3.2 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 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)