plate tectonics chapter 7 by presentermedia.compresentermedia.com
TRANSCRIPT
• 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
• Nearly 100 years ago, a scientist named, Alfred Wegener began an investigation
Alfred Wegener (VAY guh nuhr)
Wegener proposed that all the continents were once part of a
supercontinent called Pangaea (pan JEE uh).
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
PANGAEAa supercontinent that all the continents were once a part of
• Glue in the envelope, flap side up on the page across from your notes
• Pangea Activity
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
On the back of your Pangaea map
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
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.
CONTINENTAL DRIFT hypothesis suggested that continents are in constant motion on
the surface of Earth
FOSSIL CLUES
ROCK CLUES
Evidence that Continents Move
CLIMATE CLUES
THE CONTINENTS FIT TOGETHER LIKE PIECES OF A PUZZLE
• 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.
Fossil- the naturally preserved remains, imprints, or traces of organisms that lived long ago
• Fossil Clues
Lions live in Africa but not in South AmericaKangaroos live in
Australia but not on any other
continent.
Fossils of a plant called Glossopteris (glahs AHP tur us) have been discovered in rocks from South America, Africa, India, Antarctica, and Australia.
• Because oceans separate the continents, animals cannot travel from one continent to another by natural means.
• 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.
Glossopteris
Today these continents, are far apart and separated by oceans.
The plant’s seeds could not have traveled across the oceans.
South America
Africa
India
Antarctica
Austarlia
Glossopteris
• 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
• Clues to Support Continental Drift hypothesis Glue the paper bag on the page across from notes
Put both notecards in the paper bag
Climate Clues Coal forms from fossilized plants that lived long ago in warm, wet climates.
This meant that Antarctica must have been warmer and wetter when these plants were alive.
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.
Wegener found glacial grooves on many different continents.
By studying these grooves, he was able to determine the direction that the ice sheet moved across the joined continents.
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.
When the ice sheet melted as Pangaea spread apart, it left rock and sediment behind.
Wegener studied the similarities of thesesediments.
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.
Climate Clues
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.
• 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
• Clues to Support Continental Drift hypothesis
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
A. PangaeaB. continental dragC. continental movementD. continental drift
What term did Wegener use to describe the constant motion of continents on the surface of Earth?
• Lesson 1 – LR2
A. fossils B. ice sheetsC. platesD. continental coastlines
In which of these did Wegener observe similarities that suggested continents might fit together like the pieces of a puzzle?
• Lesson 1 – LR3
A. equatorB. South PoleC. North PoleD. Pacific ocean
Wegener proposed that 250 million years ago, South America, Africa, India, and Australia were located closer to what?
• What is seafloor spreading?
• What evidence is used to support seafloor spreading?
Development of a Theory
• mid-ocean ridge
• seafloor spreading
• normal polarity
• magnetic reversal
• reversed polarity
Development of a Theory
Create a 5 tab foldable for the vocabulary words (4 cuts makes 5 tabs)
• 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.
Mapping the Ocean Floor
Seafloor Topography
• 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
1. When the seafloor spreads, the mantle below melts and forms magma.
2. Magma erupts on Earth’s surface as lava, which cools and crystallizes on the seafloor, forming rock.
3. Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lavas.
4. As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge.
Seafloor Spreading (cont.)
Seafloor Spreading (cont.)
Seafloor Spreading (cont.)
What is seafloor spreading?
Scientists argued that if the seafloor spreads, the continents must also be moving.
The rugged mountains that make up the mid-ocean ridge system can form in two different ways.1. Large amounts of lava can erupt from
the center of the ridge, cool, and build up around the ridge.
2. 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.
Seafloor Spreading (cont.)
• 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 ItScientists 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.
• 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
• 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.
Development of a Theory (cont.)
Minerals in fresh lava record Earth’s magnetic signature.
• 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.
Development of a Theory (cont.)
Seafloor Spreading Theory
• 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.
Development of a Theory (cont.)
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
A. toward itB. above itC. away from itD. under it
As the seafloor continues to spread apart, the older oceanic crust moves in which direction with respect to the mid-ocean ridge?
• Lesson 2 – LR2
A. reversed polarityB. normal polarityC. magnetic reversalD. no polarity
Today’s magnetic field is described as having which of these?
• Lesson 2 – LR3
A. reversed polarityB. normal polarityC. seafloor spreadingD. magnetic polarity
In which state do magnetized objects reverse themselves to point south?