Download Sea Floor Spreading DBQ

SEA FLOOR SPREADING
Subduction Zones
Mid-Ocean Ridge
During World War II, a geologist named Harry Hess used sonar to map out the ocean floor. Sonar,
which stands for sound navigation and ranging, is a device that bounces sound waves off underwater
object and then records the distance the sound traveled.
After the war, Hess and the newly created U.S. Office of Naval Research continued to explore the ocean.
They found an enormous volcanic mountain range. This mountain range, which is about midway between
the continents became known as a mid-ocean ridge. This mountain range is the longest and highest
mountain chain on earth. We now know that the ocean ridge system snakes around the entire globe in a
continuous chain some 80,000 kilometers long and is volcanic.
In addition to the mid ocean ridge, scientists also discovered many other unusual aspects of the ocean
floor. First of all, none of the fossils found in ocean sediments were more than 180 million years old.
Also, even though oceans have been around for at least four billion years, there was very little sediment
accumulated on the ocean floor.
Based on this information, Hess proposed a groundbreaking hypothesis in 1962 that proved extremely
important in the development of plate tectonic theory. Hess theorized that the ocean floor is only 180 to
200 million years old. This is significantly younger than the continents. Hess believed the reason the
ocean floor was so much younger that the continents was because new ocean floor is being created at the
mid ocean ridge while older ocean floor is destroyed at subduction zones. Subduction zones are areas
where the ocean floor is forced under other crust and destroyed by being melted.
Hess determined the new ocean crust that was created at the mid-ocean ridge takes about 200 million
years to spread sideways beneath the ocean, and disappear back into the Earth’s deep interior at the ocean
trenches. This “recycling” process, later named “seafloor spreading,” carries off older sediment and
fossils leaving only younger ocean floor.
As a result of Harry Hess' discoveries, Alfred Wegener's Theory of Continental Drift was revisited.
Scientists theorized that if the ocean floor was moving, perhaps Wegener was correct and the continents
were also moving. From here, a revolutionary new theory called the Theory of Plate Tectonics was
formed.
Sea Floor Spreading DBQ
Answer the following questions in sentence form.
1. What is the mid-ocean ridge?
2. What occurs at the mid-ocean ridge?
3. Describe the theory of sea-floor spreading.
4. Name and explain the two pieces of evidence that support the theory of sea-floor spreading.
5. What is subduction?
Essay
Based on the reading, explain why the Atlantic Ocean is getting larger and the Pacific Ocean is getting
smaller