Earth’s Interior & Plate Tectonics: NOTES
The center of the Earth is 6,000 kilometers deep, far too deep for drilling or other
means of investigation. So, how do scientists figure out what the center of the
Earth is like?
1. rock samples – over time, rocks have been “pushed up” from the Earth’s
interior toward the surface, allowing for scientific analysis
2. seismic evidence – a seismic wave is a vibration that travels through the
Earth carrying energy released during an earthquake. Using data from
seismic waves, scientists have learned that the interior of the Earth is made
up of several layers.
4 Main Layers of the Earth:
1. crust
2. mantle
3. outer core
4. inner core
Moving down below the Earth’s surface - through the crust, mantle, and into the
core - the temperature gets hotter and the pressure rises. The heat is coming from
the Earth’s core and the pressure comes from the weight of the rock above. The
deeper into the Earth, the greater the weight of the rock above, and therefore, the
greater the pressure. (Think about what happens to the water pressure around you
when you dive into the deep end of a swimming pool.)
Let’s take a closer look at all the layers of the Earth….
Crust
The crust is the layer of rock that forms the Earth’s outer covering.
The crust includes dry land and ocean floor, and is between 5 and 40 kilometers
thick. It is the thinnest of the Earth’s 4 layers (similar to the shell of an egg).
Continental crust (dry land) is thicker, less dense, and mostly made up of granite
rock.
Oceanic crust (ocean floor) is thinner, more dense, and mostly made up of rock
called basalt.
Mantle
The mantle is the layer of hot, solid material between the Earth’s crust and core.
The mantle is about 3,000 kilometers thick and it is made mostly of silicon,
oxygen, magnesium, and iron. The mantle is divided into three layers:
1. lithosphere – the uppermost part of the mantle PLUS the crust. This cold,
rigid, rocky layer is about 100 kilometers thick.
2. asthenosphere – this middle layer of the mantle is soft and tar-like (the
molten rock can bend like plastic). The asthenosphere is hotter and under
more pressure than the lithosphere.
3. lower mantle – the lower mantle is solid rock and goes all the way down to
the Earth’s core.
Core
1. outer core – the fluid area around the solid inner core. The outer core is
mostly metallic (iron and nickel) in composition and very hot.
2. inner core - the solid center of the Earth, made up of mostly iron. The inner
core is believed to be responsible for the magnetic field that exists around
the Earth.
Continental Drift
In the early 1900s, a scientist named Alfred Wegener believed that the continents
were once all one land mass, but then they slowly drifted apart. Wegener’s theory
became known as continental drift and he called the single large land mass
PANGAEA. During his lifetime, Wegener’s theory was not fully accepted, but
evidence eventually convinced the scientific world that the theory was correct.
This evidence included:
1. shape of the continents (appear to fit together like pieces of a puzzle)
2. fossils (similar types have been found across continents)
3. land features (similar coasts, mountain ranges, and composite rocks
have been found across continents
4. climate (tropical fossils found in polar continents show that polar
continents must have been in a warmer location at one time)
Sea Floor Spreading
It wasn’t until the 1950s when scientists began examining the ocean floors that
Wegener’s theory gained support. Although most of the deep ocean floor was cold,
dark, and almost lifeless, some areas of the sea floor were found to be warm and
full of organisms. This was due to the fact that the ocean water was entering cracks
in the ocean floor and getting warmed by hot layers in the mantle. The heated
water was then vented back out into the ocean. Scientists also found that the ocean
floor had mountains and valleys. These mountain valleys were called mid ocean
ridges. A geologist named Hess proposed that mid ocean ridges were places where
the sea floor was spreading apart. As the sea floor spreads apart, magma (hot liquid
rock) from below moves upward to fill the gap created. The magma cools and
becomes new rock. As this process continues, the ocean floor moves and takes the
continents with it. This theory became known as sea floor spreading and it fully
supports Wegener’s idea of continental drift.
What proof exists to support the theory of sea floor spreading?
1. evidence of molten rock along ocean ridges
2. age of rocks - “younger” rocks are found along the ridge and rocks get
progressively older as you get further from the ridge.
3. magnetic clues – rock that contained iron was lined up along magnetic poles
(the magnetic poles of the Earth have reversed over time)
The sea floor doesn’t only spread apart, it also drops down into deep ocean
trenches in a process called SUBDUCTION. The ocean floor is like a giant
conveyor belt…hot rock comes up through the ridge, cools down and becomes
dense, drops into a trench and becomes hot again and the cycle continues.
Plate Tectonics
The Earth’s seven continents (can you name all seven?) have cracks in them.
Scientists observed these cracks and developed the theory of plate tectonics. This
theory states that the Earth’s crust and upper mantle (the lithosphere) are broken
into sections. These sections, called plates, move around on the mantle.
What causes the plates to move? The answer is convection currents. (Do you
remember what convection currents are? Hot fluids rise, cold fluids sink). Plates
move VERY slowly (a few centimeters a year). As plates move, they could pull
apart (diverge), collide (converge), or slide past (transform) one another. These
three interactions can cause mountain ranges, deep ocean trenches, volcanoes, and
earthquakes.
Plate boundaries are the places where the edges of the Earth’s plates meet. There
are three kinds of plate boundaries:
1. DIVERGENT BOUNDARIES – locations where plates are moving apart
(diverging). Sea floor spreading occurs at divergent boundaries. On land,
divergent boundaries are called rift valleys.
2. CONVERGENT BOUNDARIES – locations where plates are coming
together (converging). The plate that is more dense goes beneath the less
dense plate. Oceanic crust is usually more dense than continental crust, so
when these two collide, the oceanic crust goes under the continental crust.
When two oceanic crusts collide, deep sea trenches or volcanic islands can
be formed. When two plates carrying continental crusts collide, the plates
are pushed up into mountain ranges such as the Himalayas.
3. TRANSFORM (SLIDE) BOUNDARIES – locations where two plates are
sliding past one another, often causing earthquakes. The San Andreas fault
in California is a transform boundary.
Heat Transfer (Review)
Heat energy always moves from the warmer substance to the cooler substance.
There are three types of heat transfer: radiation, conduction, and convection
1. Radiation – the transfer of energy through space. The sun’s energy travels
through space to the Earth by radiation.
2. Conduction – the transfer of heat energy within or between materials that
are touching. When you touch a hot pot or pan, you burn your finger. The
heat transfers to your finger by conduction.
3. Convection – the transfer of heat energy by the movement of currents
within a fluid (fluids are defined as liquids or gases). The glitter lamp and
lava lamps work by convection.
Convection current is the flow that transfers heat within a fluid. As a fluid heats up
(gains energy), it becomes less dense and rises upward. As the fluid cools down
(loses energy), it becomes more dense and moves downward.
What do convection currents have to do with the layers of the Earth?
Scientists believe that differences in density cause the hot, plastic-like rock in the
asthenosphere (the soft, middle layer of the mantle) to be forced upwards towards
the Earth’s surface, where it sometimes moves horizontally, then cools off and
sinks slowly down again, taking some of the Earth’s crust with it. This cycle of
heating and rising, followed by cooling and sinking has been going on for more
than 4 billion years!