Post on 15-Jan-2016
1.2 Notes
Continents Change Position Over Time
When creating your map of
Pangaea, what clues helped you fit the pieces together?
Think About…
As far back as the
1500’s, mapmakers noticed that the coasts of Africa and South America fit together like puzzle pieces.
Continents join together and split
apart
In 1912,
Alfred Wegener proposed a hypothesis called continental drift.
Continents join together and split
apart
This hypothesis
states that Earth’s continents were once joined in a single landmass and gradually drifted apart.
Continents join together and split
apart
This hypothesis was not accepted
until the mid-1900’s.
Continents join together and drift
apart
Extraordinary claims require
extraordinary evidence! There are 3 main sources of
evidence for continental drift: fossils, climate, and geology.
Evidence for Continental Drift
Fossils
Mesosaurus is an ancient reptile, the remains of which have been found ONLY in South America and Western Africa.
Evidence for Continental Drift
Climate
Modern day Greenland is found near the Arctic Circle and is covered in ice, yet fossils of tropical plants are found on its shores.
This is clue that Greenland once existed near the equator.
Evidence for Continental Drift
Climate
South Africa today has a warm climate, yet its rocks have deep scratches clearly made by past glaciers.
This is a clue that South Africa once existed much closer to the south pole.
Evidence for Continental Drift
Geology
The type of rock found in Brazil perfectly matches rock found in Western Africa.
Evidence for Continental Drift
Geology
Limestone layers in the Appalachian Mountains match limestone layers found in Scotlands Highlands.
Evidence for Continental Drift
The continents
were once joined in a huge supercontinent called Pangaea.
Pangaea comes from the Greek for “all lands.”
Pangaea and Continental Drift
Because Wegener could not
explain HOW the continents drifted, many people disregarded his ideas for a long time.
Pangaea and Continental Drift
The theory of plate tectonics
explains how plates and their continents move.
Pangaea and Continetal Drift
Eventually, the theory of plate
tectonics arose, which built on Wegener’s idea and served to explain HOW continental drift occurs.
The theory of plate tectonics explains how
plates and their continents move
When scientists started mapping
the sea floor in the 1950’s, they expected it to be smooth and level, like a flat underwater desert.
Evidence from the Sea Floor
What scientists found on the sea
floor instead were huge underwater mountain ranges, called mid-ocean ridges.
Evidence from the sea floor
Sea-Floor Spreading
Mid-ocean ridges form along cracks in the crust.
Molten rock rises through these cracks, cools, and forms new oceanic crust.
Evidence from the sea floor
https://
www.youtube.com/watch?v=ryrXAGY1dmE
Age of the Sea Floor
We can verify that the sea floor is spreading apart by new rocks being created by testing the age of rocks.
The youngest rocks lie close to spreading centers, while older rocks lie further away.
Evidence from the sea floor
Even the oldest rocks on the ocean
floor are only 160 to 180 million years old, while continental crust can be upwards of 4 billion years old!
This is evidence that the ocean floor is constantly changing.
Evidence from the sea floor
Ocean Trenches
If the sea floor is constantly spreading, why is Earth not constantly growing larger?
Evidence from the sea floor
The answer lies in ocean trenches-
deep canyons in the sea floor where crust is being destroyed.
Evidence from the sea floor
Evidence from the sea floor
Remember, tectonic plates rest on
the asthenosphere-a layer of hot, soft rock.
Causes of Plate Movement
Rock in the asthenosphere and
mantle move by convection.
Causes of Plate Movement
Convection is the transfer of
energy (usually heat) by the movement of a material.
Causes of Plate Movement
Convection is what
happens when you boil a pot of water.
The water at the bottom heats up, becomes less dense and rises. At the surface, it cools, becomes denser, and sinks.
Causes of Plate Movement
Rock in the asthenosphere acts the
same way as a boiling pot of water. When this rising and sinking pattern
repeats, it forms a circular motion called a convection current.
Causes of Plate Movement
The movement of the mantle is
much slower than a pot of boiling water-only a few centimeters each year!
This results in the asthenosphere moving the plates like heavy boxes on rollers-but very, very slowly!
Causes of Plate Movement
Two other possible causes of plate
movement are slab pull and ridge push.
Causes of Plate Movement
Slab pull occurs where gravity
pulls the edge of a cool dense plate into the asthenosphere.
Causes of Plate Movement
Ridge push occurs when material
from a mid-ocean ridge slides downhill from the ridge.
Causes of Plate Movement
The theory of plate tectonics
encompasses scientists’ knowledge of Earth’s plates, the sea floor, and the asthenosphere.
There are 9 major tectonic plates on Earth.
Fill in your lithospheric plates map as we go! After it is filled in, shade each plate with a different color.
Putting the theory together
One plate cannot shift without
affecting the others nearby.
Putting the theory together
1. Why did many scientists reject
Wegener’s continental drift hypothesis?A. He could not explain how the
continents moved.B. The geology of continents did not
support his hypothesis.C. Fossil evidence showed that the
continents were never joined.D. The climates of the continents have
remained the same.
Review
2. What evidence from the sea floor shows that tectonic plates move?A. The sea floor is much older than any of
the continents.B. The sea floor is youngest near a mid-
ocean ridge and older farther away.C. Mid-ocean ridges circle Earth like
seams in a baseball.D. The sea floor is thinner than
continental crust.
Review
3. A mid-ocean ridge forms where platesA.Move apartB.Push togetherC.Scrape past each otherD.Subduct
Review
4. Plate motion is caused partly by A.Magnetic reversalsB.Convection currentsC.Continental driftD.Volcanic hot spots
Review