PT 1.2 seafloor spreading

What is seafloor spreading? (3)

• process where new oceanic crust is continuously formed at divergent plate boundaries (mid-ocean ridges) as magma rises, cools and solidifies.

• Older oceanic crust is pushed away from the ridge as new crust forms.

• WHERE does it occur? → At DIVERGENT plate boundaries along mid-ocean ridges, and beneath the oceans

How Does Seafloor Spreading Occur? (7)

1. Two oceanic plates diverge / move apart.

2. Cracks form between the plates.

3. Magma rises through the cracks from the mantle.

4. The magma cools and solidifies.

5. New oceanic crust is formed.

6. Newly formed crust pushes older crust away from the ridge.

7. ocean floor gradually becomes wider.

How Does Seafloor Spreading Support Plate Tectonic Theory?

• New crust is constantly being formed. Older crust moves away from the ridge.

• Oceanic plates are moving continuously.

• Therefore... Seafloor spreading proves that tectonic plates are moving, supporting Plate Tectonic Theory (PTT).

Why are Divergent Boundaries called Constructive Boundaries?

• Because new crust is created.

• Remember: Constructive = new crust is constructed

• Vs. At convergent boundaries: Crust is destroyed by subduction (that's why convergent boundaries = destructive).

Evidence for Seafloor Spreading

Evidence 1: Age of Rocks

Scientists discovered: Rocks nearest the mid-ocean ridge are the youngest. Rocks become older further away from the ridge. This pattern is found on both sides of the ridge. Why? → Because new crust is continuously formed at the ridge and pushes older crust away.

• Young rocks near the ridge show that new oceanic crust is constantly being created.

• Older rocks further away show that previously formed crust has been pushed aside.

• Thus, this proves that the ocean floor is spreading over time.

Evidence 2: Limited Sediment Accumulation

Normally, sediments (rock materials) build up slowly over millions of years. Scientists found: Very little sediment on younger oceanic crust. Oceanic crust is much younger than continental crust. Older oceanic crust is destroyed at ocean trenches through subduction and recycled into the mantle.

Why is there little sediment?

• Because newly formed crust has not existed long enough for much sediment to accumulate.

• Old oceanic crust also disappears at subduction zones before becoming extremely old