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Science Chapter 4 Sections 4 & 5 Plate Tectonics


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A break or crack in Earth's lithosphere along which the rock moves.
Deep-ocean trenches in the Atlantic Ocean
The Atlantic Ocean has fewer deep-ocean trenches and of course, the mid-ocean ridge can be found there. The spreading sea floor pushes the continental crust and the whole ocean becomes wider. The Atlantic Ocean is "growing".
Evidence for Sea-floor Spreading
In the 1960's, Harry Hess proposed the CAUSE that supports Wegener's theory of continental drift. He proposed that at the mid-ocean ridge, molten material rose from the mantle and erupted. This produces a conveyer belt effect that pushes the older rock to both sides of the ridge and adds new material to the Earth's surface (sea-floor spreading).
The process by which the ocean floor sinks beneath a deep-ocean trench.
Deep-ocean trench
A place where the ocean floor bends downward and sinks back into the mantle.
How long to renew the ocean floor?
200 million years.
Plate Boundaries
At plate boundaries, Earth's crust is broken (fault) and rocks slip past each other in one of 3 types of plate boundaries.
Scientists use sonar to map the ocean floor. Sonar is a technique that bounces sound waves off the ocean floor and records the time it takes for the wave to bounce back as a method of mapping the ocean floor. In animals and humans this technique is called echolocation or biosonar.
Continents' Slow Dance
The rate of continental movement is slow averaging about 5 centimeters a year. Page 136-137 shows the rate of movement from 225 million years ago.
Cycle Diagram showing the processes that link a trench and the mid-ocean ridge
Molten material erupts through the mid-ocean ridge forming oceanic crust which is subducted through a trench.
Transform boundaries
(a.k.a. strike slip fault) is a place where 2 plates slip past each other in opposite directions. The San Andreas Fault in California is an example of a transform boundary.
Convergent boundaries
(a.k.a. reverse fault) is a place where 2 plates come together. At places where oceanic crust converges, the denser oceanic crust will dive under the other (subduction). At places where oceanic crust and continental crust converge, the denser oceanic crust will dive under (subduction) the less dense continental crust. At places where 2 continental crusts converge, subduction does not take place. Rather, the plates crash and squeeze rock up into mountains!!!
Theory of Plate Motion
In 1965, JT Wilson proposed the idea that the Earth's lithosphere was actually broken up into plates. These plates fit together like pieces in a jigsaw puzzle. Wilson combined the evidence for sea-floor spreading, the theory of continental drift, and his theories about Earth's plates to formulate a scientific theory. This theory states that the plates are in constant, slow motion, driven by convection currents in the mantle. It explains the formation, movement, and subduction of Earth's plates.
Rift valley
A deep valley that forms where 2 plates move apart.
Deep-ocean trenches in the Pacific Ocean
The Pacific Ocean has many deep-ocean trenches. These areas swallow up the ocean crust faster than the mid-ocean ridge can add new crust and therefore, the Pacific Ocean is "shrinking".
Divergent boundaries
(a.k.a. normal faulting) is a place where 2 plates move apart. Most of these types of boundaries occur at the mid-ocean ridge. On land, this boundary is called a rift valley, such as the Great Rift Valley in East Africa and the Rio Grande in Texas.
Evidence of sea-floor spreading from Magnetic Stripes
The magnetic properties on both sides of the ridge were in the same pattern. Throughout Earth's history the magnetic poles have been reversed. Iron in the cooled rock at the ridge shows the direction of the poles at the time of cooling. The "reversed" and "normal" alignment of iron on either side of the ridge showed scientists that both sides have identical iron patterns.
Evidence of sea-floor spreading from Drilling Samples
The last piece of evidence that scientists found had to do with the age of the rock on either side of the ridge. A drilling ship brought up core samples from the ridge that were taken at varying distances and found that the rock farthest from the ridge was the oldest and the rock nearest to the ridge was youngest. This correlation (pattern from youngest to oldest starting at the center of the ridge) was the same on either side of the ridge.
What are the 3 plate boundaries?
Transform boundaries, divergent boundaries, convergent boundaries.
The separate sections of the lithosphere.
Plate Tectonics
This theory explains plate movements and how they cause continental drift.
Sea-floor spreading
The conveyer belt effect that occurs on the ocean floor and adds new material to the Earth's surface
Evidence for sea-floor spreading from Molten Material
In the 1960's, a submarine named Alvin descended to the ocean floor at the mid-ocean ridge. Scientists collected data that showed the sea floor around the ridge was made up of pillow shaped rock that could only have been formed from rapid cooling occurring over and over.

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