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 “It was very exciting in those days. We were explorers.” Marie Tharp (1920–) Oceanographer Faculty 1948–83 A pioneer of modern oceanography, Tharp was the 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 in the earth sciences. Working with pens, ink and rulers, Tharp drew the underwater cartography, longitude degree by latitude degree, based on data from sonar readings taken by pioneering earth scientist Maurice Ewing and his team.  Piecing maps together in the late 1940s and early 1950s, she and colleague Bruce Heezen discovered a 40,000-mile underwater ridge girdling the globe.  By this finding, they laid the foundation for the conclusion from geophysical data that the sea floor spreads from central ridges and that the continents are in motion with respect to one another—a revolutionary geological theory at the time.  Years later, satellite images proved Tharp’s maps to be accurate. Tharp came to Columbia in 1948 to work as Ewing’s research assistant.  Following him to the new Lamont Geological Observatory, she provided much of the data and analyses for Ewing and Hezeen’s scientific papers. In recent years, she has been honored for her contributions by the Library of Congress, the Women's Committee of the Woods Hole Oceanographic Institution, and the place where it all began, now the Lamont-Doherty Earth Observatory.

Marie Tharp's "World Ocean Floor Map” 1977

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

Echo Sounding Device Used By Hess

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 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

Sea Floor Spreading

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

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

Magnetic Reversals in the Seafloor As the rock cools, it records these magnetic reversals in the seafloor Magnetization of the sea floor

Evidence of Sea Floor Spreading

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

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 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 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 Turn and tell your neighbor the three pieces of evidence that support the theory of seafloor spreading Sea Floor Spreading Hydrothermal Vents Ocean Exploration