Sea-Floor Spreading
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Sea-Floor Spreading. Marie Tharp. Oceanographer First to map details of the ocean floor on a global scale Her observations became crucial to the eventual acceptance of the theories of plate tectonics and continental drift. Marie Tharp's "World Ocean Floor Map”. 1977. Seafloor Spreading.

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Marie Tharp

  • Oceanographer

  • First to map details of the ocean floor on a global scale

  • Her observations became crucial to the eventual acceptance of the theories of plate tectonics and continental drift



Seafloor spreading
Seafloor Spreading

  • In the 1960’s, a scientist named Harry Hess made a discovery that would vindicate Wegener

  • Using new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridges

  • Harry Hess proposed the sea-floor spreading theory



Seafloor spreading1
Seafloor Spreading

  • Hess proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges

  • Then, it flows sideways, carrying the seafloor away from the ridge in both directions


Seafloor spreading2
Seafloor Spreading

  • As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created

  • The older seafloor moves away from the ridge in opposite directions

  • This helped explain how the crust could move —something that the continental drift hypothesis could not do



Seafloor spreading the process by which molten material adds new oceanic crust to the ocean floor
Seafloor Spreading - The process by which molten material adds new oceanic crust to the ocean floor


Evidence from molten material
Evidence from Molten Material

  • Strange rocks shaped like pillows or like toothpaste squeezed from a tube have been found

    • These rocks can form only when molten material hardens quickly after erupting under water

    • These rocks showed that molten material has erupted again and again from cracks along the central valley of the mid-ocean ridge

Mid Atlantic Ridge – Molten Material


Evidence from magnetic stripes
Evidence from Magnetic Stripes

  • Scientists discovered that the rock that makes up the ocean floor lies in a pattern of magnetized “stripes”

  • 780,000 years ago, magnetic poles reversed themselves

    • If they reversed today, the needle in a compass would point south instead of north

  • The rock in the ocean is made of iron, which began as molten material

Magnetic Reversals


Evidence from magnetic stripes1
Evidence from Magnetic Stripes

  • Plates move apart

  • Magma rises and cools forming new sea-floor

    • youngest rock is near the ridge, it gets older as you move away

  • The earth’s magnetic field is recorded in the rock as it forms

  • We have a record of the earth’s magnetic reversals on either side of the ridge

100 Greatest Discoveries: Magnetic Reversals


Polarity reversal
Polarity Reversal

  • Scientists later discovered that the earth’s magnetic field changes polarity every few thousand years


Magnetic reversals in the seafloor
Magnetic Reversals in the Seafloor

As the rock cools, it records these magnetic reversals in the seafloor

Magnetization of the sea floor



Key idea
Key Idea

As distance from the ridge increases, the age of the sea-floor increases


Evidence from drilling samples
Evidence from Drilling Samples

  • When scientists sampled the rocks, they found that the further away from the ridge the rocks were the older they were

  • The younger rocks were always in the center of the ridges


Important things to remember
Important Things to Remember

  • Theory of sea floor spreading - hot, less dense material below the Earth’s crust rises toward the surface at the mid-ocean ridges. Then it flows sideways, carrying the seafloor away from the ridge in both directions

  • As distance from the ridge increases, the age of the sea-floor increases


Important things to remember1
Important Things to Remember

  • Henry Hess proposed the seafloor spreading theory

  • He proposed that hot, less dense material below Earth’s crust rises toward the surface at the mid-ocean ridges. Then, it flows sideways, carrying the seafloor away from the ridge in both directions


Quick check
Quick Check

Turn and tell your neighbor the three pieces of evidence that support the theory of seafloor spreading

Sea Floor Spreading

Hydrothermal Vents

Ocean Exploration


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