Download Earth Science Chapter 17: Plate Tectonics Chapter Overview

Earth Science
Chapter 17: Plate Tectonics
Chapter Overview
Section 1: Drifting Continents
During the average human lifespan, Earth’s surface appears to be unchanging. On the geologic time
scale, however, Earth’s surface is changing at rates almost too great to imagine. For example, South
America is moving away from Africa at 2 to 3 centimeters per year and Mount Everest is slowly rising.
Just beyond the perception of our physical senses the Earth is constantly changing.
1. Early Observations
Some of the first people to consider the idea of moving landmasses were early mapmakers. In
the late 1500s, a Dutch mapmaker noticed the apparent fit of continents on either side of the
Atlantic Ocean. He proposed that the Americas had been separated from Europe and Africa by
floods and earthquakes. In the late 1800s an Austrian geologist hypothesized that the present
southern continents had once been joined as a single landmass he named Gondwanaland. In
1912, Alfred Wegener presented the first serious, scientific hypothesis about continental
movement.
2. Continental Drift
Wegener called his hypothesis continental drift, which proposed that Earth’s continents had
once been joined as a single landmass. He called this supercontinent Pangaea, a Greek word
that means “all the earth”. Wegener proposed that Pangaea began to break apart around 200
million years ago. Wegener collected and organized rock, fossil, and climatic data to support his
hypothesis
• Evidence from Rock Formation
When Pangaea began to break apart large geologic features, like mountain ranges, would
have fractured as the continents separated. Wegener found that the Appalachians in the
United States and mountains in Greenland and Europe shared similar features
• Evidence from Fossils
Similar fossils of several different animals and plants that once lived on land had been found
on widely separated continents. This implies that at some point in time these continents were
joined together. Also, the fossilized remains of an aquatic reptile that lived in fresh water were
found on different continents. Wegener reasoned that these reptiles could swim across the
oceans to separate continents.
• Ancient Climatic Evidence
Coal deposits found in Antarctica indicate that at one time the climate on that continent was
warm enough to support large swampy areas and extensive vegetation. Evidence of glacial
deposits on Africa, India, Australia, and South America suggests that these landmasses were
once closer to the South Pole.
3. A Rejected Hypothesis
Wegener’s hypothesis had two major flaws that prevented it form being accepted. First, he could
not explain what was causing the continents to move; and second, he could not how they were
moving.
Section 2: Seafloor Spreading
Until the mid-1900s, most people thought that the ocean floor was essentially flat. Advances in
technology in the 1940s and 1950s proved this idea, and others about the nature of the see floor,
wrong.
1. Mapping the Ocean Floor
• One advance that allowed scientists to study the ocean floor was the development of echosounding methods, including sonar. Sonar uses sound waves to measure water depth by
timing how long it takes for a sound to travel to the ocean floor and back to the surface.
• Another technological advance that was used to study the ocean floor was the
magnetometer. A magnetometer is a device that can detect small changes in magnetic
fields.
2. Ocean Floor Topography
The maps made from the data collected by sonar and magnetometers revealed a sea floor
topography that included mountain ranges that are the longest on the planet, and trenches, some
almost six times as deep as the Grand Canyon. Scientists were unable to explain why these
features existed.
3. Ocean Rocks and Sediments
Scientists also collected samples of deep-sea rocks and sediments. Analysis of these sediments
produced two important discoveries. First, the ages of the rocks that make up the seafloor vary in
different places, and these variations change in a predictable way. Rock samples taken near
seafloor ridges were younger than samples taken from areas near deep-sea trenches. In fact, the
farther from the ridge, the older the rock. Scientists also discovered that the oldest rock on the
seafloor is only 180 million years old, compared to rocks 3.8 billion years old on the continents.
The second discovery was that the thickness of the sediment deposited on the ocean floor was
less than expected, only a few hundred meters thick. On the continents, however, sediment
layers could be from a few kilometers to 20 kilometers thick.
4. Magnetism
Scientists know that rocks containing iron-bearing minerals provide a record of Earth’s magnetic
field. The study of this magnetic record is called paleomagnetism.
• The Geomagnetic Time scale
Studies of continental basalt flows in the early 1960s revealed a pattern of magnetic reversals
over geologic time. A magnetic reversal is a change in Earth’s magnetic field. Comparing
the magnetic data of the seafloor with that of the continent, scientists found an interesting. In
some places, the magnetic field strength was greater than normal, while in other places it
was lower than normal. The higher than normal was explained by the combining of the
magnetic fields of the seafloor and Earth’s current magnetic field while the lower than normal
was the result of a magnetic reversal of the field on the seafloor
• Magnetic Symmetry
The positive and negative areas form a series of stripes that were parallel to ocean ridges,
and the data matched the pattern of magnetic reversals found in basalt flows on the land. By
matching the patterns on the seafloor with the known pattern of reversals on land, scientists
were able to determine the age of the ocean floor. They were able to construct isochron
maps. Isochrons are lines on a map that connect points that have the same age. These
maps revealed that the youngest seafloor was closest to the ocean ridges while the oldest
seafloor rock were close to ocean trenches
5. Seafloor Spreading
An American scientist proposed a theory that could explain the topographic, age, and magnetic
data from the seafloor. This theory, called seafloor spreading, states that new ocean crust is
formed at ocean ridges and destroyed at deep-sea trenches.
• The theory of sea floor spreading explained how landmasses could move long distances over
time and supported Wegener’s hypothesis of continental drift
Section 3: Theory of Plate Tectonics
The theory of plate tectonics states that Earth’s crust and rigid upper mantle are broken into
enormous slabs called plates. There are a dozen or so major plates and several smaller ones. These
tectonic plates move in different directions at different rates over Earth’s surface
1. Plate Boundaries
Tectonic plates interact at places called plate boundaries. At some boundaries the plates come
together, or converge. At others, plates move away from one another, or diverge. At the third type
of plate boundary the plates move horizontally past one another. Each type of boundary has
certain geologic features and processes associated with it
• Divergent Boundaries
Places where two tectonic plates are moving apart are called divergent boundaries. Most
divergent boundaries are found on the seafloor where they form ocean ridges. The formation
of new ocean crust at most divergent boundaries accounts for the high heat flow, volcanism,
and earthquakes associated with these boundaries. Occasionally divergent boundaries form
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on continents. When the continental crust begins to separate, the stretched crust forms a
long, narrow depression called a rift valley
Convergent Boundaries
Places where two tectonic plates are moving toward each other are convergent boundaries.
There are three types of convergent boundaries based on the type of crust involved. The
types are: oceanic crust converging with oceanic crust, oceanic crust converging with
continental crust, and continental crust converging with continental crust. In an oceanicoceanic boundary, one of the plates descends beneath the other in a process called
subduction. This process creates a deep oceanic trench and the formation of island arcs
that parallel the trench.
Transform Boundaries
A place where two plates slide horizontally past each other is a transform boundary. At
transform boundaries the crust is deformed and fractured. Transform boundaries are
characterized by long faults, sometimes hundreds of kilometers in length. Movement along
these faults is responsible for many of the earthquakes
Section 4: Causes of Plate Motion
1. Convection
The transfer of heated material is called convection. The heating of matter causes it to expand
and to decrease in density. The warm matter then rises because of buoyancy. As the matter rises
it cools, contracts, becomes more dense, and then sinks because of gravity. This pattern of up
and down motion is called convection. Convection currents in the mantle are thought to be the
driving mechanism of tectonic plate motion.
2. Push and Pull
During the formation of an ocean ridge forces in the mantle cause the asthenosphere to rise. The
weight of the uplifted ridge is thought to push an oceanic plate toward the trench formed at the
subduction zone in a process called ridge push. In addition to ridge push, the horizontal flow at
the top of a convection current could create drag on the lithosphere and thereby contribute to
plate motion. A sinking region of a mantle convection current could suck an oceanic plate
downward onto a subduction zone. The weight of a subducting plate helps pull the trailing
lithospheric plate into the subduction zone in a process called slab pull