What this lesson teaches
I can use rock ages and sediment evidence to explain how seafloor spreading supports plate tectonic theory.
Syllabus K229, Topic 3.1, Key Question 2. How does seafloor spreading support the plate tectonic theory?
Make a guess
Scientists collect rock samples moving away from a mid-ocean ridge. Where do they find the youngest rocks?
- At the ridge itself.
- Spread randomly across the ocean floor.
- Farthest from the ridge, near the continents.
- On one side of the ridge, where it spreads.
Show the answer
At the ridge itself.
New crust forms where magma rises and cools at the ridge. Older crust is pushed away, so rocks get older with distance on both sides.
New ocean crust forms at mid-ocean ridges and moves away, so rocks get older the farther they are from the ridge.
Where plates move apart under the ocean, a mid-ocean ridge forms. Hot mantle rises, and some of it melts. Magma rises through cracks, cools and becomes new ocean crust.
As more crust forms, older crust is pushed away. So the youngest rocks are at the ridge. Rocks get older with distance on both sides. This pattern is evidence for seafloor spreading.
Put these in order
Put the steps of seafloor spreading in order.
- Some of the rising mantle melts into magma.
- Old ocean crust sinks back into the mantle at trenches.
- Plates move apart under the ocean, and hot mantle rises below the gap.
- Older crust is pushed away on both sides, so rocks get older with distance.
- Magma rises through cracks, cools and becomes new ocean crust at the ridge.
Show the answer
- Plates move apart under the ocean, and hot mantle rises below the gap. As the hot mantle rises, the pressure on it falls, so some of it melts.
- Some of the rising mantle melts into magma. Magma is lighter than the rock around it, so it rises through cracks towards the seafloor.
- Magma rises through cracks, cools and becomes new ocean crust at the ridge. More new crust keeps forming at the ridge, so the crust made earlier is pushed aside.
- Older crust is pushed away on both sides, so rocks get older with distance. The oldest ocean crust ends up far from the ridge, where it can meet another plate.
- Old ocean crust sinks back into the mantle at trenches.
Mud, or sediment, slowly settles on the seafloor. Young crust near the ridge has little of it. Older crust has more. Old ocean crust sinks back into the mantle at trenches, so ocean crust is always fairly young. That limits how much history its sediment can hold. Sediment supply also varies, so combine sediment evidence with rock ages and magnetic stripes for a stronger explanation.
- Mid-ocean ridge
- Where new ocean crust forms.
- Youngest rocks
- At the ridge; older on both sides.
- Trenches
- Where old ocean crust is recycled into the mantle.
Step by step
New crust forms
Magma cools at the ridge into young basalt.
Older crust moves away
Spreading carries it away from the ridge.
The pattern records movement
On the diagram, ages rise from 0 to 5, 10 and 15 million years away from the ridge.
Real examples to use
Mid-Atlantic Ridge spreading rate (USGS)
The Mid-Atlantic Ridge spreads at an average of about 2.5 cm a year, or 25 km in a million years. So the Atlantic Ocean is slowly getting wider.
Iceland, where the ridge reaches land
Iceland sits across the Mid-Atlantic Ridge. Here the ridge rises above sea level, so the rift between the North American and Eurasian Plates can be seen on land.
Worked example: Describe, then explain
"The rocks get older" describes a pattern. A full answer explains it. Rocks get older with distance from the ridge on both sides. This is because new crust forms at the ridge and is carried away as plates separate.
Watch out for this
The oldest ocean rocks should be at the ridge, where the seafloor started.
The ridge keeps making new crust. Older crust has already moved away, so the ridge has the youngest rocks.
Check your understanding
Rocks at equal distances on both sides of a ridge have similar ages. Why?
- Rocks form at random places across the seafloor, then age at the same rate.
- Old crust moves inward and becomes younger at the ridge.
- Crust formed at the ridge and moved out on both sides.
Show the answer
Crust formed at the ridge and moved out on both sides.
Yes. Both sides carry away crust formed at the same time.