Earth’s Internal Heat and Geologic Activity
http://www.teachastronomy.com/astropedia/article/Internal-Heat-and-Geological-Activity
Instructions: Read the article below “Earth’s Internal Heat and Geological Activity”. As
you read, answer the questions below by highlighting the answers and placing the
number of the question next to your highlighted answer. Then answer the questions by
using your highlighted answers. Finally write a 10-15 sentence summary that
incorporates the questions and highlighted answers.
Internal Heat and Geological Activity
Heat is the ultimate source of the energy that drives geological activity on any
planet. The internal heat of the Earth, fueled by radioactivity, is the "engine" that
drives the geological activity of the planet. Volcanoes, for example, erupt because
the heat inside the Earth melts rock and creates supplies of molten magma; the
magma is less dense than surrounding rock, so it tends to rise to the surface.
Earthquakes are also caused by pressure associated with the movements of molten
rock. Without radioactivity, the Earth’s interior would have cooled long ago.
Volcanoes would be extinct, and there would be no earthquakes. Earth would be
geologically dead.
Radioactivity is one of the main sources of heat inside the Earth. How do
radioactive minerals inside the planet produce heat? Imagine an atom of a
radioactive element buried inside Earth. When the atom breaks down during the
process of radioactive decay, it shoots subatomic particles and photons of energy
outward into the neighboring material. Just as fragments of an exploding billia rd
ball might set neighboring balls into motion, the "exploding" atom’s debris hits other
atoms and increases their motions. Temperature is a measure of particles’ speed,
so increased motion results in a higher temperature. Radioactive material is
therefore an energy source, and it heats the interior of the planet in which it’s
trapped.
The heating action of radioactive elements inside the Earth explains why early
models of the Earth’s age, based simply on its cooling time, were far too short. As
Newton and Buffon showed, the Earth would have cooled completely within millions
of years - if it had no radioactivity. But with this internal heating, the depths of the
Earth have been kept hot for thousands of times longer - several billion years.
In general, planets produce heat according to their size. Radioactive atoms decay
in the interior, and conduction and convection transport this heat from the interior to
the surface. Bigger planets have more gravity, and the pressure due to gravity
helps to create a molten interior that can drive geological activity. Also, the bigger
the planet, the longer it takes internal heat to reach the surface. There’s no mystery
here - if you pull a large rock and a small rock out of a campfire, the small one will
cool off quickly, while the big one will stay warm longer. Planets are the same.
The simple idea of internal heat scaling with size tells us a lot about planetary
evolution, since the geological activity on planets is driven by heat from the interior.
A small planetary body cools off very quickly, and its interior produces little heat
because there is less radioactive material. So the surface of a small planet will
show countless crater scars from impacts that happened throughout geological
time. On the other hand, a large planetary body retains its heat longer, and its
interior produces a lot of energy from a larger mass of radioactive material. This
internal heat will drive volcanism and tectonics, which constantly reshape the
surface. Therefore, the surface of a large planet will be young and show fewer
impact craters.
To see a cutaway of the interior of the Earth showing different layers. Click here for
original source URL.
The rate and mode of heat transfer from the center of the planet to the surface
controls the rate of geological activity on the surface. If the planet is small and has
cooled off, the center is not much warmer than the surface, and the heat can be
carried outward by conduction. However, if a planet is large and has a lot of energy
from radioactive decay, the center is going to be much hotter than the surface. The
strong temperature difference between the center and surface induces convection
in the mantle - sluggish mass movement of the hot, slightly plastic material. If
they’re hot enough, the inner rock layers of a planet can actually flow slowly. Hot
plumes of convecting material rising in the asthenosphere create "hot spots" under
the lithosphere. Such hot spots produce volcanoes. The Hawaiian Islands are a
terrestrial example, and the giant volcanoes on Venus and Mars may also be
located over hot spots created by convection. A smaller planet with a colder interior
has less internal heat to create convective movement in the mantle, and thus less
internal
energy
to
drive
seismic
activity,
tectonics,
or
volcanoes.
Simple ideas of heat generation and transfer inside planets allow us to understand
the differences in geological features from one planet to another. If we list the
terrestrial planetary bodies in order of increasing size, we see exactly what we
would expect - a progression toward younger surfaces and more geological activity:
Check url for images of objects being compared.
http://www.teachastronomy.com/astropedia/article/Internal-Heat-and-Geological-Activity
.
Questions
1. What is the ultimate source of energy that drives geological activity?
2. What fuels the internal heat of the Earth?
3. Why does magma forming volcanoes rise to the surface?
4. What stopped the Earth from cooling a long time ago?
5. Highlight the steps explaining how radioactive minerals inside the planet produce
heat.
6. Radioactivity in the Earth has kept the depths of the Earth hot for how much longer ?
7. What transports the heat from the interior of the Earth to its surface?
8. What cools faster a big planet or a small planet?
9. Why do larger planets have less crater scars than small planets?
10. What induces convection in the mantle?
11. Where is an example of hotspots formed under the lithosphere?
12. Why does a smaller planet with a colder interior have less seismic, tectonic, and
volcanic activity?______
Write a 10-15 sentence summary that uses the questions and highlighted answers.
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