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Earth Science Ch. 12:Plate Tectonics
Ch. 12-1: Earthquakes
Key Terms• Fossil
• Pangea
California Content Standards for Earth Science Addressed:
Earth Science3. Plate tectonics operating over geologic time has
changed the patterns of land, sea, and mountains on Earth's surface. As the basis for understanding this concept:
3(a)Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate tectonics.
3(b)Students know the principal structures that form at the three different kinds of plate boundaries.
9(b)Students know the principal natural hazards in different California regions and the geologic basis of those hazards.
Ch 12-1:Why Do I Need To Know This?
1. Because it explains how and why earthquakes and volcanoes form.
2. Because it will help you get a great value on real estate in 160 million years or so!
Key Sections Ch. 12-1
• Earth’s Drifting Continents
• Evidence From Fossils
• Evidence From Rocks
Earth’s Drifting Continents
• Alfred Wegener was a German geologist who noticed that all the continents appeared to fit together like a giant jigsaw puzzle.
• Around 1900, Wegener published a paper that stated that all the continents were once part of a “super-continent” that he named Pangea (whole Earth) and that it broke apart in a process he called “continental drift”.
Earth’s Drifting Continents
• Because most scientists did not believe in Wegener theory, they laughed at him and called him names.
– Little did all the other scientists know, that later on, Wegener would be proven correct!
• Some geologists, though, did believe in this theory, and began to gather evidence to prove the theory. Some of the evidence they gathered came from:
1. Fossils2. Rock Formations
Evidence From Fossils
• Fossils are evidence of ancient life forms from thousands, millions or billions of years ago.
• One fossil, called Glossopteris, was a tree that lived about 250 million years ago.– Fossils from this tree were
found in South Africa, Australia, India and Antarctica.
• Other fossils were found in both South America and Africa.
• All the fossils found were too big to have moved around the world on their own.
Evidence From Rocks
• Rocks contain many types of evidence that the continents have moved.
• These include:1. Identical rock formations in
South America and Africa.2. Deposits from salts and coral
reefs at the tops of mountains.3. Mountains that match in Africa
and North America.4. Evidence from glaciers.5. Magnetic particles in rocks
showing different directions for the north and south poles.
Ch. 12-2: Earth’s Spreading Ocean Floor
Key Terms• Mid-ocean Ridge
• Ocean-Floor Spreading
• Trench
• Subduction
California Content Standards for Earth Science Addressed:
Earth Science3. Plate tectonics operating over geologic
time has changed the patterns of land, sea, and mountains on Earth's surface. As the basis for understanding this concept:
3(a)Students know features of the ocean floor (magnetic patterns, age, and sea-floor topography) provide evidence of plate tectonics.
3(b)Students know the principal structures that form at the three different kinds of plate boundaries.
Ch 12-2:Why Do I Need To Know This?
1. Because this is how plates are created and recycled.
2. It explains how and why most volcanoes form and how and why most earthquakes happen.
3. It helps us determine the outline of most of the plates.
Key Sections Ch. 12-2
• Earth’s Spreading Ocean Floor
• Mid-Ocean Ridges
• Ocean-Floor Spreading
• Magnetic Stripes
• Transform Faults
• Subduction
• Putting It All Together
Earth’s Spreading Ocean Floor
• In spite of all the evidence suggesting that the continents had moved, most scientists did not accept Wegener’s theories and had forgotten about him.– The main reason that they
rejected his theories was because he and his supporters could not provide a mechanism for how the plates moved.
• However, in one of the coolest stories from WWII, Wegener’s theories would get a fresh new start!
Earth’s Spreading Ocean Floor
• Howard Hess was a geologist who was assigned to the navy in WWII.
• His ship had sonar to detect enemy submarines and ocean floor depths to plan invasions.
• Hess kept his sonar running for all of WWII, and in the process discovered many key features about the ocean floor that proved Wegener’s theories were correct!
Mid-Ocean Ridges
• Hess discovered that in the middle of the Atlantic Ocean was a mountain chain that stretched around the world!– Hess called these ridges the
Mid-Ocean Ridges.• Hess also discovered that
around the border of the Pacific Ocean were deep trenches!
• Hess combined these two facts to provide the mechanism for Wegener’s theories to create a new theory of Plate Tectonics.
Ocean-Floor Spreading
• At the mid-ocean ridges, new oceanic plates are being created through volcanic eruptions.
• These eruptions are called “black smokers” and may have played an important role in the beginning of life on Earth.
• The lava formed from the black smokers is called “pillow basalts”. We can see the remains of ancient pillow basalts on the land today, such as in Northern California.
Ocean-Floor Spreading
Magnetic Stripes
• More evidence to prove plate tectonics was found on either side of the mid-ocean ridges.
• On either side of the ridge, the rocks contain iron particles that point to magnetic north at the time that the rocks were formed.
• However, throughout the Earth’s history, the polarity of the planet has changed (the North and South Poles have flipped).
• The rock formations on either side of the mid-ocean ridges show the same pattern for polarity which helps prove that the rocks on either side of the mid-ocean ridges were formed at the same time!
Magnetic Stripes
Transform Faults
• Because the Earth’s surface is curved, as the ocean-floor spreads, while the plates are flat, it causes parts of the Earth to appear to spread at different rates.
• Consequently, the mid-ocean ridges and plates broken in places called transform faults.
• Transform faults are strike-slip faults, like the San Andreas Fault.
• There is a major transform fault that runs from Long Beach to the Hawaiian Islands.
Transform Faults
Subduction
• At the trenches, Hess determined that old oceanic plates are being subducted.
• Subducted means “led below”, which is an accurate description of what is happening.
• Old oceanic plates (about 250 million years old) sink back down into the mantle and are melted.
• Where they sink down, we get trenches in the ocean—which can be up to 7 miles deep!
Subduction
• Hess’s Theory explains how and why volcanoes and earthquakes form.
• As the plates move, they cause earthquakes to happen.
• At the trenches, the plates are subducted and creating lots of magma.
• About 100 miles away from the trench, the magma rises, creating large numbers of volcanoes.
Putting It All Together
Ch. 12-3: Earth’s Moving Plates
Key Terms• Theory of Plate Tectonics• Plate• Lithosphere• Convergent Boundary• Divergent Boundary• Strike-Slip Boundary• Convection Current
California Content Standards for Earth Science Addressed:
Earth Science3. Plate tectonics operating over geologic time
has changed the patterns of land, sea, and mountains on Earth's surface. As the basis for understanding this concept:
3(b) Students know the principal structures that form at the three different kinds of plate boundaries.
9(e) Students know the principal natural hazards in different California regions and the geologic basis of those hazards.
Ch 12-3:Why Do I Need To Know This?
1. Because we live in California, one of the most geologically active states in the country.
2. Because California is part of the Ring of Fire.
3. To FINALLY understand plate tectonics!!!!
Key Sections Ch. 12-3
• The Earth’s Moving Plates• Lithospheric Plates• Plate Boundaries• Divergent Boundaries• Convergent Boundaries• Strike-Slip Boundaries• Plate Motion• Plate Tectonics and Evolution
The Earth’s Moving Plates
• By the 1960s, it had become clear that the surface of the Earth was more dynamic than previously thought.
• The Theory of Plate Tectonics had become widely accepted, and was used to explain how and why mountains, volcanoes, faults, earthquakes, etc. formed and helped to explain how life had evolved on the planet.
Lithospheric Plates
• There are many ways to divide up the layers of the Earth.
• For plate tectonics, the Earth is divided into 3 layers which are:
1. Lithosphere (hard rock)
2. Asthenosphere (molten rock)
3. Core
Lithospheric Plates
Lithospheric Plates
• According to plate tectonics, there are 2 types of plates:
1. Oceanic Plates2. Continental Plates
• Oceanic Plates are only about 8-10 km thick and lie underneath all the oceans.
• Continental Plates are 30-100 km thick and are the ground on which we live.
Lithospheric Plates
• According to plate tectonics, the surface of the Earth contains 7 major plates and about 20 minor plates.
• Each plate is slightly moving in different directions.
• Each plate only moves about 1 - 4 cm a year (about 0.5 - 1.5 inches a year).
Plate Boundaries
• There are 3 types of plate boundaries that occur on the surface of the Earth. They are:
1. Divergent Boundaries
2. Convergent Boundaries
3. Strike-Slip Boundaries• Each type of boundary creates a unique
type of geology and landform.• Also, the types of plates at each boundary
will also determine the geology that is likely to occur.
Divergent Boundaries
• Divergent Boundaries are areas where plates are being torn apart and new plates are being created.
• Divergent Boundaries can occur on the land or under the ocean.– On the land, a divergent
boundary is called a “rift valley”.
– In the ocean, they are the mid-ocean ridges.
• Divergent Boundaries result in lots of volcanic activity and some earthquakes.
Convergent Boundaries
• Convergent Boundaries are where 2 plates are colliding.
• What happens at a Convergent Boundary depends upon what type of plates are colliding.
• If 2 continental plates collide into each other, a mountain chain will form, such as the Himalayas.
• This will result in a lot of earthquakes and a lot of extinct volcanoes.
Convergent Boundaries
• If a continental plate and an oceanic plate collide, it will result in subduction.
• If 2 oceanic plates collide, subduction will also occur, with the older plate being subducted by the younger plate.
• This will result in a lot of active volcanoes and earthquakes.
Strike-Slip Boundaries
• If 2 plates slide by each other, a big fault will form at their boundary.
• At a strike-slip boundary, there will be many earthquakes.
• The San Andreas fault is a strike-slip boundary between the North American Plate and the Pacific Plate.
Plate Motion
• While geologists are not 100% certain as to all the details of how plates move, they do know that it has a lot to do with how heat is distributed inside the Earth.
• Inside the core, there is a lot of heat due to radioactive materials and the pull of gravity.
• The heat from deep within the Earth gets spread through a process called “convection currents”.
• The rising and spreading out of the heat moves the plates on the surface of the Earth, causing all of the different types of boundaries.
Plate Motion
Hot-Spots
• Convection currents also explain how many islands form.
• Islands such as Hawaii are known as hot spots. They form when a plate moves across a rising source of heat creating lots of volcanoes.
• As the volcanoes grow, they form islands.• However, eventually, the plate moves far
enough away from the hot-spot and a new island forms.
• Already, a new island is forming in the Hawaiian Island chain, and should appear sometime in the next 10,000 years!
Hot-Spots
Plate Tectonics and Evolution• We can see how plate
tectonics has caused major changes in evolution.
• Primates exist in both South America and Africa.– Primates in Africa do not
have prehensile tails (tails that they can use for hanging from trees).
– Primates in South America do have prehensile tails.
– There also are differences in the tooth structure between African and South American primates.
• These differences are a result of the two groups of primates being separated by the Atlantic Ocean!
