How the Earth Works:
A Peek Inside Earth’s Interior
- Last week, we discovered that movement inside the Earth causes the
surface of the Earth to be habitable. Today, we are going to use our
knowledge of temperature, pressure, and density to learn how the inside
of the Earth moves.
- The first thing to remember is that NOBODY has ever been there!
- Depending on the temperature and depth, a particular Earth
material may behave like a brittle solid, deform in a plastic–like
manner, or melt and become liquid. First, let’s take a big-picture
look at what we are talking about.
(1 mile = 1.609344 km)
How the Earth Works:
How Do We Know What We Know?
-What we think we know comes
from a study of how earthquake
(seismic) waves travel through the
earth, and how long it takes for
them to get from where the
earthquake happens to a recording
station.
-The basic idea is that different
materials transmit seismic waves at
different speeds. With a lot of
earthquakes and a lot of recording
stations, geophysicists are beginning
to get a pretty detailed picture of
what is probably down there.
How the Earth Works:
All Cracked Up
-One of the most distinctive features of the earth's interior is how it seems
to be layered by density, with the heaviest stuff in the center, and the
lightest material at the surface. The earth probably looks a lot like a hard
boiled egg if you could cut it open.
- The yellow stuff in the center (the yolk) relates to what we call
the core. Most geophysicists think that the core is composed of
high density materials like iron and nickel. The core is split into two
subcategories: a solid core surrounded by a liquid core.
- The egg's shell is like the earth's crust - a thin veneer of rigid, low
density material at the surface.
- And all the white stuff in between is like the earth's mantle - the
largest layer which, in the case of the earth, is of medium density.
The mantle is split into two subcategories: the upper and lower
mantle.
Eggs-ellent Quick Lab
How the Earth Works:
The Core
-Earth's core is thought to be composed
mainly of an iron and nickel alloy. This
composition is assumed based upon
calculations of its density and upon the fact
that many meteorites (which are thought to
be portions of the interior of a planetary
body) are iron-nickel alloys.
-The core is earth's source of internal heat
because it contains radioactive materials which
release heat as they break down into more
stable substances.
- The core is divided into two different zones. The outer core is a liquid because the
temperatures there are adequate to melt the iron-nickel alloy. However, the inner core
is a solid even though its temperature is higher than the outer core. Here, tremendous
pressure, produced by the weight of the overlying rocks is strong enough to crowd the
atoms tightly together and prevents the liquid state.
How the Earth Works:
More about the Core
•Inner core
Sphere with a radius of 3486
kilometers (2161 miles)
Material is stronger than the
outer core
Behaves like a solid
•Outer core
Composed mostly of an ironnickel alloy
Liquid layer
2270 kilometers (1410 miles)
thick
Convective flow within generates
Earth’s magnetic field
How the Earth Works:
The Mantle
-Earth's mantle is thought to be composed
mainly of olivine-rich rock. It has different
temperatures at different depths. The
temperature is lowest immediately beneath the
crust and increases with depth. The highest
temperatures occur where the mantle material is
in contact with the heat-producing core.
-This steady increase of temperature with depth
is known as the geothermal gradient. The
geothermal gradient is responsible for different
rock behaviors and the different rock behaviors
are used to divide the mantle into two different
zones.
-Rocks in the upper mantle are cool and brittle, while rocks in the lower mantle are hot
and soft (but not molten). Rocks in the upper mantle are brittle enough to break
under stress and produce earthquakes. However, rocks in the lower mantle are soft and
flow when subjected to forces instead of breaking.
How the Earth Works:
More about the Mantle
•Lithosphere (crust and upper mantle)
Earth’s outermost layer
Relatively cool, rigid shell
Averages about 100 kilometers in
thickness, but may be 250 kilometers
or more thick beneath the older
portions of the continents
•Asthenosphere (bottom of upper mantle)
Beneath the lithosphere, in the upper
mantle to a depth of about 600 kilometers
Small amount of melting in the upper
portion mechanically detaches the
lithosphere from the layer below allowing
the lithosphere to move independently of
the asthenosphere
•Lower mantle (near the core)
Rigid layer between the
depths of 660 kilometers
and 2900 kilometers
Rocks are very hot and
capable of very gradual
flow
How the Earth Works:
The Crust
- There are two different types of crust: thin oceanic crust that underlies the ocean
basins and thicker continental crust that underlies the continents.
- These two different types of crust are made up of different types of rock. The thin
oceanic crust is composed of primarily of basalt and the thicker continental crust is
composed primarily of granite. The low density of the thick continental crust allows it
to "float" in high relief on the much higher density mantle below.
How the Earth Works:
More about the Crust
-The crust of Earth is divided into two types: oceanic and continental.
- Most oceanic crust is composed of the rock basalt. Basalt is mafic
(which means it has a high magnesium and iron content), composed of
the minerals: calcium-rich plagioclase feldspar, olivine, amphibole, and
pyroxene.
Continental crust has more range in composition, but its average
composition is similar to the rock granite. Granite is a felsic rock (which
means it has a high feldspar and silica content), composed of quartz,
potassium-rich feldspar, and mica.
Mafic rocks contain denser minerals and therefore, oceanic crust is
denser than continental crust (the average density of basalt is 3.0 g/cm3
and granite is 2.7 g/cm3). On average, continental crust is much thicker
and older than oceanic crust.
Rock Examples
How the Earth Works:
A Peek Inside Earth’s Interior
-There are two different theories about how Earth’s mantle moves. One theory says
that the upper mantle and lower mantle move together as a single convection
system.
-Another theory states that the asthenosphere has its own convection system
separate from the rest of the mantle. There is good evidence for both of the
explanations. As with any scientific debate, more research and evidence must be
presented before anyone can say for sure which theory is correct.
-What we DO know is the interior of the Earth is hot! The core heats up the lower
mantle, which becomes less dense and rises to the upper mantle where it comes in
contact with cooler material.
-The warmer material loses heat and the cooler material gains heat until both are
the same temperature. As the resulting material cools, it becomes denser and sinks
back to the lower mantle. The temperature of the material is then increased by the
heat inside the Earth, decreasing density and rising again. This results in convection
cells that transfer heat and create movement inside the Earth.
How the Earth Works:
Convection Currents
Safety First! Convection Lab
How the Earth Works:
Convection in Your Daily Life
Where do you think there is evidence of
convection currents in your home?
In a two-story building, where do you
think it is warmer? Why?
Where would you want to have heater
vents in your home? Why?
What would happen if the heater vents
were located near the ceiling instead?
Announcements
Next week: Plate Tectonics!
Please read Chapter 15 of your textbook for Monday.
Also, don’t forget your 1st Quarter Homework is due
Wednesday, Sept. 14 (the beginning of class).