Inside Earth, Chapter 1, Sections 4 & 5: Seafloor Spreading and Plate Tectonics

What are three ways in which Earth's surface is changed along plate boundaries?

Volcanoes, mountain ranges, and deep ocean trenches.

What is a lithospheric plate?

A section of the lithosphere that slowly moves over the athenosphere, carrying pieces of continental and oceanic crust.

What feature form where two continental plates collide?

Mountain range.

What is seafloor spreading?

The process by which molten material from the mantle adds new crust to the oceanic floor. The seafloor spreads apart along both sides of mid-ocean ridges as new crust is added.

What is a fault?

A break in Earth's crust where masses of rock slip past each other.

What happens at convergent boundaries?

A collision.

What is a divergent boundary?

A plate boundary where two plates move away from each other.

Where do faults form?

Along plate boundaries.

Why does the ocean floor have a pattern of magnetic stripes? How does this support seafloor spreading?

As new molten rock cooled, iron within it would line up with Earth's magnetic poles, recording within the rock reversals in Earth's magnetic field. Stripes of rock that formed when the magnetic field pointed north alternate with stripes that formed when the magnetic field pointed south. The pattern is the same on both sides of mid-ocean ridges.

True or False. Crust is created at transform boundaries?

False. Crust is neither created not destroyed at transform boundaries.

Who first proposed the theory of seafloor spreading?

Harry Hess (an American geologist).

Along what feature of the ocean floor does seafloor spreading begin?

Mid-ocean ridges.

Why is the Pacific Ocean shrinking?

Subduction at the many trenches that ring the Pacific Ocean is occurring faster than new crust can be added.

What is e theory of plate tectonics?

The theory that pieces of Earth's lithosphere are in constant motion, driven by convection currents in the mantle. It explains the formation, movement, and subduction of Earth's plates.

True or False. Pangaea formed when Earth's landmasses drifted together around 260 million years ago.

True.

Define scientific theory.

A well-tested concept that explains a wide range of observations.

What is a mid-ocean ridge?

An undersea mountain chain where new ocean floor is produced. It is also a divergent plate boundary.

Where is oceanic crust the densest?

At deep ocean trenches.

Where is the oldest rock in the ocean floor?

At deep ocean trenches.

Where is oceanic crust the least dense?

At mid-ocean ridges.

Where is the youngest rock in the ocean floor?

At mid-ocean ridges.

Where do most divergent boundaries occur?

At mid-ocean ridges. But they can also occur on land (example: the Great Rift Valley in Africa).

How was sonar used in understanding the ocean floor?

It was used to map mid-ocean ridges.

Who first proposed the theory of plate tectonics?

J. Tuzo Wilson (a Canadian scientist).

How does new oceanic crust form?

Molten material from several kilometers beneath the ocean floor rises and erupts through the valleys that run along the center of mid-ocean ridges. As it cools, it forms a strip of solid rock in the center of the ridge. When new molten material erupts, the older rock moves outward on both sides of the ridge, adding to the oceanic crust.

Why are deep ocean trenches so deep?

New oceanic crust cools and becomes more dense as it moves away from mid-ocean ridges. As its density increases, it begins to sink. Eventually, gravity pulls this older, denser oceanic crust down beneath the trench.

What happens when two plates carrying continental crust collide?

The crust is squeezed into mountain ranges. Neither plate is dense enough to sink very far into the mantle.

What happens when two plates carrying oceanic crust collide?

The denser plate sinks under the other plate.

What happens at deep ocean trenches?

The oceanic crust sinks back into the mantle (subduction).

When two plates collide, what determines which plate comes out on top?

The plate that is more dense sinks under the other plate.

What is subduction?

The process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.

How did drilling samples from the ocean floor provide evidence of seafloor spreading?

They showed that the age of the rock increased the further from the mid-ocean ridge the sample was taken. The youngest rocks were always at the center of the mid-ocean ridge.

True or False. Rift valleys form at divergent plate boundaries.

True.

True or False. Some mid-ocean ridges are split by a steep-sided valley.

True.

True or false. At times in the past, a compass needle would have point south.

True.

True or False. Pangaea was the first supercontinent to form on Earth.

False. Before Pangaea, other supercontinents formed and broke up over billions of years.

True or False. Pangaea began to break up around 260 million years ago.

False. It began to break up around 225 million years ago. It formed 260 million years ago.

True or False. Mid-ocean ridges are completely underwater.

False. Most are underwater, but a few poke out (example: Ireland).

True or false. Plates can carry continents or part of the ocean floor, but not both.

False. Some carry both.

True or False. At mid-ocean ridges, subduction allows oceanic crust to sink back into the mantle.

False. Subduction occurs at deep ocean trenches.

True or False. The Atlantic Ocean is shrinking.

False. The Atlantic Ocean is expanding. The distance between New York and London grows a few centimeters every year.

True or False. The Pacific Ocean is expanding.

False. The Pacific Ocean is shrinking.

True or False. Mid-ocean ridges are found only below the Pacific Ocean.

False. The extend into all Earth's oceans.

True or false. Plates move several meters per year.

False. The move very slowly, only about 1 to 24 centimeters per year.

How did scientists discover evidence of eruptions at mid-ocean ridges?

In the 1960s, scientists dived to the deep ocean floor in a special submarine. Along the mid-ocean ridge, they saw strange-shaped rocks (shaped like pillows and toothpaste squirts).

Why are the oldest rocks in the ocean floor no more than around 200 million years old?

That's how long it takes for new oceanic crust formed at a mid-ocean ridge to move across the floor and sink into a trench.

Why is the Atlantic Ocean expanding?

The Atlantic Ocean has only a few short trenches, so the spreading floor has nowhere to go. As the floor moves, the continents on its edges also move, widening the ocean.

What are three types of evidence for seafloor spreading?

(1) Evidence of eruptions of molten material at mid-ocean ridges; (2) Magnetic stripes in the rock of the ocean floor, showing periods of pole reversals; and (3) Ages of the rock making up oceanic crust.

What is a deep ocean trench?

A deep valley along the ocean floor beneath which oceanic crust slowly bends downward and sinks back into the mantle.

What forms along divergent boundaries?

A deep valley called a "Rift Valley".

What is a rift valley?

A deep valley that forms where two plates move apart.

What is sonar?

A device that determines the distance of underwater objects by bouncing sound waves off the object. The time it takes for the echo to return indicates how far away the object is.

What is a transform boundary?

A plate boundary where two plates move past each other in opposite directions.

What is a convergent boundary?

A plate boundary where two plates move toward each other.

What do scientist think is the major force behind plate movement?

Convection currents in the mantle. As gravity pulls one edge of a plate down into the mantle, the rest of the plate also moves.

What are the three types of plate boundaries?

Divergent, convergent, and transform.

What often occurs along transform boundaries?

Earthquakes.

The ocean floor is renewed every ______ years.

Every 200 million years. That's how long it takes new rock to formed at the mid-ocean ridge to move across the ocean and sink into a trench.

According to Tuzo's theory, the lithosphere is broken into separate pieces called __________, which are separated by cracks.

Plates.

What happens when a plate carrying oceanic crust collides with a plate carrying continental crust?

Subduction. Continental crust is less dense than oceanic crust, so it can't sink under oceanic crust. The oceanic crust sinks beneath the continental plate.