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►The Earth Inside and Out

►The Theory of Continental Drift

►The Theory of Seafloor Spreading

►The Unifying Theory: Plate Tectonics

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Earth’s Internal Layers

The current theory is that the Earth’s interior consists of multiple layers: The inner core is primarily iron and nickel

plus other heavy elements. It is theorized to be solid due to enormous pressure.

The outer core is made up of the same elements. However, it is theorized that with less pressure, this layer is liquid.

The mantle is thought to contain mostly silicon and oxygen, with some iron and magnesium. It consists of the upper mantle and the lower mantle. The upper mantle itself has two layers.

The crust is composed mainly of oxygen, silicon, magnesium, and iron. It variesin thickness and is the outer layer ofthe lithosphere.

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Earth’s Internal Layers (continued)

Earth’s internal layers are further divided: The upper mantle also has two layers. The top part of the upper mantle is the

asthenosphere. The asthenosphere is also solid but much the same as old glass is a solid that flows slowly over time. Hotter, less dense material (magma) rises towards the surface where it can eventually flow from a volcano or other opening. At this point the molten rock is lava.

The lithosphere includes the uppermost, rigid part of the upper mantle and the crust. This is the solid rock portion of the outer Earth that rests on the molten asthenosphere.

Scientists separate the uppermost mantle from the crust because they think the mantle’s elemental composition changes little. The crust consists of different rock types thought to undergo change over long periods. Also differentiating the crust from the mantle are conditions such as temperature and

pressure.

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The Rock Cycle

Scientists believe that rocks form or change over long periods due to the processes of the rock cycle.

The rock cycle is like a recycling machine, endlessly converting one type of rock to another.

Three types of rock found in the crust are: 1. Igneous – rocks that form when magma/lava cools and hardens. 2. Sedimentary – rock made up of particles deposited by water flow. Sediment

consists of organic/inorganic matter particles that have been compressed into rock. This is the type of rock in which fossils are found.

3. Metamorphic– as layers build up, this rock is formed when pressure and heat become great enough to change the rock chemically.

The rock cycle is completed through the tectonic process – rocks return to the mantle, remelt, become magma, return to the crust as igneous rock.

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Isostatic Equilibrium

With the crust resting on the mantle, there must be a balance between the weight of the crust and the upward force of buoyancy. This is an application of Archimedes' Principle of buoyancy. This balance is called isostatic equilibrium.

As material adds to the oceanic crust from sedimentation, glaciers and volcanic activity or from the continental crust from erosion, this balance becomes disrupted. This is one theorized cause of earthquakes. To restore equilibrium, landmasses will sink or rise slightly along a weak area called a

fault.

The accepted theory of plate tectonicssuggests that the continents move inhorizontal directions and that earthquakesalso result from that movement.

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Alfred Wegener and Pangaea

Pangaea is the name given to the single giant continent in a proposal by Wegener (1912). He suggested that in the distant past all the Earth’s continents had been a single giant

continent.

Panthalassa is the name given to the single large ocean surrounding Pangaea. The theory of continental drift states that the continents were once a single

landmass that drifted apart and are still doing so.

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Evidence for Continental Drift

Wegener tried to support his theory with evidence: Accurate world maps. People noticed that the continents apparently fit together like a

jigsaw-puzzle. Fossils of the Glossopteris fern in South

America, Africa, Australia, India, andAntarctica. The seeds are too heavyto travel by wind and too fragile tosurvive significant sea crossings.

Distribution of other animals and fossils werestudied, especially coal.

Wegener’s theories were not accepted because he could not explain how continents could drift, also he was a meteorologist, not a geologist.

The plate tectonics theory would finally provide an explanation for how continents move, making Wegener’s theory widely accepted. Additionally, it was found that including the continental shelves filled the gaps in the

jigsaw puzzle.

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New Technology and Seafloor Knowledge

Sonar made it possible to detect an otherwise invisible object. Scientists immediately recognized that they could map the ocean bottom with sonar. The German Meteor expedition mapped the South Atlantic in 1925. This became the

basis for the first worldwide seafloor map of deep ocean ridges.

The ability to map the seafloor in greater detail revealed important new features: Mid-ocean ridges are enormous mountain ridges on the bottom of the ocean. Rift valleys are deep valleys

running through the center ofmid-ocean ridges such asthe Atlantic Ridge.

Trenches are deep ravinesin the seafloor.

Scientists began to notice patternsin the mid-ocean ridges, trenches,and rift valleys that proved pivotalin developing new theories.

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The Creation and Destruction of Seafloor

Hess and Dietz (1960) proposed an explanation of seafloor features. They hypothesized that the seafloor is in a constant state of creation and destruction through a process called seafloor spreading.

In the theory of seafloor spreading, new crust emerges from the rift valleyin a mid-ocean ridge. Magma from the asthenosphere pushes up through the riftand solidifies into new crust.

The new seafloor forms at the rift valleys and mid-ocean ridges, spreading away from the ridges until it returns as part of the rock cycle at subduction zones (trenches).

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Evidence of Seafloor Spreading

Scientists have found three forms of evidence that support the theory of seafloor spreading.

1. In 1968, scientists began drilling holes in the seafloor to gather sediment samples – the layers were thin or absent at the ridges, and thicker away from the ridges.

2. Radiometric dating was used to determine the age of rocks. Scientists have found seafloor rock to be significantly younger than rock in the center of the continents. According to the theory, continental rock doesn’t subside, where as seafloor rock does.

3. Evidence from magnetometer data. This instrument measures the polar orientation and intensity of magnetism of minerals. After towing magnetometers around the seafloor beginning in 1950, scientists discovered a symmetrical pattern in the polar orientation of magnetism of seafloor rock, the seafloor on either side of the mid-ocean ridges roughly mirrors each other’s polar orientation.

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Seafloor Spreading and Continental Drift Combine

The theory of plate tectonics united the theories of continental drift and seafloor spreading. It combines ideas from both theories, along with some of the original isostatic

equilibrium concepts.

In plate tectonic theory the Earth’s lithosphere consists of more than a dozen separate plates. The plates are rigid and float on the asthenosphere.

At a spreading or divergent boundary, two plates are moving apart. As this happens, the crust pulls apart and forms valleys. Magma flows up through the rift valleys creating new crust and widening the seafloor. Mid-ocean ridges and rift valleys mark divergent boundaries.

At a colliding or convergent boundary, two plates push together. They are also called destructive boundaries because movements along these destroy

crust.

At a transform boundary or fault, two plates slide past each other. Earthquakes result as rocks move when the plates slide next to each other.

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Seafloor Spreading and Continental Drift (continued)

Island arcs (divergent boundary). When two oceanic plates collide, the denser plate will

be subducted. Some of the material from the melting oceanic plate rises upward to form a volcanic island arc.

Subduction zones (convergent boundary). A trench forms as a more dense oceanic plate moves

under a less dense continental plate. As subduction occurs, some of the material from the melting oceanic plate rises upward to form volcanoes on the continent.

Convergent boundary (transform boundary). Mountain formation at continental plate collision.

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Hot Spots

The hot spot theory states that hot spots are small melting areas within the mantle where thermal plumes cause magma columns to push up intensely, breaking through the crust.

What makes this theory significant is that hot spots do not move with tectonic plates because they originate in the mantle.

Volcanic island chains, are the result of the plate moving over a hot spot.

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Plate Movement

Convection is the primary force driving seafloor spreading andcontinental drift. Convection currents form as hot material rises and cold material sinks. This occurs in

the mantle. This creates a current that moves the plates away from each other at the divergent

boundaries, toward each other at the convergent boundaries, and past each other at the transform boundaries.

A second driving force comes from seafloor spreading. As new seafloor forms, the plates

tend to slide away fromthe elevated mid-ocean ridges.

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Plate Movement (continued)

Predicted changes over the next 50 million years: The Baja Peninsula will have moved past and apart

from the North American Plate. Southern California will pass San Francisco as it

moves to the northwest. A new sea will form in eastern Africa. Australia will move toward Eurasia/the Equator. The Mediterranean Sea will close as Africa pushes

towards Europe. The Atlantic and Indian Oceans will continue to grow

while the Pacific will become smaller.

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