3.2 Sea-Floor Spreading3.2 Sea-Floor Spreading

Convection Currents cause the sea floor to spread

This is a mid-ocean ridge

This is an example of sea-floor spreading

Many times volcanoes are created at subduction zones

• mid-ocean ridge – long chains of underwater (ocean) mountains

• Scientists mapped mid-ocean ridges using sonar

• sonar – a device that uses sound waves to measure the distance to an object

• mid-ocean ridges are the longest mountain ranges on Earth

• sea-floor spreading – new oceanic crust is created by magma getting forced upward by convection currents and separating the old crust

– Sea-floor spreading

creates mid-ocean

ridges

3 types of evidence for sea-floor spreading3 types of evidence for sea-floor spreading

1. Evidence from ocean material:• Rocks shaped like pillows only form when molten material

hardens quickly after erupting under water

2. Evidence from magnetic stripes:• As the molten materials hardens, magnetic minerals inside

the rock line up in the direction of Earth’s magnetic poles. This forms unseen magnetic “stripes.” The magnetic poles reverse themselves. The stripes are mirror images on each side of a mid-ocean ridge.

3. Evidence from drilling samples:• Youngest rocks found near mid-ocean ridges• Oldest rocks found far away from mid-ocean ridges

• convection current – hot liquid rises while cool liquid sinks creating a circular current

– Convection currents in the mantle/asthenosphere break through the crust/lithosphere which causes the sea floor to spread and new crust to be formed

– This forms a mid-ocean ridge

• subduction– a process where oceanic crust (thin & dense) sinks under continental crust (thick & less dense) creating a trench– The oceanic crust is pushed into the mantle /

asthenosphere and burns back into molten rock

• deep-ocean trench – underwater canyons usually located near subduction zones

• An ocean surrounded by many trenches will shrink (Pacific Ocean)

• An ocean with a mid-ocean ridge will get bigger (Atlantic Ocean has the Mid-Atlantic Ridge)