Earth Science Ch 9 review : Plate Tectonics

Chapter 9 Review: PlateTectonics

Chapter 9 Review: Plate Tectonics In 1915, German scientist Alfred

Wegner proposed his theory of continental drift; that the continents had once been joined to form a single landmass; a super-continent we called Pangea.

Wegener also hypothesized that about 200 million years ago, Pangaea began breaking into smaller continents. The continents than slowly drifted to their present positions.

Chapter 9 Review: Plate Tectonics Wegener presented a variety of

evidence to support his theory of continental drift.

His evidence included Similar fossils from different

locations Matching types of rocks Traces of glaciations on widely

separated landmasses

Matching Fossils: Fossil evidence for continental drift comes from several identical types of fossil remains found on different landmasses.

Chapter 9 Review: Plate Tectonics Matching Types of Rocks: Besides

fossils, matching types of rocks in several mountain belts in different parts of the globe indicate that these areas were once connected.

Matching Types of Rocks: We find larger evidence in the forms of mountain ranges. The Appalachian mountain range that ends in Newfoundland has mountains of the same size and rock types as mountains found in northern Europe.

Ancient Climates: Wegener also found evidence for dramatic global climate changes that supported his hypothesis.

Wegener found Glacial Deposits showing that, between 220 million and 300 million years ago, ice sheets covered large areas of the southern hemispheres.

Chapter 9 Review: Plate Tectonics At first, Wagener’s

hypothesis received much criticism from scientists. The main objection was that no one could describe a mechanism by which the continents could move.

By 1967, a new theory developed that linked with Wegener’s way of thinking. The theory of plate tectonics provided explanation for the continental drifting as well as many other geological processes.

Chapter 9 Review: Plate Tectonics Data from the middle of the

Atlantic ocean indicated that a large undersea mountain range existed where we thought should have been deep ocean waters.

As scientists mapped the ocean floor, they found long, curved valleys along the edges of some ocean basins called deep ocean trenches.

Trenches form the deepest parts of Earth’s oceans. Most trenches occur around the edges of the Pacific ocean although trenches occur in the Indian and Atlantic ocean as well.

Chapter 9 Review: Plate Tectonics Mid Ocean Ridges: By the late

1950s, scientists had constructed a more complete map of the earth’s ocean floor.

This map showed a mid-Atlantic ridge; a long mountain chain that ran the length of the Atlantic ocean.

Earth’s mid-ocean ridge system forms the longest feature on Earth’s surface.

The system winds for more than 70,000 kilometers through all the major ocean basins like the seam on a baseball.

Chapter 9 Review: Plate TectonicsProcess of Sea-Floor Spreading:

In the process of sea-floor spreading, new ocean floor forms along Earth’s mid-ocean ridges, slowly moves outward across ocean basins, and finally sinks back into the mantle beneath deep-ocean trenches.

During sea-floor spreading, new oceanic lithosphere is formed, and the ocean floor gets wider.

Fractures along the central valley of a mid-ocean ridge fill with magma that wells up from within the hot mantle below.

Chapter 9 Review: Plate Tectonics As new ocean floor is added along

mid-ocean ridges, the older ocean floor moves outward and away from the ridge on both sides.

Rates on sea-floor spreading average about 5 centimeters per year; slow on human time scale but fast enough that all of Earth’s oceans could have been formed within the last 200 million years.

Although new ocean floor is constantly being added at the mid-ocean ridges, our planet is not growing larger. Earth’s total surface remains the same.

To accommodate newly created lithosphere, older portions of the ocean floor return to the mantle in a process we call subduction.

Chapter 9 Review: Plate Tectonics Evidence for sea-floor

spreading includes

Magnetic strips in ocean-floor rock

Earthquake patterns Measurements of the ages of

ocean floor rocks

Chapter 9 Review: Plate Tectonics Geophysicists learned that Earth’s magnetic

field occasionally reverses polarity.

The North magnetic pole becomes the South magnetic pole, and visa versa.

Scientists graphed these reversals over millions of years.

When Earth’s magnetic field lines up in the same direction as the present magnetic field, it is said to have normal polarity.

When the magnetic field lines up in the opposite direction, it is said to have reverse polarity.

As certain rocks form, they acquire the polarity of the Earth’s magnetic field at the time they formed.

These rocks posses paleomagnetism.

Once the rock is formed, it’s polarity remains frozen unless the rock is heated above a certain level.

Chapter 9 Review: Plate Tectonics Scientists study the magnetic polarity of

areas of the sea floor by towing instruments called magnetometers across the ocean floor.

The data they collected revealed a series of alternating strips of magnetic rock on the ocean floor.

Strips of rock with normal polarity alternated with strips with reversed polarity.

Scientists inferred from this that as new basalt formed along ocean ridges, it becomes magnetized according to the polarity of the earth at that moment.

These matching patterns of alternating magnetic strips provide evidence of sea-floor spreading.

Chapter 9 Review: Plate Tectonics More evidence for sea-floor spreading

came from studies of the depth at which certain earthquakes occur.

Scientists knew that there were many earthquakes in subduction zones.

Scientists discovered that shallow-focus earthquakes tend to occur in and around a trench.

As we see in the chart at right, the deeper the earthquake foci, the farther away it is from the ocean trench.

(green indicates shallow foci; yellow medium depth foci, red indicates deep foci)

Scientists considered the pattern of earthquakes in relation to sea-floor spreading. This data convinced scientists that slabs of ocean floor return to the mantle in subduction zones.

Chapter 9 Review: Plate Tectonics Drilling into sediment on the

ocean floor and the crust beneath it provides some of the best evidence of sea-floor spreading.

Data from drilling confirmed that the sea-floor spreading hypothesis was correct.

The ocean floor is youngest along the central valley of the mid-ocean ridge. The ocean floor is oldest in Subduction zones or near the edges of continents far from the ridge.

Chapter 9 Review: Plate Tectonics Canadian geologist J. Wilson

came up with a theory that led to a revolution in geology. Wilson suggested that the lithosphere was broken into several huge pieces, called plates. Deep faults, like cracks in an eggshell, separate the different plates.

In the theory of plate tectonics, Earth’s lithospheric plates move slowly relative to one another, driven by convection currents in the mantle.

Chapter 9 Review: Plate Tectonics According to Wilson, convection

currents within Earth drive plate motion.

Hot material deep within the mantle moves upward by convection. At the same time, cooler, denser slabs of oceanic lithosphere sink into the mantle.

Plate motion averages about 5 centimeters per year; about as fast as your fingernails grow.

The results of plate motion include earthquakes, volcanoes, and mountain building.

Chapter 9 Review: Plate Tectonics Interactions among different plates

happen along plate boundaries.

Three types of plate boundaries exist: Divergent boundaries are found when two of Earth’s plates move apart. Oceanic lithosphere is created where divergent boundaries occur and sea-floor spreading happens.

Convergent boundaries happen when two plates move together towards each other. Lithosphere can be destroyed at convergent boundaries when oceanic lithosphere sinks into the mantle during subduction.

Transform boundaries occur when two plates grind past each other. Along transform boundaries, lithosphere is neither created nor is it lost.

Chapter 9 Review: Plate Tectonics Plates may shrink or grow depending on

the locations of the convergent and divergent boundaries.

Slowly over time some plates grow over others pushing them under through subduction when borders converge. Some plates slowly expand while others shrink due to this.

Along divergent boundaries, plates move apart. Because they are the areas where sea-floor spreading begins, divergent boundaries are called spreading centers.

We think of these plate boundaries as constructive plate margins because this is where new oceanic lithosphere is produced.

Chapter 9 Review: Plate Tectonics When a spreading center forms along

land, the process can literally split a continent apart.

The process begins when forces of plate motion begin to stretch the lithosphere. At the same time, plumes of hot rock (lava) rise from the mantle. The rising plumes bend the crust upward, weakening and fracturing it.

The fractures allow magma to reach the surface. The result is a new floor of a rift valley

Examples of rift valleys include the Rhine Valley in Europe and the Great Rift valley in East Africa. The Great rift valley in East Africa may represent the first step in the process of the breakup of Africa. This process may take millions of years

Chapter 9 Review: Plate Tectonics At convergent boundaries, plates

collide and interact, producing features including trenches, volcanoes and mountain ranges.

Along convergent boundaries, older portions of oceanic plates return to the mantle. As a result, Earth’s total surface remains the same, even though new lithosphere is constantly being added at mid-ocean ridges.

Because lithosphere is destroyed at convergent boundaries, they are also called “destructive plate margins”.

As two plates slowly converge, the leading edge of one plate is bent downwards, allowing it to slide beneath the other plate. We call this sliding under the other plate subduction.

Chapter 9 Review: Plate Tectonics The type of lithosphere involved

and the forces acting upon it determine what happens at convergent boundaries.

Convergent boundaries can form between two pieces of oceanic lithosphere, between oceanic lithosphere and

continental lithosphere, and between two pieces of

continental lithosphere.

Chapter 9 Review: Plate Tectonics When the leading edge of

continental lithosphere converges with oceanic lithosphere, the less dense continental lithosphere remains floating. The denser oceanic slab sinks into the asthenosphere.

When a descending plate reaches about 100 to 150 kilometers below the Earth, some of the asthenosphere above the descending plate melts.

The newly formed magma, being less dense than the rock mantle, rises. Eventually some of it reaches the surface and becomes volcanic activity

Chapter 9 Review: Plate Tectonics A continental volcanic arc is a range of

volcanic mountains produced in part by the subduction of oceanic lithosphere.

The volcanoes of the Andes in South America are the product of magma formed during subduction of the Nazca Plate

When two oceanic slabs converge, one descends beneath the other. This causes volcanic activity similar to what happens in oceanic-continental.

The volcanoes form on the ocean floor instead of on land, however. If this activity continues, it will build a chain of volcanic structures that become islands.

This newly formed land we call a volcanic island arc.

Chapter 9 Review: Plate Tectonics When oceanic lithosphere is subducted

beneath continental lithosphere, a continental volcanic arc develops along the margin of the continent.

However, if the subduction plate also contains continental lithosphere, the subduction eventually brings two continents together.

The result is a collision between the two continental plates. Since neither sinks below the other, collision results and mountains form.

Chapter 9 Review: Plate Tectonics Before continents collide they are

separated by an ocean basin. As the continents move toward each other, the sea-floor between them is subducted beneath one of the plates.

When the continents collide, the collision folds and deforms the sediments along the margin as if they were placed in a giant vice.

A new mountain range forms that is composed of deformed and metamorphosized sedimentary rocks.

This kind of collision occurred when India rammed into Asia and produced the Himalayas.

Mountain systems such as the Alps, the Appalachians, and the Urals were formed by such a process.

Chapter 9 Review: Plate Tectonics The third type of plate boundary system

is the transform fault boundary. Pieces of lithosphere move past each other horizontally along a transform fault boundary.

At a transform fault boundary, plates grind against each other without destroying or creating lithosphere.

Most transform faults join two sections of mid-ocean ridge. These faults occur about every 100 kilometers along the ridge axis.

Chapter 9 Review: Plate Tectonics

Active transfer faults lie between the two offset ridge segments.

The seafloor produced at one ridge axis moves in a direction opposite to that of the seafloor produced at the next ridge segment.

Between the ridge segments, these slabs of oceanic crust are sliding past each other along a transfer fault.