Sea-Floor Spreading: A diagram showing the formation of new oceanic crust at a mid-ocean ridge, the spreading of the ocean floor, and subduction at a convergent boundary.

Problem: Sea-Floor Spreading

The task involves analyzing the provided diagram of sea-floor spreading and answering questions related to the process. Below is a detailed explanation and solution for each question.

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Question 1: Name and describe the feature of the ocean floor shown at A.

#### Solution:
- Feature at A: The feature labeled as "A" in the diagram is a mid-ocean ridge.
- Description: A mid-ocean ridge is a mountainous underwater range where tectonic plates diverge (move apart). It is formed by the upwelling of magma from the mantle, which cools and solidifies to form new oceanic crust. This process is a key component of sea-floor spreading.

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Question 2: Describe the process shown occurring at B, and explain what results from this.

#### Solution:
- Process at B: The arrows at "B" indicate the movement of tectonic plates away from the mid-ocean ridge. This movement is known as sea-floor spreading.
- Explanation: As the tectonic plates move apart, magma rises from the mantle into the gap created by the divergence. The magma cools and solidifies, forming new oceanic crust. Over time, this process leads to the continuous creation of new oceanic crust and the widening of the ocean basin.

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Question 3: What happens to old material as new material rises from the mantle?

#### Solution:
- What happens to old material: As new material rises from the mantle and forms new oceanic crust at the mid-ocean ridge, the older crust is pushed outward and away from the ridge. Over time, the oldest oceanic crust moves toward the edges of the ocean basin.
- Fate of old material: Eventually, the old oceanic crust reaches subduction zones (like the one shown at "C" in the diagram) where it is forced back into the mantle due to the convergence of tectonic plates. Here, the old crust is recycled through the process of subduction, melting, and potentially re-emerging as part of new volcanic activity.

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Question 4: The arrows on the figure show the ocean floor spreading from the ridge. What are three kinds of evidence scientists have found to support this idea?

#### Solution:
Scientists have gathered several lines of evidence to support the theory of sea-floor spreading. Three key pieces of evidence include:

1. Magnetic Stripes on the Ocean Floor:
- Scientists discovered that the ocean floor has symmetrical magnetic stripes parallel to the mid-ocean ridges. These stripes result from the Earth's periodic reversals of its magnetic field. As new crust forms at the ridge, it records the current magnetic orientation of the Earth. When the magnetic field reverses, the new crust records the opposite orientation. This creates alternating bands of normal and reversed magnetism on either side of the ridge, providing strong evidence for sea-floor spreading.

2. Age of Oceanic Crust:
- Studies using radiometric dating techniques have shown that the oceanic crust is youngest at the mid-ocean ridges and becomes progressively older with increasing distance from the ridge. This pattern directly supports the idea that new crust is continuously being formed at the ridges and moving outward.

3. Heat Flow Measurements:
- Heat flow measurements reveal that the highest heat flow occurs near the mid-ocean ridges, where hot magma is rising from the mantle. As you move away from the ridge, the heat flow decreases because the crust cools over time. This gradient in heat flow provides further evidence for the active upwelling and cooling processes associated with sea-floor spreading.

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Question 5: What process is shown occurring at C, and why does it occur?

#### Solution:
- Process at C: The process shown at "C" is subduction.
- Explanation: Subduction occurs when an oceanic plate converges with another plate (either oceanic or continental) and is forced beneath it into the mantle. This happens because oceanic crust is denser than continental crust. As the oceanic plate sinks into the mantle, it undergoes partial melting, releasing water and other volatiles. This process can lead to the formation of volcanoes and deep-sea trenches above the subducting plate.

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Final Answers:

1. Feature at A: Mid-ocean ridge. It is a mountainous underwater range where tectonic plates diverge, and new oceanic crust is formed by the upwelling of magma.
2. Process at B: Sea-floor spreading. Tectonic plates move apart, allowing magma to rise and cool, forming new oceanic crust.
3. Old Material: Old oceanic crust is pushed outward and eventually subducted back into the mantle at convergent boundaries.
4. Evidence for Sea-Floor Spreading:
- Magnetic stripes on the ocean floor.
- Age progression of oceanic crust away from the mid-ocean ridges.
- Heat flow measurements showing higher heat near the ridges.
5. Process at C: Subduction. It occurs because the denser oceanic crust is forced beneath another plate (oceanic or continental) into the mantle.

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Boxed Final Answer:

\[
\boxed{
\begin{aligned}
1. & \text{ Mid-ocean ridge} \\
2. & \text{ Sea-floor spreading} \\
3. & \text{ Old material is pushed outward and subducted} \\
4. & \text{ Magnetic stripes, age of crust, heat flow measurements} \\
5. & \text{ Subduction; occurs due to the density difference between oceanic and continental crust}
\end{aligned}
}
\]