Lesson 23: Earth as a Planet
In this lesson, we will talk about what causes changes in
the Earth"s surface, first learning about the internal structure of the Earth,
and then seeing how this drives geological activity. We will then learn about
the magnetosphere, processes that shape the Earth"s surface, and finish by
looking at our atmosphere and how it is critical to life on Earth.
The Earth's surface seems solid and steady but it has both dramatic and gradual
processes that reshape the Earth's surface. Dramatic processes include
volcanoes and earthquakes. They can also include rare occasions like the impact
of an asteroid or a comet slamming into Earth.
Gradual processes that
reshape the Earth's surface include erosion by wind, rain, and ice, and the
movement of continents.
The Earth, Moon, Mars, Venus, and
Mercury, referred to as the terrestrial worlds,
all share the same basic internal structure.
They all have a core which is made of the highest-density material, primarily
metals such as nickel and iron. The Earth, however, may possibly be unique in
that its core is divided into a solid inner core and a molten (liquid metal)
outer core.
Next,
all planets have a mantle that
surrounds the core. The mantle is made up of rocky material of moderate density.
Finally, the world's outer skin is its crust,
which is made of the lowest-density rock.
The core, mantle, and
crust are classifications based on the density of the material that
makes up those layers, but another way to
divide up the Earth into layers is based on rock strength.
Along those lines, the outermost layer of the Earth would be the lithosphere,
which consists of the rocky crust and a little bit of the mantle, and is
characterized as relatively cool and rigid rock floating on warmer, softer rock
below it.
Planets are layered because of a process called differentiation. Early in
each planet's formation, the interior of the planet was hot enough to melt,
allowing material to settle into layers of differing density. Dense metals like
iron sank toward the center, driving less dense rocky material toward the
surface.
The Earth is geologically active, meaning its surface is continually being
reshaped by volcanic eruptions, earthquakes, erosion and other geological
activities.
Its geological activity is driven by processes that occur below the surface.
The primary driver of geological activity is interior heat. The deeper you go
into a planet, the higher the temperature gets. At a certain point in the
mantle, it gets hot enough that it causes convection, which is where hot
material gradually expands and rises while cooler material from above contracts
and falls. Since the mantle is not quite liquid, these convection currents move
very slowly - just a few centimeters a year.
The Earth has a relatively thin lithosphere, so the convection currents reach
closer to the Earth's surface, resulting in a greater influence on volcanic and
tectonic activity. Of the other terrestrial worlds, Venus also is quite hot
inside which results in a thin lithosphere and substantial geological activity.
Mars, which is not as big as Earth and Venus, retains less internal heat but
enough for limited geological activity.
Smaller worlds such as the Moon and Mercury have lost so much heat that
molten rock has hardened and the lithosphere has grown too thick to allow for
any geological activity. Their lithospheres likely extend nearly all the way to
the core.
We have discussed how convection currents that occur in
the *mantle* drive geological activity on the surface.
The *outer core* also has its own convection currents due to that
same process of hot material rising and cool material sinking, and these
convection currents are considered to be responsible for generating Earth's
magnetic field.
As we saw earlier, the core is made of metal, primarily iron and
nickel. The outer core is liquid, so this liquid metal flows easily as a result
of convection currents. This flow of liquid iron in the outer core generates
electric currents, which in turn produce a magnetic field around the Earth.
The Magnetosphere is created by Earth's magnetic field and acts like a
protective bubble around the Earth, shielding the surface from harmful solar
wind particles that could damage living organisms and strip away atmospheric
gas. The magnetosphere successfully deflects most solar wind particles around
our planet but some of the particles still reach the North and South poles,
where they can collide with particles in our atmosphere and cause the beautiful
lights of the aurora.
The Earth's surface is shaped by four processes - impact cratering, volcanism,
tectonics and erosion.
Impact cratering is the creation of craters by asteroids or comets striking the
Earth's surface. Most planets have surfaces covered with impact craters from
the period of heavy bombardment over 4 billion years ago. On Earth, however,
geologists have identified only around 150 impact craters, indicating that most
of our craters were erased over time by geological activity such as volcanic
eruptions and erosion.
The second process shaping Earth's surface is Volcanism,
which consists of eruptions of molten rock or lava from the planet's interior
on to the surface. Earth is the most volcanically active of the terrestrial
worlds. Volcanic activity is not just responsible for the formation of
volcanoes and lava plains; Earth's atmosphere and oceans were made from gases
released from the interior by volcanic outgassing.
Tectonics is the third process that shapes Earth's surface and is the
stretching, compression, and other forces that act on the lithosphere. As
mentioned previously, the lithosphere is the outermost layer of the Earth,
consisting of the crust and a little bit of the mantle. Convection currents in
the mantle fractured Earth's lithosphere into tectonic plates and continue to
move these plates around, changing the Earth's surface through the process
called Tectonics.
Erosion is the final process that shapes Earth's surface. Wind, water and ice
cause erosion, either breaking down the Earth's surface as they carve out
valleys and canyons, or building up new geological features as they pile up
sediment into layers that eventually harden into sedimentary rock.
Earth's atmosphere supplies the oxygen we breathe and is responsible for
weather and erosion, but it also serves other functions.
One of the most important things the atmosphere does is it provides protection
on the surface from dangerous ultraviolet rays and X-rays from the Sun,
absorbing them above the ground. Solar X-rays are absorbed by atoms and
molecules high in the atmosphere while UV radiation is absorbed by a gas called
ozone in the middle layer of the atmosphere (the stratosphere). Without these
protections, life on land could not survive.
Visible light mostly gets through the atmosphere but some of it is scattered by
gases and other particles. The shorter wavelength light (the blues and the
violets) are scattered much more effectively than the higher wavelengths on the
red end of the spectrum. Since the blue light gets scattered, the sky looks
blue. This also explains why our sky is bright in the daytime. On the moon,
where there is no atmosphere to scatter the light, the sky is always black.
Another function of Earth's atmosphere is keeping the Earth warm. Certain
atmospheric gases, called greenhouse gases, trap heat in what is called the
Greenhouse effect. These gases include water vapor, carbon dioxide, and methane.
Earth's surface is much warmer than it would be otherwise, allowing water to
stay liquid over the surface. Without the greenhouse effect, the Earth's
temperature would be well below freezing.
The more greenhouse gases there are, the higher the degree of surface warming.
For example, Venus would have an average surface temperature of -40 degrees
Celsius if there were no greenhouse effect, but since it has so much carbon
dioxide in its atmosphere, Greenhouse warming results in an actual surface
temperature of around 470 degrees Celsius!