Great question! The earth has actually been
cooling down over billions of years. So, why is
there still so much heat in the earth? The heat
comes from two main sources. Some of the heat is
left over from when earth formed (about 4.6
billion years ago!), and some heat is still being
produced. The planet formed when smaller pieces of
rock, like meteors, asteroids, and small planets
(called planetesimals), collided and stuck
together. These small pieces of rock and dust were
attracted by gravity, and lot of heat was released
when they came together. Very shortly after the
earth accreted (came together from smaller pieces)
it was totally molten (liquid) rock and was all
mixed together and not layered. More heat was
released when the earth underwent differentiation.
Differentiation was when layers formed, like the
core, mantle, and crust. When the core of the
earth formed, it could have increased the
temperature of the planet by over 2000 Celsius
(Francis & Oppenheimer, 2004)!
One more important
source of heat in the earth is the decay of
radioactive elements. Some elements, like
potassium, uranium, and thorium, have isotopes
that are unstable. Because they are unstable, they
turn into other elements, and this releases a lot
of heat. These elements are concentrated in rocks
(like granites) in the crust of the earth.
Geologists think that the decay of these elements
creates enough heat to make parts of the crust
soft. This means that parts of the crust flow like
silly putty (but not quite as fast) instead of
being cold and hard like a boulder of granite that
you might see on the surface of the earth!
Francis, P. & Oppenheimer, C. (2004). Volcanoes.
New York: Oxford University Press.
The Earth heats up under the warmth of the hot sun
during the day, and even more so during the summer
time as that is when the Earth is closest to the
sun. The Earth has an atmosphere of gases which
acts like a blanket to hold in warmth. The
atmosphere includes important things like oxygen
and water vapor, but it is mostly made of
nitrogen, which is a non-reactive gas. This
atmosphere is critical to life on Earth.
Otherwise, Earth would be too cold and not have
oxygen available for breathing.
You may have heard about certain types of gases
like carbon dioxide or methane that are especially
effective at warming the Earth. The reason they
cause warming is because the bonds that hold
together the atoms in these molecules absorb and
hold a lot of the sun's heat. These gases are fine
in small amounts and they have always existed as
small fractions of the atmosphere. However, when
too much of these gases starts to fill the
atmosphere, we start getting TOO warm. Life here
evolved under a narrow average temperature range
and if it gets too hot or too cold on the planet,
we will start to see changes in habitat and
increasing extinctions. That's what everyone is
worried about--we are producing very large amounts
of carbon dioxide and methane in our agriculture,
manufacturing, transportation and electricity
production, and it's having an effect on our
atmosphere and on the health of our planet and the
plants and animals that live here.
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