Educational worksheet on plate tectonics theory and evidence, including diagrams and questions.

Let’s go through each question one at a time, using what we know about plate tectonics and the diagrams shown.

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Question 1: Based on the theory of plate tectonics, would you expect to find any very old rock at an oceanic ridge? Explain why.

Look at the diagram for “Divergent Plate Boundary” — that’s where oceanic ridges are formed. At these boundaries, plates move apart, and magma rises from the mantle to fill the gap. This magma cools and becomes new oceanic crust.

That means the rock right at the ridge is brand new — it just formed! As you move away from the ridge, the rocks get older because they were formed earlier and have been pushed farther away over time.

So, no — you would NOT expect to find very old rock at an oceanic ridge. The oldest rocks near a ridge are still relatively young (geologically speaking), maybe tens of millions of years old, but not hundreds of millions or billions like some continental rocks.

✔ Final Answer for Q1: No, because oceanic ridges are where new crust is constantly being made. The rock there is always fresh and young.

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Question 2: Based on the theory of plate tectonics, would you expect to find any very old rock in an oceanic trench? Explain why.

Oceanic trenches form at convergent boundaries — especially where one oceanic plate dives under another (subduction). The diagram shows this too.

When an oceanic plate subducts, it sinks into the mantle and eventually melts. So even if the rock was old before it reached the trench, once it goes down, it doesn’t stay around long — it gets recycled.

Also, the trench itself is where the plate begins to bend downward — so the rock there is still part of the moving plate, which may be old… BUT — here’s the key point:

The *oldest* oceanic crust on Earth is only about 200 million years old — and that’s found far from ridges, near continents or trenches. But even then, when it reaches a trench, it’s about to disappear.

However — technically, yes, you CAN find old rock in or near an oceanic trench — because that’s where the oldest surviving oceanic crust ends up before it gets swallowed.

But wait — let’s think again. The question says “in an oceanic trench.” The trench is the deep valley where subduction happens. The actual rock sitting IN the trench might be sediment scraped off the sinking plate — which could include old material. Or it might be bits of the plate itself.

Actually, scientists have drilled into ocean floors and found that the oldest oceanic crust is located near trenches — because that’s where it hasn’t been destroyed yet.

Example: The western Pacific has some of the oldest ocean floor (~180–200 million years) — and it’s near trenches like the Mariana Trench.

So — YES, you can find very old rock in or near an oceanic trench — because that’s where the last remnants of old oceanic crust exist before they’re pulled into the mantle.

✔ Final Answer for Q2: Yes, because oceanic trenches are where the oldest oceanic crust ends up before it gets subducted. Some of the oldest ocean rocks on Earth are found near trenches.

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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 oceanic crust on Earth, and the Nazca Plate. The Nazca Plate has the deepest oceanic trench on Earth, the Peru-Chile Trench. What type of plate boundary is this? How do you know?

We’re told:
- Pacific Plate = old oceanic crust
- Nazca Plate = has the Peru-Chile Trench (deepest oceanic trench)
- They meet at a boundary

Trenches form at convergent boundaries, specifically where one oceanic plate subducts under another.

Since both plates are oceanic (Pacific and Nazca are both oceanic plates), and there’s a trench involved, this must be a convergent boundary with subduction.

Which plate is going under? Usually, the older, colder, denser plate sinks. The Pacific Plate is described as having “some of the oldest oceanic crust,” so it’s likely denser and will subduct under the Nazca Plate? Wait — hold on.

Actually, looking at real-world geography: The Nazca Plate is subducting UNDER the South American Plate — forming the Andes and the Peru-Chile Trench. But here, the question says the boundary is between the Pacific Plate and the Nazca Plate.

Wait — correction: In reality, the Pacific Plate and Nazca Plate meet along a divergent boundary (the East Pacific Rise) — but the question mentions the Peru-Chile Trench, which is actually between the Nazca Plate and the South American Plate.

Hmm — perhaps the question simplified things. Let’s stick to what’s given:

It says: “plate boundary between two tectonic plates: the Pacific Plate... and the Nazca Plate. The Nazca Plate has the deepest oceanic trench...”

If the Nazca Plate has the trench, that suggests the trench is on its edge — meaning something is subducting beneath it? Or it is subducting?

Actually, trenches form on the side of the overriding plate — so if the Peru-Chile Trench exists, it’s because the Nazca Plate is subducting under South America — not under the Pacific Plate.

This seems confusing — but let’s re-read carefully:

> “In the Pacific Ocean, there is a plate boundary between two tectonic plates: the Pacific Plate... and the Nazca Plate. The Nazca Plate has the deepest oceanic trench on Earth, the Peru-Chile Trench.”

This is factually inaccurate — the Peru-Chile Trench is not between the Pacific and Nazca Plates; it’s between the Nazca and South American Plates.

BUT — since this is a homework worksheet, we should answer based on the info GIVEN, not real-world accuracy.

Given:
- Two plates: Pacific and Nazca
- Nazca Plate “has” the Peru-Chile Trench → implies the trench is associated with the Nazca Plate’s boundary
- We’re to determine the type of boundary between them

If there’s a trench involved, and it’s between two oceanic plates, it’s likely a convergent boundary where one is subducting.

But which one? Older crust tends to subduct. Pacific Plate has “some of the oldest oceanic crust” → so it’s probably denser → so it would subduct under the Nazca Plate? That would make the trench on the Nazca side — which matches “Nazca Plate has the trench.”

Wait — no: When Plate A subducts under Plate B, the trench forms on Plate B’s side — the overriding plate.

Example: If Pacific Plate subducts under Nazca Plate → trench forms on Nazca Plate’s edge → so Nazca “has” the trench → fits!

Therefore: Convergent boundary, with Pacific Plate subducting under Nazca Plate.

But again — in reality, it’s the opposite — Nazca subducts under South America. But per the question’s setup, we’ll go with their description.

Alternatively — maybe the question meant to say the boundary is between Nazca and South America — but wrote Pacific by mistake.

To avoid confusion — let’s focus on clues:

Clue 1: There’s a trench → indicates convergent boundary (specifically subduction zone).

Clue 2: One plate has very old crust → likely the one that’s subducting (because old crust is cold and dense).

Clue 3: The trench is “on” the Nazca Plate → so Nazca is the overriding plate → Pacific is subducting.

Thus: Type of boundary = Convergent (subduction)

How do we know? Because trenches form at convergent boundaries where one plate dives under another. Also, the presence of old crust on one plate suggests it’s dense enough to sink.

✔ Final Answer for Q3: This is a convergent plate boundary. I know because there’s an oceanic trench, which only forms where one plate is forced under another. Also, the Pacific Plate has old, dense crust, making it likely to subduct under the Nazca Plate.

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Final Answers:

1. No, because oceanic ridges are where new crust is constantly being made. The rock there is always fresh and young.

2. Yes, because oceanic trenches are where the oldest oceanic crust ends up before it gets subducted. Some of the oldest ocean rocks on Earth are found near trenches.

3. This is a convergent plate boundary. I know because there’s an oceanic trench, which only forms where one plate is forced under another. Also, the Pacific Plate has old, dense crust, making it likely to subduct under the Nazca Plate.