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 SpreadingIn the 1960’s, a scientist named Harry Hess made a discovery that would vindicate WegenerUsing new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridgesHarry Hess proposed the sea-floor spreading theory

Echo Sounding Device Used By Hess

Seafloor SpreadingHess 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 SpreadingAs the seafloor spreads apart at a mid-ocean ridge, new seafloor is createdThe older seafloor moves away from the ridge in opposite directionsThis 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 waterThese 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 StripesScientists 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 StripesPlates 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

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

Polarity Reversal

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

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 SamplesWhen scientists sampled the rocks, they found that the further away from the ridge the rocks were the older they wereThe 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