Plate Tectonics Chapter 7 By PresenterMedia.com

Alfred Wegener (VAY guh nuhr) Nearly 100 years ago, a scientist named, Alfred Wegener began an investigation He wanted to know if Earth’s continents had always been in the same place, or if they had moved. He looked at the coastlines of continents that are now separated by oceans. He saw similarities in their shapes. For instance, Africa and South America seemed to fit together like the pieces of a puzzle Wegener proposed that all the continents were once part of a supercontinent called Pangaea (pan JEE uh).

PANGAEA 250 million years ago a supercontinent that all the continents were once a part of PANGAEA 250 million years ago Over time, Pangaea broke apart, and the continents slowly drifted to their present location. To explain this Wegener proposed the hypothesis of continental drift

Pangea Activity On the back of your Pangaea map Glue in the envelope, flap side up on the page across from your notes Use your notes to write a paragraph summarizing why and how Alfred Wegener came up with his concept of Pangaea. Cut out the continents Put puzzle together and move the continents apart like Wegener proposed Trade puzzles with a partner Put the puzzle together paragraph side up Read your partners paragraph Put your puzzle in the envelope on next page On the back of your Pangaea map

CONTINENTAL DRIFT hypothesis suggested that continents are in constant motion on the surface of Earth Wegener knew that he needed evidence to support his hypothesis of continental drift. The most obvious evidence was how the continents fit together like pieces of a puzzle. But other scientists were doubtful of his hypothesis. Wegener needed more evidence.

Evidence that Continents Move the continents fit together like pieces of a puzzle Fossil Clues CLIMATE Clues ROCK CLUES

Fossil Clues Fossil- the naturally preserved remains, imprints, or traces of organisms that lived long ago Lions live in Africa but not in South America There are many animals and plants that live only on one continent. However, fossils of similar organisms have been discovered on several continents that are now separated by oceans. Kangaroos live in Australia but not on any other continent. Because oceans separate the continents, animals cannot travel from one continent to another by natural means. Fossils of a plant called Glossopteris (glahs AHP tur us) have been discovered in rocks from South America, Africa, India, Antarctica, and Australia.

Glossopteris Today these continents, are far apart and separated by oceans. The plant’s seeds could not have traveled across the oceans. This figure shows how some of the continents were joined as part of Pangaea 250 million years ago. The green area on the map shows where Glossopteris fossils have been found. Because these plants grew in a swampy environment, this region, including Antarctica, was different from how it is today. NOW, most of Antarctica is covered in ice sheets with no swampy environments. Africa India South America Austarlia Antarctica Notice that the plant once grew in parts of five continents—South America, Africa, India, Antarctica, and Australia. Glossopteris

Clues to Support Continental Drift hypothesis Glue the paper bag on the page across from notes NOTECARD 1 On blank side write Continents fit like a puzzle On lined side summarize notes on fitting like a puzzle from todays notes NOTESCARD 2 On blank side write Fossil Clues On lined side summarize notes on fossil Clues from todays notes Put both notecards in the paper bag

Coal forms from fossilized plants that lived long ago in warm, wet climates. Climate Clues Wegener found glacial grooves on many different continents. When the ice sheet melted as Pangaea spread apart, it left rock and sediment behind. There are Coal beds in Antarctica, a polar climate today. Wegener also studied glacial grooves, deep scratches in rocks made as ice sheets move across the land. When Wegener pieced Pangaea together, he proposed that 250 million years ago South America, Africa, India, and Australia were located closer to the South Pole and suggested that a large ice sheet covered much of the continents. This meant that Antarctica must have been warmer and wetter when these plants were alive. By studying these grooves, he was able to determine the direction that the ice sheet moved across the joined continents. Wegener studied the similarities of these sediments.

Climate Clues Wegener suggested that a large sheet of ice covered the continents. Wegener studied the sediments left behind and the glacial grooves that formed when the ice sheets melted and Pangaea spread apart. This provided climate evidence for continental drift.

ROCK CLUES Wegener observed that mountain ranges and rock formations on different continents had common origins, providing rock evidence for continental drift. Volcanic rock that is identical in chemistry and age has been found on both the western coast of Africa and the eastern coast of South America.

Rock Clues The Caledonia mountain range in northern Europe and the Appalachian Mountains in eastern North America are similar in age, structure, and rock type.

Clues to Support Continental Drift hypothesis NOTECARD 3 On blank side write Climate Clues On lined side summarize notes Climate Clues from todays notes NOTESCARD 4 On blank side write Rock Clues On lined side summarize notes on Rock Clues from todays notes Put both notecards in the paper bag

What was missing? Wegener’s ideas were not widely accepted until nearly four decades later. Scientists questioned continental drift because it was a slow process and Wegener could not measure how fast continents moved or how they moved. Scientists could not understand how continents could push their way through the solid rock of the mantle and the seafloor.

Lesson 1 - VS All continents were once part of a supercontinent called Pangaea.

Lesson 1 - VS Alfred Wegener proposed that continents move around on Earth’s surface.

Lesson 1 – LR1 What term did Wegener use to describe the constant motion of continents on the surface of Earth? A. Pangaea B. continental drag C. continental movement D. continental drift

Lesson 1 – LR2 In which of these did Wegener observe similarities that suggested continents might fit together like the pieces of a puzzle? A. fossils B. ice sheets C. plates D. continental coastlines

Lesson 1 – LR3 Wegener proposed that 250 million years ago, South America, Africa, India, and Australia were located closer to what? A. equator B. South Pole C. North Pole D. Pacific ocean

Development of a Theory What is seafloor spreading? What evidence is used to support seafloor spreading?

Development of a Theory Create a 5 tab foldable for the vocabulary words (4 cuts makes 5 tabs) mid-ocean ridge seafloor spreading normal polarity magnetic reversal reversed polarity

Mapping the Ocean Floor During the late 1940s scientists were able to determine the depth of the ocean using a device called an echo sounder.

Mapping the Ocean Floor Once ocean depths were determined, scientists used these data to create a topographic map of the sea floor that revealed vast mountain ranges, called mid-ocean ridges, that stretch for many miles deep below the ocean’s surface.

Seafloor Topography

Seafloor Spreading By the 1960s, scientists discovered the process of seafloor spreading. Seafloor spreading is the process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge.

Seafloor Spreading (cont.) When the seafloor spreads, the mantle below melts and forms magma. Magma erupts on Earth’s surface as lava, which cools and crystallizes on the seafloor, forming rock. Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lavas. As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge.

Seafloor Spreading (cont.)

Seafloor Spreading (cont.) Scientists argued that if the seafloor spreads, the continents must also be moving. What is seafloor spreading?

Seafloor Spreading (cont.) The rugged mountains that make up the mid-ocean ridge system can form in two different ways. Large amounts of lava can erupt from the center of the ridge, cool, and build up around the ridge. Or, as the lava cools and forms new crust, it cracks and the rocks move up or down along these cracks in the seafloor, forming jagged mountain ranges.

Seafloor Spreading (cont.) The abyssal plain, the smooth part of the seafloor, is made when the layer of sediment that accumulates far from the mid-ocean ridge becomes thick enough.

Seafloor Spreading (cont.) Continents move as the seafloor spreads along a mid-ocean ridge.

LAB- How do rocks on the seafloor vary with age from a mid-ocean ridge? Scientists discovered that ocean crust forms at a mid-ocean ridge and spreads away from the ridge slowly over time. This process is called seafloor spreading. The age of the seafloor is one component that supports this theory. Learn It Scientists use models to represent real-world science. By creating a small three-dimensional model of volcanic activity along the Mid-Atlantic Ridge, scientists can model the seafloor spreading process. They can then compare this process to the actual age of the seafloor. In this skill lab, you will investigate how the age of rocks on the seafloor changes with distance away from the ridge.

Development of a Theory The first evidence used to support seafloor spreading was discovered in rocks on the seafloor. Scientists studied the magnetic signature of minerals in these rocks. Earth’s magnetic field today is described as having normal polarity—a state in which magnetized objects, such as compass needles, will orient themselves to point north.

Development of a Theory (cont.) Sometimes a magnetic reversal occurs and the magnetic field reverses direction. The opposite of normal polarity is reversed polarity: a state in which magnetized objects reverse direction and orient themselves to point south.

Development of a Theory (cont.) Volcanic rock on the seafloor contains iron-rich minerals that are magnetic. Magnetic minerals in cooling lava from the mid-ocean ridge record the direction of Earth’s magnetic field. Scientists have discovered parallel patterns in the magnetic signature of rocks on either side of a mid-ocean ridge.

Minerals in fresh lava record Earth’s magnetic signature.

Development of a Theory (cont.) Scientists studied magnetic minerals in rocks from the seafloor using a magnetometer to measure and record the magnetic signature. They discovered parallel magnetic stripes on either side of the mid-ocean ridge.

Development of a Theory (cont.) Each pair of stripes has a similar composition, age, and magnetic character. The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges is carried away from the center of the ridges in opposite directions.

Seafloor Spreading Theory

Development of a Theory (cont.) Other measurements made on the seafloor confirm seafloor spreading. Measuring the amount of thermal energy (from magma) leaving the Earth shows that more thermal energy leaves Earth near mid-ocean ridges than is released from beneath the abyssal plains.

Development of a Theory (cont.) Sediment collected from the seafloor can be dated to show that the sediment closest to the mid-ocean ridge is younger than the sediment farther away from the ridge.

Video- Our Planet Earth The Ocean Floor

QUIZ TOMORROW

New ocean crust forms along mid-ocean ridges. Mid-ocean ridges are large mountain ranges that extend throughout Earth’s oceans.

A magnetic reversal occurs when Earth’s magnetic field changes direction.

Lesson 2 – LR1 As the seafloor continues to spread apart, the older oceanic crust moves in which direction with respect to the mid-ocean ridge? A. toward it B. above it C. away from it D. under it

Lesson 2 – LR2 Today’s magnetic field is described as having which of these? A. reversed polarity B. normal polarity C. magnetic reversal D. no polarity

Lesson 2 – LR3 In which state do magnetized objects reverse themselves to point south? A. reversed polarity B. normal polarity C. seafloor spreading D. magnetic polarity