Sea Floor Spreading Lab

Sea Floor Spreading Lab:

Name: Date: Period:

Background:

In the last few decades, scientists have discovered both age and magnetic patterns in the seafloor, which is further evidence for plate tectonics. This evidence shows that new seafloor has been forming for millions of years at mid-ocean ridges throughout the oceans. Magma melted within the crust rises to the seafloor, cools and solidifies into new rock. In some places this new seafloor is pulled apart by movement of the asthenosphere under the plates, forming two rock masses which move slowly apart from the ridge. Geologists call this seafloor spreading.

Elsewhere, one plate slides under another, or subducts, and deep ocean trenches are formed. This is called a subduction zone and happens at converging boundaries. Rock on the subducting plate becomes part of the asthenosphere. Oceanic crust is denser and thinner than continental crust, so all seafloor rock is eventually destroyed in this way. Thus, the oldest seafloor rocks are only 180 million years old, but the oldest continental rocks, which cannot be pulled into the trenches, are as old as 4,000 million years, or 4 billion years.

Background Questions:

1. What pulls the new seafloor apart in opposite directions?______

2. Where are trenches formed? ______

3. How old is the oldest seafloor? ______

4. How old is the the oldest continental rocks? ______

5. Why are the continental rocks so old? ______

Procedure:

1. Cut out the strips from the paper and then from each other. Tape the strips together at the label point so that they face each other. 2. On the model base, cut the lines at A, B, and C. 3. Thread the strips up through slit B and out to slits A and C. 4. Pull the strips apart and watch what is occurring at the slits.

Analysis Questions:

1. Is the rock labeled A the oldest or youngest? ______Explain: 2. Is the mid-oceanic ridge at SLIT B or SLITS A& C? ______Underline one: At this boundary are plates coming together or pulling apart? Is this called converging or diverging? ______

3. Is the rock layer labeled B the oldest or youngest? ______

4. Is the subduction zone with ocean trenches at SLIT B or SLITS A& C? ______

Are the plates at this boundary converging or diverging? ______

5. New seafloor rock is continually being formed at mid-ocean ridges and destroyed at trenches in subduction zones. Rock is formed on the continents, but then pushed up into mountains. Which rock will be older, continental or ocean floor? ______Explain:

6. Ocean plates slide under continental plates. Which are denser, oceanic plates or continental plates? ______

Which are thicker (Review background material )? ______

7. Look at the map, which is part of the lab and look at the map key at the bottom. Pick a color for convergent boundaries and use it on the symbol in the key. Color all convergent boundaries that color on the map.

Five of these features are formed at convergent boundaries. Remember that earthquakes can happen wherever plates move. Circle the 5 features:

volcanic islands trench mid-ocean ridge earthquakes

volcanic mountain chain rift valley non-volcanic mountain range

8. Repeat step 7 for divergent boundaries.

Four of these features are always formed at divergent boundaries. Circle them:

volcanic islands trench mid-ocean ridge earthquakes

volcanic mountain chain rift valley non-volcanic mountain range

9. Color the transform boundaries a third color on the map and key.

10. What kind of mountains are formed when from the magma when 2 pieces of ocean crust separate, volcanic, or non-volcanic? ______

11. Circle the Mid-Atlantic Ridge, and Iceland on your map. What kind of boundary is this, divergent or convergent? ______12. One of the keys to plate tectonics was the discovery that the Earth's magnetic field has reversed its polarity 170 times in the last 80 million years. As new basaltic material is squeezed up into the mid-ocean cracks and solidifies, it is magnetized according to the polarity of the Earth's magnetic field. If the field reverses its polarity, the strip of new material is magnetized in an opposite sense. As the oceanic floor continues to spread, the new strips of rock are carried away on either side like a conveyer belt.

On your model, the alternating black and white bands show the reverses in polarity. Describe what it looks like on both sides of the Mid-Ocean Ridge and why it is that way?

Rock Layer A Rock Layer A

Rock Layer B Rock Layer B

Tape Here Tape Here Slit A

Slit B

Slit C