9.2: Sea-Floor Spreading in the early 1900s, scientists using sonar (SOund Navigation And Ranging) discovered deep-ocean trenches deep-ocean trenches: long, curved valleys along the edges of some ocean basins

trenches form the deepest parts of the ocean

deepest is the Mariana Trench

ridges form as magma rises to create new ocean floor made of basalt in the late 1950s, scientists discovered mid-ocean ridges mid-ocean ridges: long, undersea mountain chains that have a steep, narrow valley at its center ridges form as magma rises to create new ocean floor made of basalt

at the center of some mid-ocean ridges are a crack in Earth’s crust where magma can rise this crack is a rift valley

Harry Hess (1963) proposed the idea of sea-floor spreading where a) magma rises at mid-ocean ridges to create new ocean floor b) ocean floor slowly moves outward away from the rift c) ocean floor sinks back into the mantle at deep-ocean trenches

when ocean plates sink back into the mantle, it is called subduction rift and then cools to form new rock.

trenches where the ocean plates sink back into the mantle are called subduction zones rift and then cools to form new rock.

1960s brought more data to support the idea of sea-floor spreading scientists learned that Earth’s magnetic field occasionally reverses polarity north magnetic pole becomes the south magnetic pole – called reversed polarity

paleomagnetism: study of the alignment of magnetic minerals that rock gains during formation scientists discovered a striped magnetic pattern on the ocean floor on each side of a mid-ocean ridge

pattern on one side of the ridge is a mirror image of the pattern on the other side

mid-ocean ridges contain young rocks – none older than 180 million years rocks get older towards the trenches thin rocks – covered with less sediment – at top of ridge, getting thicker with more sediment as move towards the trenches

sea-floor spreading was the mechanism to explain continental drift! rift and then cools to form new rock.