Chapter 9-4 Sea Floor Spreading

Layers of the Earth  Crust—Oceanic –Basalt—Very Dense Continental---Granite—Less Dense  Mantle—Thick slowly flowing rock. Has the property of plasticity (solid that flows like a liquid).  Outer Core —Liquid metal  Inner Core —Solid Iron & Nickel due to intense pressure.

Explaining the Push or Pull  Arthur Holmes –British Geologist (1919)  Found that inside of the Earth the material in the mantle moved in a cycle due to changes in density. Convection Current  Hot Material---less dense & rises in the mantle  Cooler Material---more dense & sinks deeper in mantle.

New Technology  Sonar---designed for military (WW2)  Sonar was used after the war to map the ocean floor.  In 1952 scientists discovered a long volcanic ridge that went through all of Earth’s oceans. It was the Mid Ocean Ridge.

SONAR

Mid Ocean Ridge  Longest Mountain Range in the world  Runs through all of Earth’s oceans  Most is hidden under water but parts are above ocean surface  The top is split into a valley that is deeper than the Grand Canyon.

Sea Floor Spreading  Harry Hess –American Geologist (1962)  Theory: oceanic crust moves away from long volcanically active ridges. The ocean crust moves due to convection currents in the mantle.

Evidence of Sea Floor Spreading  Molten Material  Magnetic Stripes  Drilling Samples

Evidence from Molten Material  They found rocks in the central valley of the mid-ocean ridge that had cooled very quickly forming igneous rock.  There was also evidence that these eruptions had occurred again and again.

Evidence from Magnetic Stripes  By studying the patterns in rocks on the ocean floor they found that rocks align themselves in magnetic stripes.  The molten material contains the element iron (Fe). As the material erupts out of the earth the iron atoms line up facing the magnetic poles, when it hardened it left a permanent pattern in the rock.  As basaltic rocks crystallize (type of rock in oceanic crust (ocean floors), some minerals align themselves with Earth’s magnetic field, as it exists at that time, imparting a permanent magnetic field, called paleomagnetism, to the rock.  Periodically Earth’s magnetic field polarity (direction) reverses poles.

Evidence from Drilling Samples Scientists found that the rock further away from the mid-ocean ridge was older than the rock closest to the mid ocean ridge.

Other Features  Deep Ocean Trenches– form where oceanic crust is pushed beneath continental or oceanic crust.  Subduction– when oceanic crust sinks back into the mantle and melts into magma again.  Subduction– when oceanic crust sinks back into the mantle and melts into magma again. Old crust will be destroyed as denser oceanic crust (basaltic crust) moves below less dense Continental crust (granitic crust). This process is known as subduction!

Subduction

Evidences of Sea Floor Spreading… 1) Age of rock samples from drilling on the sea floor 2) Magnetic Striping 3) Mapping of the ocean floor using sonar