Seafloor Spreading

What Is It & How Does It Happen?

What is happening?

1. 2 oceanic plates are moving (spreading) apart

2. Magma is rising to fill in the gap = new crust being formed

3. Mid-ocean ridges are being formed

Mid-ocean ridge

Mid-ocean ridge

What does it look like?

Newest/youngest crust

Oldest crust

Ridge Push

• Warm, less dense rock pushed up at ridge
• Cools down, becomes more dense (heavier)
• Slides down ridge
• Think of a “rock slide”

How & Why Does It Happen?

1. Magma from the asthenosphere rises because it is hot & low in density

2. As it reaches the crust, it is forced to turn and drag the crust with it.

3. The magma cools down and sinks

4. Magma reheats and rises once again (convection cell!!!)

Slab Pull

• Denser plate sinks under less dense plate
• Pulls rest of plate behind it

​

Where Do We Find Seafloor Spreading Zones on Earth?

• Underwater mountain ranges = mid-ocean ridges
• One example: Mid-Atlantic Ridge
• Runs through center of Atlantic Ocean

Evidence for a Moving Seafloor

Crust on either side of a mid-ocean ridge has alternating bands of magnetic mineral crystals

Switches between normal and reverse polarity as N & S poles flip over time

Paleomagnetism

Normal polarity

Reverse polarity

Put it all together and you get…

• Large Scale Seafloor Spreading

Continental Drift Connection?

• How does this all relate to the continental drift hypothesis?
• Scientists confirmed the Earth was covered in plates
• Mantle convection is moving these plates
• Seafloor spreading is evidence of this movement
• New & improved idea…The THEORY OF PLATE TECTONICS

What does Plate Tectonics Tell Us?

• The Earth’s surface is divided into plates that move
• Earth’s surface resembles a broken eggshell
• These plates “float” on top of the asthenosphere
• Approximately 14 major plates
• The plates move at varying speeds over time
• Some move ½ inch per year, some move up to 4 inches per year

​