Unit 3: Earth S Dynamic Geosphere

Name: Date: Period: Unit 3: Earth’s Dynamic Geosphere Section 1: Plate Tectonics

Plate tectonics is a scientific theory. Remember that a scientific theory is a well-tested concept that is supported by experimental or observational evidence. It explains a wide range of observations. The theory of plate tectonics explains the formations, movement, and changes of the outer, rigid layer of the Earth, called the lithosphere.

The Interior Structure of the Earth (G68) The thin, outermost layer of the Earth is called the crust. There are two kinds of crust:

1. Continental – Forms the Earth’s continents, between 30-50 km thick, and very old in geological time. (~ 4 billion years old)

2. Oceanic – Forms under the Earth’s bodies of water, only 5-10 km thick, and young in geological time. (~ 200 million years old)

*Notice the vertical and horizontal scales are different. This was done to exaggerate the vertical view so you could see the separation better. If drawn to scale, the layers would look much thinner.

Beneath the Earth’s crust is the mantle. The rocks of the mantle are very different is composition from the crust. The division between the crust and the mantle is well defined. The upper part of the mantle is a cool, rigid block and the crust rests on top. They move as one and together they are called the lithosphere. The lower part of the mantle is hot enough that it can flow very slowly like a very stiff liquid. That part is called the asthenosphere.

Measuring Plate Motions with GPS The Global Positioning System (GPS) consists of 24 satellites that orbit the Earth. Receivers at stations on Earth use the signals from satellites to calculate the location of the station. Geoscientists have set up a network of targets all over the world in order to monitor the movement of the lithospheric plates. Changes in horizontal and vertical positions can be detected within several millimeters.

1 GPS data collected at stations all over the world confirm that the surface of the Earth is moving. However, data has only been collected for the past decade. In order to answer the question whether or not the Earth has been moving in the past, we must look elsewhere for evidence.

Sea-Floor Spreading (G70) All magnets and materials that have magnetism have a north and south direction, or magnetic polarity. Rocks with normal magnetic polarity match that of the Earth’s magnetic field and point toward the north pole of the Earth. The other group has magnetic minerals with reversed polarity and point toward the south pole.

It was known that as lava cools to form basalt, its iron minerals become magnetized and “lock in” the polarity of the Earth’s magnetic field. Beginning in the 1950’s, scientists began to noting patterns in the magnetism of rocks on the ocean floor. The alternating belts were of higher (normal polarity) and lower (reverse polarity) than the average magnetic field strength.

In 1963, two scientists, Vine and Matthews, proposed the sea-floor spreading theory to explain this pattern. According to their theory, the matching patterns on either side of the mid-ocean ridge could be explained by new ocean crust forming at the ridge and spreading away from it. As the ocean crust forms, it obtains the polarity of the Earth’s magnetic field at the time. Since the Earth’s polarity is always changing, it explains the magnetic “stripes”.

Since the theory was proposed, samples of volcanic rock have been taken in order to measure their age. The results supported the theory, with the rocks becoming older as you moved to the left and right of the ridge. The mid-ocean ridge zig-zags between the continents, winding its way across the globe like the seams on a baseball. (G71)

Define the following terms using your textbook.

Lithosphere – the outermost layer of the Earth, consisting of the Earth’s crust and part of the upper mantle. The lithosphere is a rigid layer.

Crust – the thin outermost layer of the Earth. There are two types: continental and oceanic crust.

Mantle – the zone of the Earth below the crust and above the core. It is divided into the upper mantle and lower mantle with a transition zone between.

Asthenosphere – the part of the mantle beneath the lithosphere. The asthenosphere behaves like a thick liquid that undergoes slow flow. 2 Global Positioning System – a satellite-based system for accurate location of points on the Earth.

Basalt – a kind of volcanic igneous rock, usually dark colored, with a high content of iron.

Mid-Ocean Ridge – a chain of undersea ridges extending throughout all of the Earth’s ocean basins, and formed by sea-floor spreading.

Review Questions 1. Using the map on page G63 of your text, which named GPS station is closest to us? WES2

2. How does GPS provide evidence that the surface of the Earth moves over time? GPS allows scientists to make very precise and accurate measurements of the position of an object on the Earth’s surface. When its tracked over time, data reveals that the object moves.

3. Describe at least one advantage of using GPS technology to gather evidence of plate motion. Accuracy, GPS measures even the slightest movements over time.

4. How do studies of the magnetism of rocks on the sea floor provide evidence that the surface of the Earth moves over time? By revealing a symmetrical pattern of magnetic reversals in rocks on each side of the mid-ocean ridge. The similar ages and magnetic patterns in the rocks on opposite sides of the ridge indicate that the sea floor has been spreading over time.

5. Predict: How might the difference in motion of these plates affect the Earth’s lithosphere? Cracks or fractures might happen along with subductions, earthquakes and volcanoes.

6. Using the map of the plates on page G66, answer the following questions: a. Look at the names of the plates. On what basis does it appear that the plates were named? Named for continents and ocean basins. b. Write down the name of the plate we live on. North American Plate c. Describe the motion of your plate relative to all the plates that border it. Western motion

7. On the map provided on the following page, use colored pencils to draw and label the major lithospheric plates. Then add their direction of movement with arrows.

3 4 5 New plates are created at certain places on Earth, and existing plates are consumed at certain other places. The total surface area of the planet stays the same, so the creation of new plates has to be exactly equal to the consumption of existing plates.

Types of Plate Boundaries Geologists use three descriptive terms to classify the boundaries between plates: 1. divergent boundaries – two plates move away from each other 2. convergent boundaries – two plates move toward each other 3. transform boundaries – two plates slide parallel to each other

1. Divergent Plate Boundaries a. Characterized by basaltic volcanoes and shallow focus earthquakes.

b. As the plates move apart, the mantle QuickTime™ and a asthenosphere rises slowly upward, and a rift TIFF (Uncompressed) decompressor are needed to see this picture. valley is created. c. Rift valleys are usually found atop mid-ocean ridges such as the Mid-Atlantic Ridge.

2. Convergent Plate Boundaries There are three types of convergent plate boundaries: a. Ocean – Ocean Convergence  Occurs when two oceanic plates collide.  One plate stays at the surface and the other gets pushed underneath. This process is called subduction.  The very deepest ocean depths are at subduction zones.  Forms volcanic islands called a volcanic island arc.

b. Ocean – Continent Convergence  Occurs when an oceanic plate collides with a continental plate.  The ocean plate always gets subducted because it is less dense than the continental plate.

 Forms a chain of volcanoes on the continent called a continental volcanic arc. (Example: The Andes Mountains in western South America) c. Continent – Continent Convergence  Occurs when two continental plates collide.  Continental plates cannot be subducted because they are too dense.  Creates a suture zone which is a place where two continents have met and become welded into a single continent.

6 3. Transform Plate Boundaries  The place where two plates slide past each other is called a transform fault.  The boundary between the North American and Pacific plates is a transform fault called the San Andreas Fault. California has many earthquakes because of this fault line.

Divergent Plate Boundary – a plate boundary where two plates move away from one another.

Rift Valley – a large, long valley on a continent, formed where the continent is pulled apart by forces produced when mantle material rises up beneath the continent.

Convergent Plate Boundary – a plate boundary where two plates move toward each other.

Subduction – the movement of one plate downward into the mantle beneath the edge of the other plate at a convergent plate boundary. Only oceanic plates subduct.

Suture Zone – the zone on the Earth’s surface where two continents have collided and have been welded together to form a single continent.

Transform Plate Boundary – a plate boundary where two plates slide parallel to one another.

Transform Fault – occurs when two lithospheric plates slide parallel to each other and there is an offset between two segments of a separating ridge.

7 Review Questions

1. Name the three types of boundaries between lithospheric plates. Convergent, Divergent, and Transform

2. How and where are rift valleys formed? Mantle convection causes doming and fracturing of existing continental lithosphere.

3. Convergent plate boundaries can be in three different settings. What are they? a. Ocean-Ocean b. Ocean-Continent c. Continent-Continent

4. Why does the oceanic plate always get subducted? Because it is less dense and thinner than the continental plate.

5. a. Why is it that transform faults can be used to figure out the directions of plate movements? b. Why can’t subduction zones be used for that? Remove this Question

6. What happens when two continents collide along a convergent plate boundary? They are both thrust upward with one being more horizontal and the other being pushed more vertically.

Evidence of Earth’s Layered Structure

1. The density of a substance refers to how concentrated the mass of the material is. Less dense materials tend to rise upward and float on more dense material. Rocks in the Earth’s crust are less dense than the rocks of the underlying mantle. The crust floats on the more dense interior material.

Density = Mass / Volume Example: D = 100 g / 25 cm3 D = 4.0 g/cm3

 Heating materials decreases their density, thus magma rises to the surface.  Earth’s average density is 5.5 g/cm3 with its surface being 2.8 g/cm3  Earth’s core is much more dense than the crust because it’s under great pressure and made mostly of iron.

2. Seismic waves are waves of motion generated within the Earth by earthquakes. Waves travel faster the deeper they are within the Earth due to increased density. By studying the changes in wave speed, scientists concluded that the Earth’s interior structure is layered.

3. The last piece of evidence has to do with the major layers of the Earth. From the inside moving out they are: inner core, outer core, mantle, and crust. Thermal convection is movement in a fluid caused by heating from below and cooling from above.

8 Mantle material is heated up deep inside the Earth and then rises to the crust. When the crust cools it becomes more dense and therefore sinks back down into the mantle.

Building Features on Earth’s Surface The building of the features on Earth’s surface due to deformation caused by plate movements is called plate tectonics. Plate movements create mountain ranges, trenches, rift valleys, and cause earthquakes and volcanoes.

 Subduction at plate boundaries forms trenches that can be 10,000 meters deep.

 Magma rises at subduction zones forming volcanic arcs which together are called the Ring of Fire. However, not all “hot spots” are located near the edges of plates but sometimes occur elsewhere like Yellowstone National Park where magma rises up close to the surface.

 Friction caused by plates subducting or sliding past each other cause earthquakes.

 Mountain ranges are formed when two plates are pushed together and instead of one subducting, they both thrust upward.

Density – the mass per unit volume of a material or substance.

Core – the solid, innermost part of the Earth consisting mainly of iron.

Thermal Convection – a pattern of movement in a fluid caused by heating from below and cooling from above.

Plate tectonics – the field of study of plate motion.

Continental Accretion – the growth of a continent along its edges.

Pangea – Earth’s most recent Supercontinent which was rifted apart about 200 million years ago.

9 Supercontinent – a large continent consisting of all of the Earth’s continental lithosphere. Supercontinents are assembled by plate-tectonic processes of subduction and continent- continent collision.

Paleomagnetism – the record of the past orientation and polarity of the Earth’s magnetic field recorded in rocks containing the mineral magnetite.

Review Questions 1. How is the density of the Earth calculated? Divide the mass of the Earth by the volume of the Earth.

2. Name the four major layers of the Earth from the exterior to the interior. Crust, Mantle, Inner and Outer Core

3. Why is the inner core of the Earth solid even though it is hot? Great pressure, iron melts at a lower temperature when under pressure.

4. How do convection currents work? Mantle material gets hot and rises to the surface where it cools and then sinks, this causes a continuous slow current.

5. What part of the Earth’s interior layers are in motion due to the density difference? Mantle asthenosphere

6. What geographic features would you expect to see at plate boundaries? (3 features) mountain ranges, trenches, rift valleys

7. In your own words, explain the process of continental accretion. (G103) ______

Section 1 Vocabulary ______Lithosphere Basalt Asthenosphere Crust Mantle Mid-Ocean Ridge Global Positioning System (GPS) ______

Divergent Plate Boundary Rift Valley Subduction Convergent Plate Boundary Suture Zone Transform Plate Boundary Transform Fault ______

Thermal convection Density Core Plate tectonics Pangea Supercontinent Continental accretion Paleomagnetism