Evidence for plate tectonics (Oceanic Crust)

Question 1

Ocean topography

Answer 1

• Huge, linear, underwater mountain ranges with deep central valleys exist on the floor of ocean basins.
• These submarine mountain chains run continously for 50,000km around the Earth.

Question 2

Age and pattern of ocean basin geology

Answer 2

-Rocks at the Mid-Atlantic Ridge are dated at around 0-5.5 million years old, bu at the coastlines of North America and Africa, rocks are dated far older at 135-180 million years old.

Question 3

Palaeomagnetism

Answer 3

• Particles in molten volcanic rock are magnetised as the rock cools and solidifies.
• Particles retain the direction of the Earth’s magnetic field at the time of cooling; therefore the rocks keep a permanent record of this direction and angle of dip which can be used to locate where the rock was formed.
• The earth’s magnetic field will reverse at certain points in geological history
• bands of rock parallel to the ridge have alternating magnetic polarity which reflects the repeated reversal of the earth’s magnetic field.

Question 4

Distribution of Earthquakes and Volcanoes

Answer 4

• Volcanic and seismic activity occur in long and narrow linear bands, sometimes along coastlines or through oceans.
• The patterns of submarine volcanoes and earthquakes haave revealed active processes at work.

Question 5

How can these all be explained?

Answer 5

SEAFLOOR SPREADING

Question 6

How does seafloor spreading work?

Answer 6

• Mid Ocean ridges= weak zones in ocean crust where the floor is pulled apart along the ridge crust.
• Magma from within the mantle rises through these weak zones, erupting along the crest of ridges.
• Molten rock cools, creating new ocean crust
• Convection currents in the mantle causes newly forming crust to spread away from the ridge.
• Millions of years later the crust is subducted and destroyed at deep ocean trenches.