4Continents join together and split apart As far back as the 1500’s, mapmakers noticed that the coasts of Africa and South America fit together like puzzle pieces.
5Continents join together and split apart In 1912, Alfred Wegener proposed a hypothesis called continental drift.
6Continents join together and split apart This hypothesis states that Earth’s continents were once joined in a single landmass and gradually drifted apart.
7Continents join together and drift apart This hypothesis was not accepted until the mid-1900’s.
8Evidence for Continental Drift Extraordinary claims require extraordinary evidence!There are 3 main sources of evidence for continental drift: fossils, climate, and geology.
9Evidence for Continental Drift FossilsMesosaurus is an ancient reptile, the remains of which have been found ONLY in South America and Western Africa.
10Evidence for Continental Drift ClimateModern 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.
11Evidence for Continental Drift ClimateSouth 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.
12Evidence for Continental Drift GeologyThe type of rock found in Brazil perfectly matches rock found in Western Africa.
13Evidence for Continental Drift GeologyLimestone layers in the Appalachian Mountains match limestone layers found in Scotlands Highlands.
14Pangaea and Continental Drift The continents were once joined in a huge supercontinent called Pangaea.Pangaea comes from the Greek for “all lands.”
15Pangaea and Continental Drift Because Wegener could not explain HOW the continents drifted, many people disregarded his ideas for a long time.
16Pangaea and Continetal Drift The theory of plate tectonics explains how plates and their continents move.
17The theory of plate tectonics explains how plates and their continents move Eventually, the theory of plate tectonics arose, which built on Wegener’s idea and served to explain HOW continental drift occurs.
24Evidence from the sea floor Age of the Sea FloorWe 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.
25Evidence 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.
26Evidence from the sea floor Ocean TrenchesIf the sea floor is constantly spreading, why is Earth not constantly growing larger?
27Evidence from the sea floor The answer lies in ocean trenches-deep canyons in the sea floor where crust is being destroyed.
29Causes of Plate Movement Remember, tectonic plates rest on the asthenosphere-a layer of hot, soft rock.
30Causes of Plate Movement Rock in the asthenosphere and mantle move by convection.
31Causes of Plate Movement Convection is the transfer of energy (usually heat) by the movement of a material.
32Causes 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.
33Causes 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.
34Causes 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!
35Causes of Plate Movement Two other possible causes of plate movement are slab pull and ridge push.
36Causes of Plate Movement Slab pull occurs where gravity pulls the edge of a cool dense plate into the asthenosphere.
37Causes of Plate Movement Ridge push occurs when material from a mid-ocean ridge slides downhill from the ridge.
38Putting the theory together 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.
48Putting the theory together One plate cannot shift without affecting the others nearby.
49ReviewWhy did many scientists reject Wegener’s continental drift hypothesis?He could not explain how the continents moved.The geology of continents did not support his hypothesis.Fossil evidence showed that the continents were never joined.The climates of the continents have remained the same.
50Review2. What evidence from the sea floor shows that tectonic plates move?The sea floor is much older than any of the continents.The sea floor is youngest near a mid-ocean ridge and older farther away.Mid-ocean ridges circle Earth like seams in a baseball.The sea floor is thinner than continental crust.
51Review 3. A mid-ocean ridge forms where plates Move apart Push togetherScrape past each otherSubduct
52Review 4. Plate motion is caused partly by Magnetic reversals Convection currentsContinental driftVolcanic hot spots