Sea Floor Spreading and Continental Drift

Continental Drift In 1915, Alfred Wegener (1880-1930), a German meteorologist, wrote a book titled The Origin of Continents and Oceans.

Continental Drift Wegener proposed the idea of Pangaea A supercontinent that drifted apart Wegener had some evidence to support his theory which he called Continental Drift

Evidence of Continental Drift The continents fit together like a puzzle There are fossils of ancient animals and plants on different continents and there were no land bridges The mountain ranges on different continents align with one another Climate changes: Tropical plant fossils found in non-tropical regions

Pangaea and its break-up

Fossil Evidence of Continental Drift

Sea Floor Spreading In the early 1960s, Henry Hess (1906-69), a geologist and former commander of a Navy ship equipped with an echo sounder, used the profile of the sea floor to propose that it was spreading at the mid- ocean ridges.

Sea Floor Spreading Hot fluid from the mantle (called magma) enters the rise or valley and cools, creating new sea floor (also called oceanic crust).

Sea Floor Spreading The advent of Sonar allowed for mapping of the ocean floor. Hess, using sonar, discovered significant depth differences along ocean bottom Hess explained the difference by proposing mountain ranges and Rift Valleys. He stated that the ocean floor must be spreading at the Rift Valleys, creating the depth differences

Mapping the Mid-Ocean Ridge The mid-ocean ridge is the longest chain of mountains in the world. Scientists mapped the mid-ocean ridge using sonar. Sonar is a device that bounces sound waves off underwater objects and records it.

Evidence of Sea Floor Spreading Age of the crust at mid ocean ridges Older crust further away from the ridge, young crust at the ridge Magnetic reversals over geological time Alternating patterns that align on either side of the ridge Pillow rocks at rift valleys Evidence of magma rising

In the 1960s scientists found evidence that new material is erupting along the mid-ocean ridge. The presence of pillow shaped rocks has showed that molten material has erupted again and again. Pillow rocks are created when magma cools quickly at a mid ocean ridge. Pillow Rocks

Magnetic Patterns Over geologic time, the magnetic polarity of Earth has switched. Scientists believe the poles switch because of a magnetic interaction between the planet’s inner and outer core.

Magnetic Reversals in Crust Evidence in the rocks shows that Earth’s magnetic poles have reversed. The iron bits line up in an alternating pattern. The alternating pattern demonstrates crust movement

Age of the Crust Drilling into the sea floor shows evidence of the ages of the rocks The closer you get to the mid-ocean ridge the younger the rock. The farther you get, the older the rock. This demonstrates a movement away from the ocean ridge

Age of the crust

Forces that shape the earth

Ocean Floor Spreading

Convection at Mid-Ocean Ridge At a mid-ocean ridge, molten material rises from the mantle and erupts. The molten material spreads out, pushing the older rock away on either side of the ridge. This process continually adds to the ocean floor

Subduction in Deep-Ocean Trenches As the ocean spreads apart, it also plunges into deep water canyons called deep- ocean trenches. Subduction is the process by which the ocean floor sinks beneath a deep-ocean trench and back into the mantle.

Subduction The processes of subduction and sea-floor spreading can change the size and shape of the oceans. Because of these processes, the ocean floor is renewed about every 200 million years.

Subduction in the Oceans The Pacific Ocean is shrinking! This is due to the fact that a deep-ocean trench is swallowing more crust than the mid-ocean ridge can produce. The Atlantic Ocean however, is expanding!

How Plates Move The Lithosphere is separated into 14 major plates and 32 minor plates These plates move due to the convection currents occurring beneath them The direction of their movement will have an effect on the surface of our planet

Plate map

Three Types of Plate Boundaries Divergent Convergent Transform

Describing Plate Boundaries There are three main kinds of plate boundaries: divergent convergent transform

Plate Boundaries Place where 2 lithospheric plates meet Transform Boundaries Place where 2 plates are sliding past each other in opposite directions Shearing stresses are caused Divergent Boundary Place where 2 plates are moving away from each other. Creates Rift Valleys and new crust Occur mostly at ocean floor

Convergent Boundaries Boundary where 2 plates move towards each other and collide. There are 3 types of convergent boundaries They can occur as follows: Ocean crust to ocean crust Ocean crust to continental crust Continental crust to continental crust

Convergent Boundaries Ocean to Ocean crust collide Volcanic Island Arcs created Ocean to Continental crust collide Volcanoes are created at the Subduction zones Continental and Continental crust collide Mountain ranges form due to similar densities of the continental crust (no Subduction)

Plate Boundary animations Plate Boundaries