🌍 Week 2: Seafloor Spreading & Continental Drift 🧩

🏔️ The Challenge: What's on the Ocean Floor?

In the 1950s, a geologist named Marie Tharp was mapping the Atlantic Ocean floor using sonar depth measurements. Everyone assumed the ocean floor was flat. What she found shocked the world.

Look at the depth data in the form below. You are going to do what Tharp did — find the pattern hidden in the numbers.

✋ Stop & Think — Before You Open the Form

If you were in a submarine traveling away from the center of the Atlantic Ocean, what would you expect the depth of the water above you to do? Pick one prediction:

• Get deeper as you move away from the center
• Stay the same depth the whole way
• Start shallow in the middle, then get deeper toward the edges

Write your prediction on your worksheet, then open the form to see the real data.

📖 The Scenario

Scientists drilled rock samples from the ocean floor at different distances from the Mid-Atlantic Ridge. They measured the age of the rock and the magnetic polarity recorded in the iron minerals. The table below shows what they found on both sides of the ridge.

🔢 Step 1: Notice the Pattern on Each Side

Read down the "Left Side Polarity" column. The pattern is: N → R → N → R (alternating). Now read the "Right Side Polarity" column. The pattern is: N → R → N → R — exactly the same.

Key observation: The left side and right side are mirror images of each other. Every stripe on the left has an identical twin stripe at the same distance on the right.

🔢 Step 2: Connect Polarity to Earth's Magnetic History

Why does the polarity alternate? Earth's magnetic field has flipped direction many times in its history — this is called a magnetic reversal. When lava erupts at the ridge and cools into solid rock, the iron minerals inside lock in whichever direction Earth's field is pointing at that moment.

Think of it like a tape recorder: each new stripe of rock records a snapshot of Earth's magnetic field at the time it formed. Older rocks (farther from ridge) recorded older reversals.

🔢 Step 3: Use the Pattern to Prove Spreading

Look at the ages in the table. The rock at the ridge is 0–1 million years old. Rock at 150 km is 14–16 million years old. The rock gets older as you move away from the ridge.

This means the ridge is constantly making new rock. Old rock gets pushed outward. The seafloor is literally spreading apart — like a conveyor belt moving in both directions from the center.

🔢 Step 4: State the Claim

Here is how a scientist would write up this finding:

"The symmetric magnetic stripe pattern on both sides of the Mid-Atlantic Ridge, with rock age increasing with distance from the ridge, provides evidence that new seafloor is continuously formed at the ridge and spread outward in both directions. This supports the hypothesis of seafloor spreading."

⚠️ Common Mistake — Don't Fall for This!

Wrong thinking: "The stripes formed randomly, and it's just a coincidence that the two sides look similar."

Why it's wrong: Random processes don't produce perfect mirror images across thousands of kilometers. The symmetry is exact because both sides of the ridge are made of the same rock that split apart. One piece went left, one piece went right — they started as identical twins.

The rule: Symmetry = the ridgeline is the center where new crust forms. Period.

✏️ Now You Try

Imagine a new ridge where scientists collect the following data:

Fill in the "?" values and write one sentence explaining why you chose those values. Record your answer on your worksheet before moving to Station 1.

💻 Simulation: Pangaea Puzzle

🧩 What You Are Doing

You just learned in the Worked Example how to read magnetic stripe patterns. Now you will apply that skill to real data from the Mid-Atlantic Ridge. You will decide whether the evidence supports or does not support the claim that seafloor spreading is happening.

Remember: good scientists look at multiple features of the data — the pattern of stripes, the symmetry between sides, and how age changes with distance.

📊 Reference: Magnetic Stripe Data — Mid-Atlantic Ridge

Near continents (2,000+ km from ridge), rock can be up to 200 million years old.

💡 Support: Sentence Starters

Use these if you are not sure how to write your answers:

• "The magnetic stripe pattern on the left side is _____ and the right side is _____, which shows that the pattern is [symmetric / asymmetric]."
• "As distance from the ridge increases, the rock age [increases / decreases / stays the same]. This supports seafloor spreading because _____."
• "This evidence [supports / does not support] the claim that new crust is forming at the ridge because _____."

💻 The Big Idea

About 200 million years ago, all of Earth's continents were joined into one giant landmass called Pangaea. They slowly drifted apart — and they are still moving today. How do scientists know this? Not from one piece of evidence, but from five completely different types that all point to the same conclusion.

Study the table below carefully. You will use this information to answer the form questions.

📊 Five Lines of Evidence for Continental Drift

💡 Support: Sentence Starters for Your Arguments

• "The _____ evidence supports continental drift because _____."
• "If the continents had never been connected, we would not expect to find _____ in both [location A] and [location B]."
• "The strongest evidence is _____ because _____. The weakest evidence alone would be _____ because _____."
• "Using multiple types of evidence is important because one piece of evidence could be [coincidence / explained another way], but when all five agree, _____."

🛠️ The Engineering Challenge

We know plates are moving. Scientists have proved it with magnetic stripes, fossils, and matching coastlines. But those are all clues from the past. Can you measure plates moving right now?

Your task: design a monitoring system to detect plate movement at three different plate boundaries. You have a budget and precision requirements to work within.

📊 Available Technologies

Hint: You can mix technologies! Not every boundary needs the same tool.

🎓 Before You Submit

You have done a lot of thinking today. The exit ticket will ask you to pull it all together. Here is what you should be able to explain on your own:

• Why magnetic stripe symmetry is evidence for seafloor spreading
• How rock age patterns at ridges support the idea that new crust is forming
• Why scientists need multiple lines of evidence to support a big claim like continental drift
• How spreading at ridges and subduction at trenches keep Earth's total crust area roughly constant

If you are not sure about any of these, look back at your notes from today before starting the ticket.