Seafloor Spreading

Seafloor Spreading

Middle School

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Learning Objectives

• Describe the key features of the ocean floor, including the mid-ocean ridge.

• Explain the theory of seafloor spreading and the role of Harry Hess.

• Differentiate between divergent, convergent, and transform plate boundaries and their effect on oceanic crust.

• Summarize the key pieces of evidence that support the theory of seafloor spreading.

Key Vocabulary

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Mid-ocean Ridge

A long, underwater mountain range where new oceanic crust is formed by volcanic activity.

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Seafloor Spreading

The process by which new oceanic crust forms at mid-ocean ridges and pushes older crust away.

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Subduction

The process where denser oceanic crust sinks below less dense continental crust at a convergent boundary.

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Divergent Boundary

A location where two tectonic plates are moving apart from each other, allowing magma to rise.

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Convergent Boundary

A location where two tectonic plates are colliding with each other, resulting in subduction.

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Transform Boundary

A location where two tectonic plates slide past each other in opposite directions horizontally.

Key Vocabulary

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Rifting

The process of the Earth's crust being pushed apart, creating a valley or new ocean floor.

Unveiling the Ocean Floor

• The ocean floor is not flat; it has many topographic features.

• Oceanic crust is significantly younger than continental crust.

• Older oceanic crust is constantly being destroyed.

Multiple Choice

Which statement best describes the surface of the ocean floor?

• It is a completely flat and sandy surface.

• It has many different topographic features.

• It is a smooth, gently sloping plain.

• It is uniform and without any mountains or valleys.

Multiple Choice

How does the age of oceanic crust compare to the age of continental crust?

• Oceanic crust is much younger than continental crust.

• Oceanic crust is slightly older than continental crust.

• Both types of crust are the same age.

• Continental crust does not have a measurable age.

Multiple Choice

Based on the information provided, what is the best explanation for why oceanic crust is so much younger than continental crust?

• Because new continental crust forms much faster.

• Because older oceanic crust is constantly being destroyed.

• Because the ocean's water protects the crust from aging.

• Because continental crust is much softer than oceanic crust.

Harry Hess and Seafloor Spreading

• Harry Hess developed the seafloor spreading hypothesis in the early 1960s.

• Seafloor spreading happens at mid-ocean ridges where molten rock from the mantle rises.

• Magma cools to form new oceanic crust, pushing the existing seafloor apart.

Multiple Choice

What is the central idea of the seafloor spreading hypothesis?

• The process where continents drift apart over time.

• The process where mountains are formed by colliding plates.

• The process where new oceanic crust is formed at mid-ocean ridges, pushing the existing seafloor apart.

• The process where old oceanic crust sinks back into the mantle.

Multiple Choice

What causes the formation of new oceanic crust at a mid-ocean ridge?

• The weight of the ocean water presses down on the seafloor.

• Underwater earthquakes shake the seafloor apart.

• Molten rock rises from the mantle and cools to form new crust.

• Continents moving on either side pull the seafloor apart.

Multiple Choice

If new oceanic crust forms at mid-ocean ridges during seafloor spreading, what pattern of crust age should we observe?

• The crust closest to the ridge is the newest, and it gets older farther away.

• The crust is the same age all along the seafloor.

• The crust closest to the continents is the newest.

• The age of the crust is random and shows no pattern.

Plate Boundaries: Creation, Destruction, and Sliding

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Convergent Boundaries

• Plates collide with each other at these boundaries.

• The denser oceanic crust sinks under the lighter continental crust.

• This crust recycling process is called subduction.

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Divergent Boundaries

• Tectonic plates move away from each other at these boundaries.

• Magma rises up from the mantle to fill in the gap.

• New oceanic crust is created as the magma solidifies.

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Transform Boundaries

• Two plates slide horizontally past one another at these locations.

• Crust is not newly created at this type of boundary.

• Existing crust is also not destroyed at this boundary.

Multiple Choice

Which statement best describes the three main ways tectonic plates interact at their boundaries?

• They can collide, move apart, or slide past each other.

• They only form volcanoes when they interact.

• They only cause mountains to form when they meet.

• They are static and do not interact with each other.

Multiple Choice

What is the key difference between what happens to the Earth's crust at convergent and divergent boundaries?

• New crust is formed at divergent boundaries, while existing crust is recycled at convergent boundaries.

• Crust is recycled at both convergent and divergent boundaries.

• New crust is created at transform boundaries, and recycled at convergent boundaries.

• Crust is neither created nor destroyed at divergent or convergent boundaries.

Multiple Choice

Based on the processes of crust creation and destruction, which location would be the most logical place to find the oldest oceanic crust?

• Near a convergent boundary, because this is where older crust is destroyed through subduction.

• Near a divergent boundary, because this is where new crust is actively being created.

• At a transform boundary, because the crust is not changed at these locations.

• Directly in the center of a continent, far from any plate boundary.

Evidence for Seafloor Spreading

• New oceanic crust is formed at mid-ocean ridges.

• The youngest rocks are found at the ridge, getting older further away.

• This process slowly pushes the continents apart over millions of years.

Multiple Choice

Where does the formation of new oceanic crust primarily occur?

• At mid-ocean ridges

• Along the coastlines of continents

• In deep ocean trenches

• At the center of continents

Multiple Choice

What is the relationship between the age of oceanic rocks and their distance from a mid-ocean ridge?

• The rocks are youngest at the ridge and get older as you move away.

• The rocks are oldest at the ridge and get younger as you move away.

• The age of the rocks is the same everywhere on the ocean floor.

• The age of the rocks changes randomly with distance from the ridge.

Multiple Choice

Based on the evidence provided, what is the long-term effect of continuous seafloor spreading on the continents located on either side of a mid-ocean ridge?

• The continents will be pushed further apart.

• The continents will be pulled closer together.

• The mid-ocean ridge will eventually disappear.

• The ocean between the continents will become shallower.

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Common Misconceptions

Misconception	Correction
The ocean floor is a flat, featureless plain.	The ocean floor has many features, like the mid-ocean ridge.
The seafloor is ancient, just like the continents.	Oceanic crust is much younger because it is continually recycled.
All plate boundaries create or destroy crust.	Crust is neither created nor destroyed at transform boundaries.

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Summary

• Seafloor spreading starts at mid-ocean ridges, creating new oceanic crust.

• Older crust is recycled back into the mantle through subduction.

• At transform boundaries, crust is neither created nor destroyed.

• Evidence includes continental fit, fossil records, and seafloor rock ages.

Poll

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