquid outer core and a solid inner
core. In 1936, she published this discovery in an
international science journal, and (later) she was
honored with many awards for her achievements.
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