Objectives • Describe one piece of early evidence that led people to suggest that Earth’s continents may have once been joined. Drifting Continents • Discuss evidence of continental drift. • Explain why continental drift was not accepted when it was first proposed. – continental drift – Pangaea Vocabulary
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Objectives
• Describe one piece of early evidence that led people to suggest that Earth’s continents may have once been joined.
Drifting Continents
• Discuss evidence of continental drift.
• Explain why continental drift was not accepted when it was first proposed.
• In the late 1500s, Abraham Ortelius, a Dutch mapmaker, noticed the apparent fit of continents on either side of the Atlantic Ocean.
Drifting Continents
• In the late 1800s, Eduard Suess, an Austrian geologist, hypothesized that the present southern continents had once been joined as a single landmass that he named Gondwanaland.
• The first time that the idea of moving continents was proposed as a serious scientific hypothesis was in 1912 by a German scientist named Alfred Wegener.
• Wegener’s hypothesis, continental drift, proposed that Earth’s continents had once been joined as a single landmass.
Drifting Continents
• Wegener proposed that Pangaea began to break apart about 200 million years ago and that the continents had continued to slowly move to their present positions.
• Pangaea, a Greek word that means “all the earth,” refers to the combined landmass.
– Similar fossils of several different animals and plants that once lived on land had been found on widely separated continents.
– The ages of different fossils predated Wegener’s time frame for the breakup of Pangaea.
– Fossils of Glossopteris, a seed fern that resembled low shrubs, have been found on many continents, indicating that the areas had a single climate that was close to the equator.
• In the early 1900s, most scientists rejected Wegener’s hypothesis of continental drift.
Drifting Continents
• Two unanswered questions—what forces could move continents and how continents could move without shattering—were the main reasons that the hypothesis of continental drift was rejected.
1. How did Pangaea differ from the present layout of the continents?
Drifting Continents
During Pangaea, the continents were joined and essentially located in one hemisphere from the north to the south poles. Presently, the continents have separated and are distributed in both the eastern and western hemisphere, with the majority of landmass in the northern hemisphere.
• Analysis of deep-sea rocks and sediments produced two important discoveries.
Seafloor Spreading
1. The ages of the rocks that make up the seafloor vary in different places, and that the age of oceanic crust consistently increases with distance from a ridge.
– The oldest part of the seafloor is geologically young at about 180 million years old.
2. The thickness of ocean-floor sediment was, in general, much less than expected and that the thickness of the sediments increases with distance from an ocean ridge.
– The magnetic data collected from the ocean floor matched the pattern of magnetic reversals that had been found in basalt flows on land.
– From this match, scientists were able to determine the age of the ocean floor from a magnetic recording and quickly create isochron maps of the ocean floor.
– An isochron is a line on a map that connects points that have the same age.
– Each cycle of spreading and the intrusion of magma results in the formation of another small section of ocean floor, which slowly moves away from the ridge.
– When the magma hardens, a small amount of new ocean floor is added to Earth’s surface.
– Subduction occurs when one of the two converging plates descends beneath the other.
– A subduction zone forms when one oceanic plate, which has become denser as a result of cooling, descends below another plate creating a deep-sea trench.
– The subducted plate descends into the mantle and melts.
– Some of the magma forms new oceanic crust at the ridge or is forced back to the surface, forming an arc of volcanic islands that parallel the trench.
– When an oceanic plate converges with a continental plate, the denser oceanic plate is subducted.
– Oceanic-continental convergence produces a trench and a series of volcanoes along the edge of the continental plate.
– Two continental plates collide when an ocean basin between converging oceanic and continental plates is entirely subducted.
– Because continental rocks are too buoyant to be forced into the mantle, the colliding edges of the continents are crumpled and uplifted to form a mountain range.
2. What happens to an oceanic plate once it is subducted? What is created with the material?
Theory of Plate Tectonics
The subducted plate melts in the mantle. Some of the resulting magma is forced to the surface creating a series of volcanoes that are parallel to the subduction zone. Some of the magma is recycled into new oceanic crust at the ridge.
3. Why does uplift occur when two continental plates converge? Give an example of this process.
The rocks that make up a continental plate are too buoyant to be forced into the mantle. As a result, the colliding edges of the continents are crumpled and uplifted to form a mountain range such as the Himalayas.
– During the formation of an ocean ridge, forces in the mantle cause the asthenosphere to rise.
– In a process called ridge push, the weight of the uplifted ridge is thought to push an oceanic plate toward the trench formed at the subduction zone.
– 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 similar set of questions surround the formation of divergent continental plate boundaries.
– One hypothesis is that large continental masses ultimately cause their own breakup by acting as insulating blankets.
– The underlying mantle then becomes warmer and causes the upward leg of a convection current to develop, which eventually causes the continent to split.
1. How might a convection current cause a divergent boundary?
Causes of Plate Motions
An upward flow in the mantle causes the asthenosphere to rise. This force causes the lithosphere to rise and split. As the plates separate, material rising from the mantle supplies the magma that hardens to form new ocean crust.
2. How might a convection current cause a convergent boundary?
Causes of Plate Motions
A sinking region of a mantle convection current could suck an oceanic plate downward into a subduction zone. The weight of a subducting plate helps pull the trailing lithosphere into the subduction zone in a process called slab pull.
3. How are slab push and slab pull related processes?
Slab pull is thought to be the most important process driving tectonic plate motions. The material that is subducted through slab pull enters the convection current that drives slab push.
• The matching coastlines of continents on opposite sides of the Atlantic Ocean suggest that the continents were once joined.
• Continental drift states that Earth’s continents were joined as a single landmass that broke apart and sent the continents adrift.
• Wegener supported his hypothesis of continental drift with rock types, fossils, and ancient climatic data. His hypothesis was not accepted at first because he couldn’t explain how the continents moved or what caused their motion.
Section 17.1 Study Guide
Section 17.2 Main Ideas
• Sonar and magnetic studies of ocean rocks and sediments led to the proposal of the theory of seafloor spreading.
• Magnetic patterns on the seafloor are symmetric in relation to ocean ridges, indicating that ocean crust on either side of the ridge is moving away from the ridge at essentially the same rate.
• During seafloor spreading, magma rises and hardens to form new crust, which becomes 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 ridge.
Section 17.2 Study Guide
Section 17.3 Main Ideas
• Plate tectonics states that Earth’s crust and rigid upper mantle are broken into large slabs of rock called plates, which move in different directions and at different rates over Earth’s surface.
• At divergent plate boundaries, plates move apart. At convergent boundaries, plates come together. At transform boundaries, plates slide horizontally past each other.
• High heat flow, volcanism, and earthquakes are associated with divergent boundaries; trenches, island arcs, and folded mountains with convergent boundaries; and faults and earthquakes with transform boundaries.
Section 17.3 Study Guide
Section 17.4 Main Ideas
• Convection is the transfer of energy via the movement of heated matter. Convection currents in the mantle are the result of energy transfer between Earth’s hot interior and cooler exterior.
• Ridge push occurs when the elevation of a ridge pushes a plate toward a subduction zone. Slab pull occurs as the weight of the subducting plate pulls a plate into a subduction zone.
Section 17.4 Study Guide
1. Pangaea begin to break apart ____ years ago?
a. 65 million c. 200 million
b. 135 million. d. 400 million
Multiple Choice
Chapter Assessment
Alfred Wegener hypothesized that Pangaea began to break apart 200 million years ago based on geologic evidence.
2. The oldest oceanic crust is generally located near what feature?
a. ridge c. abyssal plain
b. trench d. rift
Subduction occurs in a trench, meaning that oceanic crust at a trench is older than any crust closer to a ridge. A rift is a valley that forms along the axis of a ridge. It is in the rift that new seafloor is created.
Multiple Choice
Chapter Assessment
Multiple Choice
3. The San Andreas Fault is an example of what kind of boundary?
a. divergent c. transform
b. convergent d. rift
Chapter Assessment
Two plates sliding past each other form the San Andreas Fault. The San Andreas Fault is a rare example of a transform boundary that occurs on a continent.
Multiple Choice
4. Which device was instrumental in determining the age of the ocean floor?
a. magnetometer c. sonar
b. isochron d. seismometer
Chapter Assessment
A magnetometer allowed scientists to match patterns magnetic patterns in the seafloor with known patterns and ages of land-based material. From this they could determine the age of the ocean floor.
Multiple Choice
5. The oldest part of the seafloor is approximately ____ years old.
a. 70 million c. 260 million
b. 180 million d. 430 million
Chapter Assessment
The discovery that the seafloor is geologically young was one of the major factors in proposing the theory of seafloor spreading. There are examples, for comparison, of continental rocks that are 3.8 billion years old.
Short Answer
6. Why are there more volcanoes around the rim of the Pacific Ocean than the Atlantic Ocean?
Chapter Assessment
There are subduction zones located around the periphery of the Pacific Ocean. In the Atlantic Ocean, the seafloor is spreading, but with the exception of Caribbean, there are no subduction zones.
Short Answer
7. Why was Wegener’s hypothesis of continental drift rejected when it was first proposed?
Chapter Assessment
Two unanswered questions—what forces could move continents and how continents could move without shattering—were the main reasons that the hypothesis of continental drift was rejected when it was first proposed.
True or False
8. Identify whether the following statements are true or false.
______ Wegener called his hypothesis Pangaea.
______ Magnetic reversal events are longer in duration than epochs.
______ There are around a dozen major plates.
______ Slab push is a process that is associated with an ocean ridge.
______ Folded mountain ranges are the result of a convergent boundary involving oceanic crust and continental crust.
Chapter Assessment
false
false
true
true
false
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