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Plate Tectonics

Science, Grade 6

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Table Of Contents: Plate Tectonics

1. Earth's Interior

2.1. Seismograms and the Earth's Structure
Scientists use machines called seismographs to study seismic waves caused by earthquakes. Data gathered from seismograms has provided a relatively detailed picture of Earth’s interior structure.
2.2. Layers of the Earth
The Earth is composed of three main layers—the crust, mantle and core. These layers vary in composition, size, temperature and pressure.
2.3. The Earth's Crust
The crust is the outermost layer of the Earth and is 5 to 70 kilometers thick. There are two different types of crust—oceanic and continental.
2.4. Oceanic Crust
Oceanic crust is primarily composed of basalt that forms when magma pours out from the mantle onto the ocean floor. Oceanic crust is denser, but also thinner than continental crust.
2.5. Continental Crust
Continental crust forms the landmasses called continents. This crust layer is composed of rocks such as granite. It is less dense, but thicker than oceanic crust.
2.6. The Earth's Mantle
The mantle is found below the crust and is approximately 3,000 kilometers thick. The mantle is made of solid, hot rock. The uppermost layer of the mantle and the crust make up what is known as the lithosphere.
2.7. Upper and Lower Mantle
The asthenosphere is found below the lithosphere and is called the upper mantle. It is made up of hotter, weaker rock. The lower mantle called the mesosphere is a thick layer of strong, solid rock.
2.8. The Earth's Core
The core is the densest layer of the Earth and is composed of nickel and iron. It is divided into a liquid outer core and a solid inner core.

2. Heat Transfer and Convection Currents

3.1. Types of Heat Transfer
There are three types of heat transfer. Radiation is heat transferred through space. Conduction is heat transferred within a material, and convection is heat transferred by the movement of liquids.
3.2. What Causes Convection?
Convection is caused when a fluid has differences in temperature and density. When a liquid becomes cooler, its particles move more slowly and the liquid becomes denser.
3.3. Hot Mantle Rises
In the Earth, the mantle is the hottest near the core and cooler near the crust. Because it has a lower density, the hot mantle material near the core starts to rise toward the crust.
3.4. Cool Mantle Sinks
As the mantle rises, it also cools. When it cools, it becomes denser and then descends back towards the center of the Earth.
3.5. Mantle Convection Currents
This circular motion creates convection currents. Convection currents in the mantle cause the crust to move over Earth’s surface.
3.6. Core Convection Currents
There are also convection currents in the outer core. These currents contribute to the Earth's magnetic field.

3. Pause and Interact

4.1. Earth's Interior
Click on the Terms button. Then click and drag each term to the correct box. Use the reset button to clear the terms and start over. Use the gear button to customize the draggable terms.

4. Continental Drift

5.1. What Is the Continental Drift Theory?
In the early 1900s a scientist named Alfred Wegener proposed the continental drift theory, which suggests that the continents were once together as a single landmass that broke up and drifted apart over millions of years.
5.2. Basis of the Continental Drift Theory
Wegener based his theory on the observation that the boundaries of the continents appear to look like pieces of a large puzzle.
5.3. Evidence for Continental Drift
Studies of fossils and glacial striations in rocks on different continents also supported continental drift.
5.4. Pangaea
According to the continental drift theory, all of the continents once formed a large, single landmass called Pangaea. We now know that Pangaea existed 245 million years ago, when dinosaurs were roaming the Earth.
5.5. Laurasia and Gondwana
About 180 million years ago, Pangaea began to split into two smaller continents called Laurasia and Gondwana.
5.6. Modern Continents
Approximately 65 million years ago, these two landmasses had broken apart to form the modern continents.
5.7. Theory History
The continental drift theory was an accepted part of geology for about 50 years until it was replaced by the more complete theory of plate tectonics.

5. Sea-Floor Spreading

6.1. Mid-Ocean Ridges
In the middle of the Atlantic ocean is a submerged mountain range called a mid-ocean ridge. It is part of a worldwide system of mid-ocean ridges formed by sea-floor spreading.
6.2. How Sea-Floor Spreading Occurs
Sea-floor spreading occurs when two tectonic plates pull away from each other, and a crack forms in the ocean floor. Magma from the Earth's mantle pours onto the ocean floor through the crack and creates new oceanic crust.
6.3. New and Old Oceanic Crust
A mid-ocean ridge is formed along the crack where magma is emerging. As the sea floor spreads, new oceanic crust pushes the older crust away from the ridge.
6.4. Supports Plate Tectonics Theory
Sea-floor spreading is part of the plate tectonics theory. In the Atlantic, the plates move away from the mid-ocean ridge at a rate of about one centimeter per year. As the sea floor spreads, the Earth's crust is changing and moving the continents as well.

6. Theory of Plate Tectonics

7.1. What Is the Theory of Plate Tectonics?
The theory of plate tectonics was proposed in the 1960s. It states that the lithosphere is broken into pieces called tectonic plates that are moving on top of the upper mantle. The plates carry both the continents and the ocean floors.
7.2. Tectonic Plates in Motion
According to the theory, the tectonic plates are in motion due to the movement of convection currents in the mantle.
7.3. Tectonic Plate Boundaries
The tectonic plates move against each other at their boundaries, resulting in intense geologic activity, such as volcanoes, earthquakes and mountain formation. There are three types of plate boundaries—divergent, convergent and transform.
7.4. Divergent Plate Boundaries
New crust is created at divergent boundaries, where two plates are moving apart. This type of boundary creates mid-ocean ridges in the oceanic crust and rift valleys on land.
7.5. Convergent Plate Boundaries
Convergent boundaries, where two plates are moving together, can occur between an oceanic and continental plate, two oceanic plates or two continental plates.
7.6. Subduction Zone
A subduction zone occurs when a dense oceanic plate is pushed down into the mantle below another plate. Over time, the subducted crust melts in the mantle. This geologic activity creates volcanic islands in the oceans and volcanic arcs on land.
7.7. Mountain Range Formation
Subduction does not occur at the convergent boundary of two continental plates. Instead, the crust is pushed upward and mountain ranges are formed.
7.8. Transform Boundary
A transform boundary occurs when plates slide past each other, such as the San Andreas fault in California. These boundaries are called conservative because plate material is neither created nor destroyed.

7. Pause and Interact

8.1. Review
Use the whiteboard tools to complete the activity.
8.2. Continent Movement Over Time
Roll over each box to see continental movement over geologic time. Drag and drop the boxes into the correct sequence, from the oldest continental arrangement to the modern continents.

8. Vocabulary Review

9.1. Vocabulary Matching Review

9. Virtual Investigation

10.1. Tectonic Plate Boundaries
In this virtual investigation, you will explore the geologic activities and formations that occur at the boundaries of moving tectonic plates. Real-world examples of the different types of plate boundaries are also provided.

10. Assessment

11.1. Plate Tectonics
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