Sea-Floor Spreading
Where does sea-floor spreading begin?
at the mid-ocean ridge
What force is acting on the oceanic crust to pull it down beneath the trench ?
Gravity
What is the theory of plate tectonics?
The theory of plate tectonics states that the Earth's tectonic plates are constantly moving in slow motion.
Why is the country of Iceland unique?
The mountains of the mid-ocean ridge rarely are found above the surface of the ocean, but such is the case in Iceland. A part of the Mid-Atlantic Ridge rises above the surface in the North Atlantic Ocean and forms the country of Iceland.
When oceanic crust collides with continental crust, what happens? Explain why it happens.
The oceanic crust will subduct (go under) the continental crust because it is more dense.
Plates at different types of boundaries move
differently.
Most divergent boundaries occur along
mid-ocean ridges where sea-floor spreading occurs
Every 200 million years new rock forms at the mid ocean ridge, moves across the ocean, and sinks into a trench... recyling itself.
200 million years
Tuzo Wilson, a Canadian scientists, developed the theory of plate tectonics based on what three (3) observations?
1) that there are cracks in the continents similar to the cracks found on the ocean floor 2) sea-floor spreading 3) continental drift
What do we call it when two plates converge?
A collision
What is a rift valley? Where can a good example be found?
A rift valley is formed when a divergent boundary develops on land (the Earth's plates in that area move apart) and a deep valley forms. One of the most well known examples of a rift valley can be found in East Africa.
How does the concept of sea-floor spreading help support the idea that continents drift (thereby supporting Wegener's theory)?
As magma is thrust up and hardens forming new crust, the ocean floor on both sides of the mid-ocean ridge move outward carrying the continents along with them.
What "radical idea" did Hess propose?
He proposed that a process he called sea-floor spreading was continually adding new material (crust) to the ocean floor.
Who was Harry Hess and what did he study?
He was an American geologist who studied mid-ocean ridges.
What happens to oceanic crust at deep ocean trenches ?
I t eventually sinks back into the mantle.
The ocean floor has mountain chains that are as impressive as those on land. What are these underwater mountain chains called?
Mid-ocean ridges
Where do mid-ocean ridges form?
Mid-ocean ridges form along divergent boundaries (where two plates are pulling away from one another) in every part of the world's ocean.
Explain the process of sea-floor spreading (how it happens).
Sea-floor spreading occurs when the sea floor spreads apart along divergent boundaries and forms the mid-ocean ridge. Magma is pushed up through cracks in the crust along the mid-ocean ridge. As the magma is thrust up and hardens it forms new crust and the ocean floor on both sides of the mid-ocean ridge move outward.
The processes of sea floor spreading and subduction can change size and shape or the ocean
Size and shape
How did scientists determine the location and size of these underwater mountain chains?
Sonar
What happens to the density and temperature of oceanic crust as it spreads away from the mid-ocean ridge?
The density increases and the temperature decreases (the crust gets cooler)
Plate movement is measured by instruments mounted onto satellites that orbit the Earth.
The plates move at a rate of 1 to 24 centimeters per year.
These different types of boundaries occur on land and in the ocean all around the world. You are not expected to know each area, but should be able to use symbols provided to identify the different types of boundary. These are some you SHOULD know and that we have discussed in class:
a. convergent boundaries between the Eurasian and Pacific Plates and the Eurasian and Indo- Australian plates b. divergent boundaries between the North American and Eurasian Plates and South American and African Plates c. transform boundaries between part of the North American and Pacific Plates (northern California)
The North American Plate and the Eurasian plates are moving
apart at a rate of about 2.5 centimeters per year.
Compare the thickness and density of continental crust to oceanic crust
continental crust is thicker, but oceanic crust is more dense.
What force do geologists believe is responsible for the movement of the continents?
convection currents in the mantle
What technology/technologies existed during Hess's lifetime that did not exist during Wegener's that allowed Hess to "prove" the theory of sea-floor spreading?
sonar, submersibles (submarines), tools for deeper oceanic drilling
Earth's lithosphere, its solid outer shell, is broken into separate sections called
tectonic plates
What is a deep-ocean trench and how does it form?
A deep ocean trench is an underwater canyon that forms when older, more dense oceanic crust begins to bend downward (subducts).
What evidence did scientists discover to support the theory of sea-floor spreading when they examined rock samples taken from the sea floor by the drilling ship, Glomar Challenger?
Rock samples from different areas of the sea floor were brought up through pipes and then scientists determined the age of each rock sample. They discovered that the oldest rock/crust was farthest away from the mid-ocean ridge and that the youngest rock/crust was on the mid-ocean ridge.
The Pacific Ocean covers approximately 30% of the Earth's surface, but it is shrinking. Why?
Subduction in the many deep-ocean trenches is occurring faster than new crust is being added (more crust is sinking back into the mantle at the deep-ocean trenches than is being created along the mid-ocean ridges).
What is this process called ?
Subduction.
Why is the Atlantic Ocean expanding?
The Atlantic has fewer deep-ocean trenches so more crust is created along the mid-ocean ridge than is subducted (destroyed) in the trenches.
How do mid-ocean ridges form?
Underneath the ocean, deep in the earth convection currents bring molten material up to the upper mantle. This material rises through faults (cracks) between oceanic plates that are moving away from one another. This material fills the cracks, hardens, and forms new crust. This process repeats many times and eventually forms an underwater mountain range.
What happens to the rock (oceanic crust) along the mid-ocean ridges when new crust is formed?
When new crust is formed along the mid-ocean ridge, the old rock/crust is pushed out away from the ridge. So, the further away you get from the ridge, the older the rock/crust gets.
When two plates collide, what determines which one comes out on top?
When two plates collide the density of the plates determines which one comes out on top. The plate with the greater density will subduct (go under) the less dense plate.
Is sea-floor spreading occurring today? Give an explanation for your answer.
Yes, sea-floor spreading continues today. We know this for several reasons: 1) current GPS, satellite, and computer technology allow us to tell the exact position/location of all of Earth's landforms to within mm; and we have evidence of sea-floor spreading in the seismic activity (earthquakes and tremors) that occur around the world all of the time.
There are three kinds of tectonic plates, explain the direction plates move at the following boundaries:
a. convergent boundary plates come together or move toward each other b. divergent boundary plates move apart or away from each other in opposite directions c. transform boundary plates slide past one another going in opposite directions (like rubbing hands together)
What type of landforms (trenches, volcanic mountains, volcanic islands, rift valleys) are created when:
a. two ocean plates diverge mid-ocean ridges, mountain ranges, central valley (new crust is formed) b. two continental plates diverge rift valley, volcanoes c. an oceanic and a continental plate converge subduction zone, volcanic islands, trenches (crust is destroyed) d. two oceanic plates converge trench