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Chapter 17: Plate Tectonics

Chapter Worksheet

Ch. 17.1 Drifting Continents

The shape and geology of the continents suggests that they were once joined together.

Early Observations

On the geologic time scale Earth's surface has changed dramatically. Cartographers were among the first to notice this.
In late 1500's Abraham Ortielius noticed the apparent fit of continents on either side of the Atlantic Ocean. He proposed that North America and South America had been separated from Europe and Africa by earthquakes and floods.

First scientific hypothesis of moving continents was proposed in the early 1900's. In 1912 Alfred Wegener presented his ideas about continental movement.

Continental Drift

Wegener proposed continental drift. Continental drift- proposed that Earth's continents had once been joined as a single landmass that broke apart and sent the continents adrift.
Pangea- means all the Earth, a super continent that began to break apart about 200 mya.

Wegener was first to base his ideas on more than just the puzzle-like fit of continental coastlines. He also collected and organized rock, climatic, and fossil data to support his hypothesis.

Evidence From Rock Formations

Wegener reasoned that as rock formations broke apart there should be areas of similar rock types on opposite sides of the Atlantic Ocean. He observed similar rock layers in the Appalachians, Greenland, and Europe.

Evidence From Fossils

Fossils of similar land-dwelling organisms (both plants and animals) were found on widely separated continents. Wegener reasoned that they could not have swum the distances that now exist between continents. (Lystrosaurs, Cynognathus)
Fossils of a fresh water reptile found on both continents could not have crossed the oceans.

Climatic Evidence

Wegener used the fossil Glossopteris , a seed fern that resembled low shrubs, as further support. The plant grew in temperate climates, so places that the fossils had been found were once closer to the equator. This led him to conclude that the rocks containing these fossils had once been joined.

Coal Deposits

Coal forms from the compaction and decomposition of accumulations of ancient swamp plants that grew in warm, wet regions. Coal deposits are found in Antarctica and other high latitude locations. Wegener reasoned that the existence these coal beds indicated that Antarctica must have been much closer to the equator in the geologic past.

Glacial Deposits

Glacial deposits found in Africa, India, Australia, and South America suggested to Wegener that these areas were once covered by a thick ice cap. These regions now exist in areas too warm for ice caps to form.

Wegener proposed that these areas were once located near the South Pole.
He suggested either the South Pole had shifted its position, or these landmasses had once been closer to the South Pole.

A Rejected Notion

Although Wegener compile a large collection of data, his continental drift hypothesis was not accepted by the scientific community during his lifetime.

The hypothesis had two major flaws:

  1. It did not explain what force could be strong enough to push the large masses over such great distances.
  2. How could the land masses move through the solid ocean floor.

In 1960's new technology revealed more evidence about how continents move. Advances in seafloor mapping and understanding Earth's magnetic field provided the necessary evidence to show how continents move, and the source of the forces involved.

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Ch. 17.2 Seafloor Spreading

Oceanic crust forms at ocean ridges and becomes part of the seafloor.

Mapping the Ocean Floor

Advances in technology during th e1940's and 1950's changed many old, accepted ideas. One device is the magnetometer.
Magnetometer- a device that can detect small changes in magnetic fields. Can be towed behind a ship to record the magnetic field generated by ocean floor rocks.

Echo-sounding methods also helped in the mapping of the ocean floor.
Sonar uses sound waves to measure distance by measuring the time it takes for sound waves sent from the ship to bounce off the seafloor and return to the ship. Sonar allowed us to map the topography of the ocean floor.

Ocean-Floor Topography

The maps created by sonar and magnetometers revealed underwater mountain chains called ocean ridges that run along the ocean floors. They form the longest continuous mountain range on Earth.
Volcanism and earthquakes are common along the ridges.

Mapping also revealed deep-sea trenches; a narrow, elongated depression in the seafloor.
Deepest trench, the Mariana Trench is in the Pacific Ocean and is more than 11 km deep.

These two features confused geologists for many years.

Ocean Rocks and Sediments

Samples of deep-sea sediments and the underlying oceanic crust revealed that the ages of the rocks that make up the seafloor vary across the ocean floor, these variations are predictable.
The samples showed that the age of oceanic crust consistently increases with distance from a ridge. This pattern was symmetric and parallel to the ocean ridges.
Scientists also learned that the sea floor is geologically young, about 180 million years old compared to continental rocks that are at least 4 billion years old.

Scientists also learned that the ocean sediments is a relatively thin layer compared to the continental sediment layers. Additionally, the layer is thicker with increasing distance from the ocean ridge.
The pattern of thickness across the ocean floor was symmetrical across the ocean ridges.


Earth has a magnetic field generated by the flow of molten iron in the outer core.
Magnetic reversal- when the flow in the outer core changes and Earth's magnetic field changes direction.
Magnetic reversals have occurred many times in Earth's history.

Magnetic Polarity Time Scale

Paleomagnetism- the study of the history of Earth's magnetic field.
When lava solidifies, iron-bearing minerals such as magnetite crystallize, as they do they behave like tiny compasses and align with Earth's magnetic field.
Scientists have constructed a magnetic polarity time scale.

Magnetic Symmetry

Oceanic crust is mostly basaltic rock, which contains large amounts of iron-bearing minerals of volcanic origin.
Regions of ocean floor with normal and reverse polarity forms a series of stripes across the floor parallel to the ocean ridges.
Ages and widths of the stripes were the same on both sides of the ridges.

Matching the ocean floor patterns with known patterns of reversal on land, scientists were able to determine the age of the ocean floor to create a isochron.
Isochron- an imaginary line on a map that shows point that have the same age; or formed at the same time.

Seafloor Spreading

Using topographic, sedimentary, and paleomagnetic data from the seafloor, seafloor spreading was proposed.
Seafloor spreading- the theory that explains how new oceanic crust is formed at ocean ridges, slowly moved away from ocean ridges, and destroyed at the deep-sea trenches.

Seafloor spreading:

  1. Magma is forced toward the surface of the crust along the ridge.
  2. Two sides of the ridge spread apart.
  3. Gap fills with magma.
  4. Magma solidifies, adding a small amount of ocean floor which slowly moves away from the ridge.

Seafloor spreading was the missing piece that Wegener needed to complete his model of continental drift.
Continents are not pushing through the ocean crust, but are passengers while ocean crust moves away from the ocean ridges.

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Ch. 17.3 Plate Boundaries

Volcanoes, mountains, and deep-sea trenches form at the boundaries between the plates.

Theory of Plate Tectonics

Seafloor spreading suggested that continental and oceanic crust move as enormous slabs.
Tectonic plates- huge pieces of crust and rigid upper mantle that fit together at their edges to cover Earth's surface.

Eight large plates and several smaller ones that move only a few centimeters each year.
Plates move in different directions and at different rates relative to one another interacting with one another at their boundaries.

  • Divergent boundary- occurs where tectonic plates move away from each other.
  • Convergent boundary- occurs where tectonic plates move toward each other.
  • Transform boundary- occurs where plates move horizontally past each other.

Divergent Boundaries

Most divergent boundaries are found along the seafloor in rift valleys.
The process of seafloor spreading along a divergent boundary can cause an ocean basin to expand.
Most divergent boundaries form ridges on the ocean floor.
When continental crust begins to separate the stretched crust forms a long, narrow depression called a rift valley.

Convergent Boundaries

Convergent boundaries- two tectonic plates are moving toward each other.
Subduction- when two plates collide, the denser plate eventually descends below the other, less-dense plate.
The differences in density of the crustal material affects how they converge.


Occurs when one oceanic plate, which is denser as a result of cooling, descends below another oceanic plate creating an ocean trench.
Molten material often forms an arc of volcanic islands that parallel the trench.


A subduction zone in which the denser oceanic plate converges with the less dense continental plate.
Produces a trench and volcanic arch the results in a chain of volcanoes along the edge of the continental plate creating a mountain range with many volcanoes.


Occurs long after a Oceanic crust has converged and been subducted beneath the a continental crust. The oceanic crust drags the continental behind it as it is subducted.
The continental plates then collide and become crumpled, folded, and uplifted forming vast mountain ranges.

Transform Boundaries

Transform boundaries- a region where two plates slide horizontally past each other.
Characterized by long faults and shallow earthquakes. The crust is only deformed or fractured.

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Ch. 17.4 Causes of Plate Motions

Convective currents in the mantle cause plate motions.


Scientists think that large-scale motion in the mantle, Earth's interior between the crust and the core, is the mechanism that drives the movement of tectonic plates.

Convection Currents

The cooling of matter causes it to contract slightly and increase in density. The cooled matter then sinks as a result of gravity. Warmed matter is displaced and force to rise.
This up-and-down flow produces a pattern of motion called a convection current.

Convection in the Mantle

The part of the mantle that is too cold and stiff to flow lies beneath the crust and is attached to it, moving as a part of tectonic plates.

Cooler mantle material is denser than hot mantle material. It is thought that these convection currents are set in motion by subducting slabs.

Plate Movement

The rising material in the convection current spreads out as it reaches the upper mantle and causes both upward and sideways forces.
These forces lift and split the lithosphere at divergent plate boundaries.
As the plates separate, material rising from the mantle supplies the magma that hardens to form new ocean crust.
Sinking force pulls tectonic plates downward at convergent boundaries.

Push and Pull

As oceanic crust cools and moves away from a divergent boundary, it becomes denser and sinks compared to the newer, less-dense oceanic crust.
As the older portion of the seafloor sinks, the weight of th uplifted ridge is thought to push the oceanic plate toward the trench at the subduction zone; known as ridge push.

Slab pull- the weight of the relatively cool, dense subducting plate pulls the trailing slab into the subduction zone.
Slab pull is thought to be at least twice as important as ridge push in moving an oceanic plate away from an ocean ridge.

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Page last updated April 3, 2017.