Science Chapter 7 Lesson 1 and Lesson 3

What do plate boundaries in oceans mark?

Plate boundaries in oceans mark the mid-ocean ridge.

Continental Drift

A hypothesis by Alfred Wegener which suggested that continents are in constant motion on the surface of Earth.

Reversed Polarity

A state in which magnetized objects would reverse direction and orient themselves to point south.

Normal Polarity

A state in which magnetized objects, such as compass needles, will orient themselves to point north.

Plate Tectonics

A theory that states that Earth's surface is made of rigid slabs of rock, or plates, that move with respect to each other.

Tectonics

A word geologists use to describe the forces that shape Earth's surface and the rock structures that form as a result.

Pangaea

All the continents that were once part of a supercontinent.

What is an abyssal plain?

An abyssal plain is a layer of sediment that is thick enough to make the seafloor smooth, and it is far from the mid-ocean ridge. However, close to the mid-ocean ridge, there is almost no sediment.

Explain how rocks on the seafloor record a magnetic signature.

Basalt on the seafloor contains iron-rich minerals that are magnetic. Each mineral acts like a small magnet. These magnetic minerals align themselves with Earth's magnetic field. When lava erupts from a vent along a mid-ocean ridge, it cools and crystallizes, and this permanently records the direction and orientation at the time of the eruption. Scientists have discovered parallel patterns in the magnetic signature of rocks on either side of a mid-ocean ridge.

What causes convection?

Convection currents occur because radioactive elements heat Earth's interior. When materials such as solid rock are heated, they expand and become less dense, causing hot mantle material to rise upward and come in contact with the crust. Thermal energy is transferred from the hot mantle material to the colder surface above. As the mantle cools, it becomes denser and then sinks, forming a convection current.

Explain why magnetic stripes on the seafloor are parallel to the mid-ocean ridge.

Magnetic stripes on the seafloor are parallel to the mid-ocean ridge because when lava erupts from the mid-ocean ridge, it creates basalt, which is a magnetic rock. Since the rock originally came from the mid-ocean ridge and it magnetized when it formed, the magnetic stripes are parallel to the mid-ocean ridge.

Is Earth's magnetic field currently normal or reversed polarity. How often does Earth's magnetic field change direction, on average?

Earth's magnetic field is currently described as having normal polarity. Magnetic reversals occur every few hundred thousand to every few million years.

What are Earth's outermost layers called?

Earth's outermost layers are called the lithosphere, which is made up of the crust and the solid, uppermost mantle.

How are earthquakes and volcanoes related to the theory of tectonic plates?

Earthquakes and volcanoes are related to the theory of tectonic plates because when plates separate, collide, or slide past each other along a plate boundary, stress builds. A rapid release of energy can result in earthquakes. Volcanoes form where plates separate along a mid-ocean ridge or a contiental rift or collide along a subduction zone.

How do mountain ranges form along the mid-ocean ridge?

Mountains form along the mid-ocean ridge when large amounts of lava erupt from the center of the ridge, cool, and build up around the ridge. Or, as the lava cools and forms new crust, it cracks. The rocks move up or down along these cracks in the seafloor, forming jagged mountain ranges.

Examine the evidence used to develop the theory of plate tectonics. How has new technology strengthened the theory?

New technology has strengthened the theory of plate tectonics by explaining how the continents move and how fast they move. For example, the GPS can measure the movement of the continents and track how far they move every year, and an echo sounder measures the depth and topography of the seafloor, which can help scientists figure out where mid-ocean ridges are located. Also, a magnetometer was used by scientists to measure and record the magnetic signature of rocks on the seafloor, and that led them to finding parallel stripes on both sides of the mid-ocean ridge, and that confirmed that the crust formed at the ridge is being carried away from it. Finally, an anisotropy is a new technology that scientists are using to create 3-D images of of seismic wave velocities in a subduction zone.

Why did scientists argue against Wegener's continental drift hypothesis?

Scientists argues against Wegener's continental drift hypothesis because he wasn't able to explain how the continents moved, and he wasn't able to measure how fast they moved because it is such a slow process.

Describe how seafloor spreading helps to explain the continental drift hypothesis.

Seafloor spreading helps to explain the continental drift hypothesis because when the seafloor moves, the continent(s) on top of it move as well. Seafloor spreading is powerful enough to move continents, and that helps explain the continental drift hypothesis because is explains how continents move.

How were similar rock types used to support the continental drift hypothesis?

Similar rock types were used to support the continental drift hypothesis because the Caledonian mountain range in northern Europe and the Appalachian Mountains in eastern North America would fit together and form one mountain range if the two continents were places side by side. Also, there were volcanic rocks found on the western coast of Africa and the eastern coast of South America that are identical in both chemistry and age.

Why do tectonic plates move?

Tectonic plates move because of convection currents in the mantle.

Subduction Zone

The area where a denser plate descends into Earth along a convergent plate boundary.

Convection

The circulation of material caused by differences in temperature and density.

How did the climate in Antarctica change between 280 and 225 million years ago?

The climate in Antarctica changed between 280 and 225 million years ago from being cold and covered with glaciers to warm and wet.

Lithosphere

The cold and rigid outermost rock layer of Earth. It is made up of the crust and the solid, uppermost mantle.

Explain the continental drift hypothesis and the evidence used to support it.

The continental drift hypothesis is a hypothesis proposed by Alfred Wegener that suggests that all continents are in constant motion on the surface of Earth. The four pieces of evidence used to support the hypothesis are that the continents seem to fit together like puzzle pieces, there are fossils of the same plants and animals on continents that are now separated by oceans, there are rock formations and mountain ranges on different continents that are similar in age and structure and are composed of the same rock type, and climate clues of different continents and regions. For example, the fossil of the Glossopteris, which is a plant, have been discovered in rocks from South America, Africa, India, Australia, and Antarctica. But now, these continents are separated by oceans, and the plant's seeds couldn't have traveled that far. So, the continents must have previously been together and then split apart in order for this to have happened. Also, the Glossopteris lived in a swampy environment, which means that Antarctica had a warm and wet climate long ago. An example of the same rocks on different continents are the Caledonian mountain range in northern Europe and the Appalachian Mountains in eastern North America.They are composed of the same rock types, and they are similar in age and structure. If you placed the two continents next to each other, these mountains would meet and form one long, continuous mountain belt. Also, volcanic rocks have been found on the east coast of South America and the west coast of Africa that are the same in chemistry and age. This shows that Africa and South America were most likely connected some time in the past when these volcanic explosion occurred. An example of climate clues are that Wegener discovered glacial grooves in continents that are now warm and he discovered that the climate of the Southern Hemisphere was much cooler when Pangaea existed. This suggests that the continents were closer to Antarctica and glaciers from Antarctica slid across to other continents, which means that the continents had to have been connected to Antarctica.

What is the evidence used to support the seafloor spreading theory?

The evidence used to support the seafloor spreading theory are magnetic minerals on the seafloor, the high thermal energy near mid-ocean ridges, and dated sediment on the seafloor. Magnetic minerals help to support the seafloor spreading theory because the magnetic stripes are parallel to the mid-ocean ridge and each magnetic stripe represents crust that formed and magnetized at a mid-ocean ridge during a period of either normal or reversed polarity. The pairs of magnetic stripes confirm that the ocean crust formed at mid-ocean ridges is carried away from the center of the ridge in opposite directions. Also, by drilling a hole into the seafloor, scientists can measure the amount of thermal energy leaving Earth, and the measurements show that more thermal energy leaves Earth near mid-ocean ridges than is released from beneath the abyssal plains. Finally, sediment from the seafloor is dated; so sediment closer to the mid-ocean ridge is younger than sediment farther away. Also, the thickness of sediment increases with distance away from the ridge.

Basal Drag

The force produced by convection currents in the mantle.

Where is the lithosphere thick and where is it thin?

The lithosphere is thin below mid-ocean ridges and thick below continents.

Mid-Ocean Ridge

The mountain ranges in the middle of oceans.

WHAT IS THE THEORY OF PLATE TECTONICS?

The plate tectonics theory states that Earth's surface is made up of rigid slabs of rock, or plates, that move with respect to each other. Continental drift was a theory proposed by Alfred Wegener that suggested that the continents are in constant motion on the surface of the Earth. Seafloor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and older crust moves away from the ridge. Some evidence that was used to support the theory of plate tectonics was the fact that most earthquakes occurred and volcanoes and mountains formed along plate boundaries, which proves that the plates are constantly colliding, separating, and sliding past each other. Also, the theory of seafloor spreading helps to support the theory of plate tectonics because it explains how the continents move. Furthermore, a GPS satellite in space can be used to measure how far and how fast the continents move each year. Also, convection currents in the mantle help explain the forces that move tectonic plates. Finally, the interaction between the plastic mantle and the lithosphere help to explain plate tectonics because the lithosphere "floats" on top of the asthenosphere.

Seafloor Spreading

The process by which new ocean crust forms along a mid-ocean ridge and older crust moves away from the ridge.

Why does the theory of plate tectonics continue to change?

The theory of plate tectonics continues to change because there are still so many unanswered questions about it.

What are the three types of plate boundaries and what does each of them do? What are the effects of the three types of plate boundaries?

The three types of plate boundaries are convergent, divergent, and transform. Convergent plates collide, divergent plate separate, and transform plates slide past each other. When two plates collide, the denser plate sinks below the more buoyant one in a process called subduction. When an oceanic plate and a continental plate collide, the denser oceanic plate subducts under the continental plate. This creates a deep ocean trench. A line of volcanoes form above the subduting plate on the edge of the continent. This process can also occur when two oceanic plates collide; and this forms a deep ocean trench and a line of volcanoes called an ocean arc. When two continental plates collides, neither plate subducts and and mountains such as the Himalayas in southern Asia form from uplifted rock. When two plates separate, the seafloor spreads at a mid-ocean ridge, and lava erupts, cools, and forms new oceanic crust. Divergent plate boundaries can also exist in the middle of the continent. They pull continents apart and form rift valleys. The East African Rift is an example of a continental rift. When two plates slide past each other, the plates can get stuck and stop moving. Stress builds up where the plates are "stuck." Eventually, the stress is too great and the rocks break, suddenly moving apart. This results in a rapid release of energy as earthquakes. An example of transform plate boundaries is the San Andreas Fault.

Subduction

This happens during converging plates when the denser plate sinks below the more buoyant plate.

Slab Pull

When a slab sinks and pulls on the rest of the plate with a force.

Magnetic Reversal

When the magnetic field reverses direction.

Ridge Push

When the rising mantle material at mid-ocean ridges creates the potential for plates to move away from the ridge with a force.

Explain the process of seafloor spreading.

When the seafloor spreads, the mantle below melts and forms magma. Because magma is less dense than solid mantle material, it rises through cracks in the crust along the mid-ocean ridge. When magma erupts on Earth's surface, it is called lava. As this lava cools and crystallizes on the seafloor, it forms a type of rock called basalt. Because the lava erupts into water, it cools rapidly and forms rounded structures called pillow lavas. As the seafloor continues to spread apart, the older oceanic crust moves away from the mid-ocean ridge. The closer the crust is to a mid-ocean ridge, the younger the oceanic crust is. The oldest oceanic crust is farthest away from the mid-ocean ridge and it eventually gets melted during subduction and it is recycled.

Convergent Plate Boundary

When two plates collide.

Divergent Plate Boundary

When two plates separate from each other, causing the seafloor to spread at a mid-ocean ridge and for lava to erupt, cool, and form new oceanic crust.

Transform Plate Boundary

When two plates slide past each other. The San Andreas Fault was produced by transform plate boundaries and they often cause earthquakes.