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Earth’s Interior
Natural Disasters: Part B
Earth’s Spheres & Systems
What is the solid Earth made of?
Planet Earth is a rocky planet. It is made up of rocks
and minerals. This sphere is called the “geosphere”.
The Earth is made up of a number of elements.
The interior of the Earth is quite different than the
exterior (continental crust).
The Earth can be compared to a hard-boiled egg. Its
major layers (crust, mantle, core) are like the shell,
egg-white and yolk of an egg.
The deepest man has dug into the Earth is 5 km in
the South Africa gold mines. (This is within the
crust.) Here the temperature increases by 10 to 15
˚C for every kilometer down.
We have not actually been to the center of the Earth.
So how can we infer what the composition of the
Earth’s interior is like?
Earthquakes: Seismic Waves
• If Earth were made of the same materials
throughout, seismic waves (energy released from
earthquakes) would travel in straight lines at a
constant speed.
• However, seismic waves speed up and refract
(bend) when they reach the mantle as it is more
dense (due to increased pressure).
• Scientists can infer the composition of the Earth’s
interior based on the behavior of seismic waves.
Uncertainties:
• This is our “best guess” as we have never actually
seen the mantle or core.
• Our “best guess” scenario cannot explain the
location of “hot spots” and why they occur where
they do.
• We think rock within the asthenosphere (mid to
lower mantle) melts due to a decrease in pressure
of surrounding rock. However, our “best guess”
cannot explain what causes this change in
pressure.
Earth’s Interior based on Seismic
Wave Behavior:
The Earth is divided into 3 concentric layers:
•The crust (upper Lithosphere)
•The mantle (lower Lithosphere and upper
Asthenosphere)
•The core
Each layer has different properties.
The Crust
The crust is a very thin skin which surrounds the
planet (like the shell on an egg).
There are two types of crust:
Oceanic crust: which is thin (≈7 km) and is
made up of dense volcanic rock (basaltic).
Oceanic crust is “young” (180 million years
old).
Continental crust: which is thick (≈ 35 km)
and is less dense than oceanic crust. It is rich
in quartz and feldspar (granitic) and much
older (4 billions years old). It is thicker under
mountains.
The Lithosphere
The crust and the upper part of mantle form the
Lithosphere.
It is fragmented into a series of segments called
plates.
These plates move with respect to each other and
are the cause of most earthquakes and volcanoes.
The lithosphere is usually 100 km thick.
The Mantle
The mantle is located between the crust and the core. It is
mainly solid, but acts as a “fluid-like” solid (putty).
The upper part of the mantle is cooler. This part of the mantle
is joined together with the crust to form the lithosphere. Its
temperature reaches about 1300˚C.
The mantle, directly beneath the lithosphere, is much hotter.
This is called the asthenosphere. Its temperature reaches
about 1700 ˚C.
The lithosphere slides over the asthenosphere as it is less
dense.
The area below this is called the lower mantle
Convection currents in the mantle
We think the mantle acts like hot, sticky tar (putty-like).
It is heated by the core below it (decay of radioactive elements
and enormous pressure create intense heat) and cooled by the
crust above.
When you heat a fluid, it’s density drops and it expands.
If you cool it, it will contract and become more dense.
Therefore, the hotter material at the base rises toward the top
while the cooler material at the top sinks.
This generates slow flowing convection currents in the mantle.
The Core
•The core is divided into two parts:
outer core which is liquid
inner core which is solid
•The core is made up of mainly iron and nickel
which makes it very dense.
•The movement of the liquid outer core over the
solid inner core generates the Earth’s magnetic
field. (?)
Earth’s Magnetic Field
• Like a bar magnet, the Earth has two magnetic
poles. The lines of force of the Earth’s magnetic
field extend between the North Magnetic Pole and
the South Magnetic Pole.
• The source may be the liquid iron in the Earth’s
outer core. Because iron is a good conductor,
scientists hypothesize that motions within the core
produce electrical currents that in turn create the
Earth’s magnetic field.
• Deflects UV radiation from the sun (protects us).
Uncertainties:
• Research has shown that both the sun and
moon have magnetic fields as well.
• The sun contains no iron and the moon does
not have a liquid core.
• Discovering the sources of the magnetic
fields of the sun and moon may help
scientists identify the source of the Earth’s
magnetic field.
The interior of the Earth has a major impact
on other Earth systems.
•Convection currents in the mantle cause the movement of
the plates.
•The internal heat is responsible for volcanoes.
•The outer core generates the Earth’s magnetic field. (?)
•Earth’s early atmosphere and water was probably formed
from volcanic gases derived from inside the Earth.
•The position of the continents effects ocean currents which
impact the Earth’s climates.
•Plates also effect the geographic distribution of organisms
as well as their evolution.