V: Ocean Floor Dynamics

Most of the geologic processes occurring on land are linked, directly or indirectly, to the dynamics of the ocean floor.

(Info gathered from:http://pubs.usgs.gov/publications/text/developing.html)

Four major scientific developments encouraged the formulation of the plate-tectonics theory:

1. Demonstration of the ruggedness and youth of the ocean floor

In 1947, seismologists on the U.S. research ship Atlantis found that the sediment layer on the floor of the Atlantic was much thinner than originally thought. Scientists had previously believed that the oceans have existed for at least 4 billion years, so therefore the sediment layer should have been very thick. Why then was there so little accumulation of sedimentary rock and debris on the ocean floor? The answer to this question, which came after further exploration, would prove to be vital to advancing the concept of plate tectonics.

[ Thickness of the Ocean Floor ]

(Info. from the NOAA/National Geophysical Data Center, Boulder, CO, USA )

2. Confirmation of repeated reversals of the Earth's magnetic field in the geologic past

Grains of magnetite -- behaving like little magnets -- can align themselves with the orientation of the Earth's magnetic field. Scientists have discovered that the earth's magnetic field reverses over time. When magma (molten rock containing minerals and gases) cools to form solid volcanic rock, the alignment of the magnetite grains is "locked in," recording the Earth's magnetic orientation or polarity (normal or reversed) at the time of cooling.

[ Animation representing scientists gathering data about the magnetism of the ocean floor. ]

Example of flowing magma becoming magnetized:

[ Mid-Atlantic Ridge with magnetic stripes ]

3. Emergence of the Seafloor Spreading hypothesis and associated recycling of oceanic crust

This lead to the theory of "Sea Floor Spreading"


[ Age of the seafloor from Strait of Juan de Fuca ]

HOWEVER - As the seafloor spreads is the Earth expanding?

NO! – the lithosphere is being reabsorbed in the oceanic trenches along the edges of the Pacific Ocean, resulting in the Atlantic Ocean expanding and the Pacific Ocean shrinking. The oceanic trenches in the Pacific Ocean run parallel to a series of volcanoes and are the site of numerous earthquakes. The area of the trenches/volcanoes is called "The Ring of Fire"

[ Flash interactive animation of subduction ]

[ Flash interactive map, Ring of Fire ]

4. Precise documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges

Mid-Ocean ridge – Though hidden beneath the ocean surface, the global mid-ocean ridge system is the most prominent topographic feature on the surface of our planet.

[ Flyover Movie, Blue Marble... COMING SOON! ]

Scientists conjecture that the Mid-Ocean Ridge is the site of volcanic eruption, lava flows, and that the plates move away from ridge.

[ Flash interactive animation of underwater volcanic activity ]

The volcanic country of Iceland, which straddles the Mid-Atlantic Ridge, offers scientists a natural laboratory for studying on land the processes also occurring along the submerged parts of a spreading ridge. Iceland is splitting along the spreading center between the North American and Eurasian Plates, as North America moves westward relative to Eurasia.


[ Iceland and its Divergent Boundary ]