PLATE TECTONICS

Chapter 17

Earth Science

17.1 Drifting ContinentsAlthough it is difficult to imagine, the Earth’s

surface is constantly changing. – South America is moving away from Africa at a rate of

2-3 cm per year. – What makes up the Hawaiian Islands are migrating

northwest at a rate of 8-9 cm per year! – In addition to this, Earth’s highest point (Mt. Everest)

continues to slowly rise.

When continents slowly move to their current positions, we call this continental drift.

Early Observations

Early mapmakers first considered the idea of moving landmasses because they found many of the continents had matching coastlines.

• 1500’s - Abraham Ortelius (Dutch) noticed fit of continents surrounding Atlantic Ocean.Incorrectly proposed that N America and S America were separated from Europe and Africa by earthquakes and floods.

• 1800’s – Eduard Suess (Austrian) proposed southern continents had once been joined by a single landmass named Gondwanaland.

• 1900’s – Alfred Wegener (German) hypothesized the theory of continental drift.

Continental Drift

The theory of continental drift proposed that Earth’s continents were once joined as a single landmass, Pangaea.

• Pangaea is a Greek word that means “All Earth.”

• Pangaea is proposed to have begun breaking apart approximately 200 million years ago.

• Continents then have slowly moved to their current positions.

Beginning with a view of Earth with the continents in their present positions, the continents move back in time to reunite as Pangaea, are labeled, and locations of fossil evidence that Wegener used to argue in

favor of continental drift are added.

Evidence of Continental DriftRock Formations

Similar bands of rocks were found oceans apart.

For example, parts of the Appalachian Mountains share similar features with rocks found in Greenland and Europe.

Evidence - FossilsSimilar fossils of several different animals and plants that once lived on

land had been found on widely separated continents. Support is justified as land animals could not have possibly swam across large bodies of water.

Glossopteris (fossil fern) representing continental drift due to finding these on several continents despite climate changes.

Conclusion is that rocks joining these fossils had to once be joined.

Evidence - Climate

Evidence of large climate changes were found on some continents. For example, coal deposits were found on Antarctica. Coal requires temperate, wet climate and wet, spongy land. In addition, glacial deposits were found in Africa, India, Australia and South America. The deposits were estimated to be 290 million years old, which signifies that these lands were once covered by ice.

Flaws to the hypothesis of Continental Drift

1. Wegener could not satisfactorily explain what was actually causing the continents to move.

Wegener suggested the rotation of the Earth; however, physicists were able to show this force was not great enough to move continents.

2. Wegener could not satisfactorily explain how the continents were moving.

Wegener proposed that landmasses were plowing through the ocean floor; however, scientists were able to argue that there would be evidence of fracture on the ocean floor… which there was not.

17.2 Seafloor Spreading

Most people originally believed that the ocean floor was flat and unchanging. Beliefs also indicated that the ocean floor was older than the continental crust surrounding it.

Technology advancements during the 1940’s and 1950’s proved all of these misconceptions wrong.

Technology Assistance

To detail specifics of the ocean floor, scientists discovered that they needed to study what was going on within the ocean floor. To do this, many tools were developed to study the rocks and sediments on the ocean floor.

These tools included the following:• Sonar – an echo sounding technique used to map topography.

Travel times of waves were used to determine distances to the ocean floor.

• Magnetometer – Device used to detect small changes in magnetic fields.

These devices revealed deep-sea trenches and mid-ocean ridges active in volcanoes and earthquakes

By testing rocks and sediments from the seafloor, they found those near the mid-ocean ridges are younger and those near deep-sea trenches are older.

Oceanic crust rocks are much younger than continental crust rocks

Paleomagnetism: study of the magnetic record in earth’s rocks.

Iron bearing minerals in rocks become oriented to earth’s magnetic field during rock formation.

Magnetic field reversal occurs over time. The magnetic pattern is symmetric around the mid-ocean ridge.

Seafloor SpreadingUpon new findings of the ocean floor (to include

topographic, sedimentary, age and magnetic data) an American scientist came up with a theory to explain all prior observations.

Harry Hess (American) – indicated that magma rises and hardens to form new crust, which then becomes a part of the ocean floor. Each cycle of spreading and intrusion results in the formation of another small section of ocean floor, which slowly moves away from the mid-ocean ridge.

As result, continents are not pushing through ocean crust but are moving with the ocean crust as it pushes away from the ridge

Seafloor Spreading Animation

Magma rises and hardens to form new crust, which then becomes a part of the ocean floor. Each cycle of spreading and intrusion results in the formation of another small section of ocean floor, which slowly moves away from the mid-

ocean ridge.

17.3 Theory of Plate Tectonics

The Earth’s crust and rigid upper mantle are broken into large slabs of rock called plates, which move in different directions at different speeds. These plates interact with one another at plate boundaries.

Each type of boundary has specific characteristics and processes associated with it.

Divergent BoundariesAt divergent boundaries, two tectonic plates

are moving apart from one another. Most are found on the seafloor, where they form ocean ridges.

The formation of new ocean crust at these boundaries accounts for high heat flow, which results in volcanism and earthquakes.

Some create rift valley’s, which occur on land and create long, narrow depressions.

Example of Divergent Plate Boundary

Askja Central Volcano is situated at the divergent plate boundary in the North Volcanic Zone in Iceland

Convergent BoundariesTwo tectonic plates come together at a

convergent boundary.Convergent boundaries are classified

according to the type of crust involved within the specific boundary.

Three types of convergent boundaries:1. Continental / Continental2. Oceanic / Oceanic3. Continental / Oceanic

Convergent boundaries are associated with trenches, island arcs, and very large, folded mountains.

Example of Convergent Plate Boundary

The Himalayas is an example of a Convergent plate boundary

Transform BoundariesTwo tectonic plates slide horizontally past one

another at a transform boundary.

At transform plate boundaries, the Earth’s crust is only deformed or fractured (not destroyed.)

Characterized by long faults and shallow earthquakes

Most are offset sections of ocean ridges, rarely are they found on continents. San Andreas Fault is best-known exception. This fault is responsible

for nearly every earthquake to strike California.

Example of Transform Plate Boundary

The San Andreas Fault in Southern CA, is the best-known exception of a transform boundary and is responsible for

most earthquakes to strike California.

17.4 Causes of Plate MotionsThe transfer of thermal energy by the movement of

heated matter is called convection. The heating of matter causes it to expand and decrease in density. As it warms, it rises (buoyancy). The cooler part then sinks (gravity). This up and down pattern is a convection current.

Convection currents are thought to be the driving force of plate tectonics.

In the mantle, convection currents are set in motion by the transfer of energy between the Earth’s hot interior and its cooler exterior.

Ridge Push / Slab Pull

The upper part of the mantle, the asthenosphere, can flow like a soft plastic. This flowing movement drives plate movement.

During formation of ocean ridges, mantle forces cause the asthenosphere to rise. This uplifting is thought to push oceanic plates toward trenches (ridge push)

At subduction zones, a sinking region of a mantle convection current could suck an oceanic plate downward into a subduction zone. The weight of a subduction plane helps pull the trailing lithosphere into the subduction zone (slab pull)

Unanswered QuestionsMost scientists agree that convection currents in

the mantle are related to the movement of plates. There are still questions regarding how the currents originate:

1. Are mantle convections currents permanent features?

2. Do they shift their positions through geologic time?

3. How does a convection current start?

4. What causes the movement to stop?

5. Does convection only take place in the upper mantle, or do they occur in the lower mantle as well?

6. Does the convection current cause subduction or vice versa?

7. What causes an upward convection current to form beneath a continent?