Objectives

Summarize the evidence that led to the discovery of seafloor spreading.

Explain the significance of magnetic patterns on the seafloor.

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on the seafloor.

Explain the process of seafloor spreading.

Mapping the Ocean Floor

Until the mid-1900s, many scientists thought that the ocean floors were essentially flatand that oceanic crust was unchangingand was much older than continental crust.

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and was much older than continental crust.

Advances in technology during the 1940s and 1950s showed that all of these widely accepted ideas were incorrect.

How do we know what is on the ocean floor?

We have mapped the magnetic fields and the topography of the ocean floor using sonar and

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topography of the ocean floor using sonar and magnetometers.

Tools to Map the Ocean Floor

1. magnetometer

can detect small changes in magnetic

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can detect small changes in magnetic

fields.generated by ocean floor rocks.

2. sonar

can measure water depth and map

the topography of the ocean floor.

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Ocean-Floor Topography

These maps showed underwater mountain chains (ridges) and deep-sea trenches.

Ocean-Floor Topography

The deepest trench, the Marianas Trench, is more than 11 km deep. Mount Everest, the world’s tallest mountain, stands at 9 km

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world’s tallest mountain, stands at 9 km above sea level, and could fit inside the Marianas Trench with six Empire State buildings stacked on top.

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What evidence did these tools give us?

Concept 1. The age of oceanic crust increases with distance from a ridge.

Proof: The sediment is thicker the closer you are to the continents.you are to the continents.

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What evidence did these tools give us?

Concept 2: Magnetic north has changed over time.

Proof: The magnetic record is in the rocks! The study of this is called Paleomagnetism.

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Magnetism

Earth has a magnetic field generated by the flow of molten iron in the outer core. This field is what causes a compass needle to point to the North.

A magnetic reversal happens when the flow in the outer core changes, and Earth’s magnetic field changes direction.

What happens?

1. Lava comes up through the mid-ocean ridge.

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Paleomagnetism

2. Lava cools and the iron-bearing minerals crystallize.

3. As they crystallize, these minerals behave like tiny compasses and align with Earth’s magnetic field.

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Magnetism

What does this look like on the ocean floor?

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By matching the magnetic patterns on the seafloor with the known pattern of magnetic reversals on land, scientists were able to determine the age of the ocean floor from magnetic recording and to create maps of

Magnetic symmetry

magnetic recording and to create maps of the ocean floor.

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Seafloor Spreading

Seafloor spreading is the theory that explains how new ocean crust is formed at ocean ridges and destroyed at deep-sea at ocean ridges and destroyed at deep-sea trenches.

How Seafloor Spreading Occurs

1. Magma is hotter and less dense than surrounding mantle material

2. It is forced toward the surface of the crust along an ocean ridge.

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3. the two sides of the ridge spread apart, the rising magma fills the gap that is created.

4. When the magma solidifies, new ocean floor is added to Earth’s surface.

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Result of Seafloor Spreading

The cycle of spreading and the intrusion of magma continues the formation of ocean floor,

The newest crust is by the mid-ocean ridge.The newest crust is by the mid-ocean ridge.

The oldest crust is by the continents.

Activity

• Get a partner – sit in two desks next to each other.

• Practice pulling the two pieces of paper out of the “ridge” at the same SLOW speed.

• Get one colored marker.

Answer the questions based on the activity and your notes.