Evidence of Plate Tectonics | Worksheet - Free Printable

Certainly! Let’s solve each of the three questions based on the Theory of Plate Tectonics and the provided diagram.

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Question 1:

> Based on the theory of plate tectonics, would you expect to find any new rock or very old rock in an oceanic ridge? Explain why.

✔ Answer:

You would expect to find new rock at an oceanic ridge.

🔹 Explanation:
Oceanic ridges (also called mid-ocean ridges) are divergent boundaries, where tectonic plates move away from each other. As the plates separate, magma from the mantle rises to fill the gap. This magma cools and solidifies to form new oceanic crust — primarily basaltic rock. Therefore, the rocks found at or near the ridge are the youngest in the ocean floor.

As you move away from the ridge toward the continents, the oceanic crust gets progressively older — which is strong evidence for seafloor spreading, a key part of plate tectonics.

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Question 2:

> Based on the theory of plate tectonics, would you expect to find any very new rock or very old rock in an oceanic trench? Explain why.

✔ Answer:

You would expect to find very old rock in an oceanic trench.

🔹 Explanation:
Oceanic trenches are convergent boundaries, typically where an oceanic plate is being subducted (pushed down) beneath another plate (either oceanic or continental). The oceanic plate that is being subducted has been moving away from its birthplace (the mid-ocean ridge) for millions of years — sometimes over 100–200 million years.

Therefore, by the time it reaches the trench and begins to subduct, it is old oceanic crust. In fact, the oldest oceanic crust on Earth (about 200 million years old) is found near trenches like those around the Pacific Ocean (e.g., Mariana Trench).

New rock does not form at trenches — instead, old rock is destroyed as it sinks into the mantle.

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Question 3:

> In the Pacific Ocean, there is a plate boundary between two tectonic plates, the Pacific Plate, which contains some of the oldest ocean crust on Earth, and the Mariana Plate. The Mariana Trench, the deepest oceanic trench on Earth, is located at this boundary. There is an arc of volcanic islands called the Mariana Islands. Based on this information, what type of plate boundary is this? How do you know?

✔ Answer:

This is a convergent plate boundary — specifically, an oceanic-oceanic convergent boundary.

🔹 How we know:
- The presence of the Mariana Trench — the deepest trench on Earth — indicates subduction, where one plate is forced under another.
- The Mariana Islands are a volcanic island arc — these form when magma rises due to melting of the subducting plate and erupts at the surface, creating a chain of volcanoes parallel to the trench.
- The Pacific Plate contains very old oceanic crust, which is typical of plates that have been moving away from their ridge for a long time and are now being subducted.
- The Mariana Plate is smaller and likely being overridden by the Pacific Plate, leading to the formation of the trench and volcanic arc.

These features — deep trench + volcanic island arc — are classic indicators of an oceanic-oceanic convergent boundary.

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✔ Summary:

| Question | Answer |
|----------|--------|
| 1 | New rock — because oceanic ridges are sites of seafloor spreading where magma creates new crust. |
| 2 | Very old rock — because trenches are where ancient oceanic plates are subducted after traveling far from their origin. |
| 3 | Convergent boundary — evidenced by the deep trench and volcanic island arc, which result from subduction. |

Let me know if you’d like a diagram explanation or further details!