Sea-Floor Spreading Diagram: Shows mid-ocean ridge formation, seafloor movement, and subduction.

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 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 a subduction zone (not explicitly shown in the diagram but implied), where it is forced back into the mantle due to the convergence of tectonic plates. This process is called subduction, and it recycles the old crust back into the mantle.

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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. Here are three key pieces of evidence:

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 magnetic field reversing over geological time scales. As new crust forms at the ridge, it records the Earth's magnetic field at the time of its formation. This creates a "bar code" of magnetic reversals 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 feature labeled "C" shows the interaction between the oceanic crust and the continental crust. This process is known as subduction.
- Why it occurs: Subduction occurs when an oceanic plate converges with a continental plate. Because the oceanic crust is denser than the continental crust, it is forced beneath the continent into the mantle. This process is driven by gravity and the dynamics of plate tectonics. Subduction zones are typically associated with deep ocean trenches (not explicitly shown in the diagram but implied) and can lead to the formation of volcanic arcs and mountain ranges on the overriding continental 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: As new material rises, old oceanic crust is pushed away from the ridge and eventually subducted back into the mantle.
4. Evidence for Sea-Floor Spreading:
- Magnetic stripes on the ocean floor.
- Age progression of oceanic crust away from the ridge.
- Heat flow measurements showing higher heat near the ridge.
5. Process at C: Subduction. It occurs because the denser oceanic crust is forced beneath the less dense continental crust due to gravitational forces.

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Boxed Final Answer:

\[
\boxed{
\begin{array}{l}
1. \text{Mid-ocean ridge} \\
2. \text{Sea-floor spreading; new crust forms as plates diverge.} \\
3. \text{Old crust is pushed away and eventually subducted.} \\
4. \text{Magnetic stripes, age of crust, heat flow.} \\
5. \text{Subduction; denser oceanic crust sinks under continental crust.}
\end{array}
}
\]