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Transcript of Plate Tectonics
in the early 1900’s proposed the hypothesis that continents were once joined together in a single large land mass he called
(meaning “all land” in Greek).
He proposed that Pangea had split apart and the continents had moved gradually to their present positions - a process that became known as
The heat building up underneath Pangea made the continent
, or split apart, around 200 million years ago.
Oceans filled the areas between these new sub-continents.
The land masses continued to move apart, riding on separate plates, until they reached the positions they currently occupy.
These continents are still on the move today
According to the hypothesis of
, continents have moved slowly to their current locations.
Continents fit together like a
(meaning he looked at the order of rock layers) in South America, Africa, India, Antarctica, and Australia show remarkable similarities. Wegener showed that the same three layers occur at each of these places.
Fossils of plants and animals of the
But, Wegener could not come up with an acceptable way to explain how the continents moved. Scientists had a hard time with Wegener’s theory because there was no explanation for
the continents moved.
suggested a process that could explain Alfred Wegener's theory of continental drift: the power of
Currents of heat and thermal expansion in the Earth's mantle, he suggested, could force the continents toward or away from one another, creating new ocean floor and building mountain ranges.
In the 1960’s, a scientist named
made a discovery that would prove Wegener correct.
Using new technology, radar, he discovered that the seafloor has both trenches and mid-ocean ridges.
Henry Hess proposed the
seafloor spreading theory
Hess proposed that
hot, less dense material below Earth’s crust rises toward the surface
at the mid-ocean ridges.
Then, it flows sideways,
carrying the seafloor away from the ridge in both directions
As the seafloor spreads apart at a mid-ocean ridge, new seafloor is created.
The older seafloor moves away from the ridge in opposite directions.
This helped explain how the crust could move—something that the continental drift hypothesis could not do.
In 1968, scientists aboard the research ship Glomar Challenger began gathering information about the rocks on the seafloor.
Scientists found that the youngest rocks are located at the mid-ocean ridges.
Evidence for Spreading
Both Hess’s discovery and Wegener’s continental drift theory combined into what scientists now call the Plate Tectonic Theory.
Plate Tectonic Theory
The Earth’s crust is not a solid shell.
It is made up of thick, interconnecting pieces called tectonic plates that fit together like a puzzle.
They move atop the underlying mantle, a really thick layer of hot flowing rock.
Theory of plate tectonics:
The Earth’s crust and part of the upper mantle are broken into sections, called plates which move on a plastic-like layer of the mantle
The Earth’s solid crust lies over a layer of melted material called the
As the material deep in the mantle is heated, it becomes less dense and rises. At the same time, material nearer Earth’s surface spreads out, cools, and becomes denser. This denser material sinks below the hotter, less dense material.
Uneven heating causes material in the mantle to constantly and slowly rise and fall in a
Earth has two kinds of crust:
Continents are made of continental crust, which is made up of rocks that are less dense than those of oceanic crust. Because it is less dense, continental crust rides higher on the mantle than the oceanic crust.
The plates are all
moving in different directions
(from 2 cm to 10 cm per year--about the speed at which your fingernails grow) in relationship to each other.
The plates are moving around like cars in a demolition derby, which means they sometimes
The place where the two plates meet is called a
Boundaries have different names depending on how the two plates are moving in relationship to each other crashing:
Crashing: Convergent Boundaries
Pulling Apart: Divergent Boundaries
Side Swiping: Transform Boundaries
All the continents have moved and are still moving
Places where plates crash or crunch together are called
Plates only move a few centimeters each year, so collisions are very
and last millions of years.
Just as the front ends of cars fold and bend in a collision, so do the "front ends" of colliding plates. The edge of the continental plate in the drawing has folded into a huge mountain range, while the edge of the oceanic plate has bent downward and dug deep into the Earth. A trench has formed at the bend.
Folding and bending
of this type of collision:
Where the oceanic Nazca Plate is crashing into the continent of South America. The crash formed the Andes Mountains, the long string of volcanoes along the mountain crest, and the deep trench off the coast in the Pacific Ocean.
Are They Dangerous Places to Live? Mountains, earthquakes, and volcanoes form where plates collide. Millions of people live in and visit the beautiful mountain ranges being built by plate collisions. For example, the Rockies in North America, the Alps in Europe, the Pontic Mountains in Turkey, the Zagros Mountains in Iran, and the Himalayas in central Asia were formed by plate collisions. Each year, thousands of people are killed by earthquakes and volcanic eruptions in those mountains. Occasionally, big eruptions or earthquakes kill large numbers of people. In 1883 an eruption of Krakatau volcano in Indonesia killed 37,000 people. In 1983 an eruption-caused mudslide on Nevada del Ruiz in Columbia killed 25,000 people. In 1976, an earthquake in Tangshan, China killed an astounding 750,000 people.On the other hand, earthquakes and volcanoes occurring in areas where few people live harm no one. If we choose to live near convergent plate boundaries, we can build buildings that can resist earthquakes, and we can evacuate areas around volcanoes when they threaten to erupt. Yes, convergent boundaries are dangerous places to live, but with preparation and watchfulness, the danger can be lessened somewhat.
Places where plates are coming apart are called
As shown in the drawing above, when Earth's brittle surface layer (the lithosphere) is pulled apart, it typically breaks along parallel faults that tilt slightly outward from each other. As the plates separate along the boundary, the block between the faults cracks and drops down into the soft, plastic interior (the asthenosphere).
Magma (liquid rock) seeps upward to fill the cracks. In this way, new crust is formed along the boundary.
Earthquakes along the faults
Volcanoes form where the magma reaches the surface
of Divergent Boundaries:
East Africa rift in Kenya and Ethiopia
Rio Grande rift in New Mexico.
Places where plates slide past each other are called
Transform boundaries are marked in some places by:
linear valleys along the boundary where rock has been ground up by the sliding.
features like stream beds that have been split in half and the two halves have moved in opposite directions.
Most famous transform boundary in the world is the San Andreas fault, shown in the drawing below. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year.
In about ten million years, the two cities will be side by side!
Transform Boundaries sliding motion causes:
The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city was destroyed by the fires that followed. More than 600 people died as a result of the quake and fires. Recent large quakes along the San Andreas include the Imperial Valley quake in 1940 and the Loma Prieta quake in 1989.
Crustal Plates move due to Sea-Floor Spreading and because of Convection Currents in the mantle.
Why are the plates moving?
As convection currents move the molten material sideways, large portions of the crust, called
, ride on top across Earth’s surface.
The lithosphere is the rocky outer shell of Earth, which is made up of the crust and the rigid upper part of the mantle.
Earth's crust is made up of seven major and several minor lithospheric plates.
The seven major plates are:
The edges of the most plates lie beneath the oceans
The plates are named for the surface features that lie on top of them