+~Plates Of The Earth~+

Adora Mazoch || 1-23-2015 || 7th Period


Plate Boundaries

"1. Convergent boundaries: where two plates are colliding.

Subduction zones occur when one or both of the tectonic plates are composed of oceanic crust. The denser plate is subducted underneath the less dense plate. The plate being forced under is eventually melted and destroyed.

i. Where oceanic crust meets ocean crust
Island arcs and oceanic trenches occur when both of the plates are made of oceanic crust. Zones of active seafloor spreading can also occur behind the island arc, known as back-arc basins. These are often associated with submarine volcanoes.

ii. Where oceanic crust meets continental crust
The denser oceanic plate is subducted, often forming a mountain range on the continent. The Andes is an example of this type of collision.

iii. Where continental crust meets continental crust
Both continental crusts are too light to subduct so a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas."

"2. Divergent boundaries – where two plates are moving apart.

The space created can also fill with new crustal material sourced from molten magma that forms below. Divergent boundaries can form within continents but will eventually open up and become ocean basins.

i. On land
Divergent boundaries within continents initially produce rifts, which produce rift valleys.

ii. Under the sea
The most active divergent plate boundaries are between oceanic plates and are often called mid-oceanic ridges."

"3. Transform boundaries – where plates slide passed each other.

The relative motion of the plates is horizontal. They can occur underwater or on land, and crust is neither destroyed nor created.

Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it exceeds the threshold of the rocks, the energy is released – causing earthquakes."

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Alfred Wegener- The Theory of Continental Drift

"While at Marburg, in the autumn of 1911, Wegener was browsing in the university library when he came across a scientific paper that listed fossils of identical plants and animals found on opposite sides of the Atlantic. Intrigued by this information, Wegener began to look for, and find, more cases of similar organisms separated by great oceans. Orthodox science at the time explained such cases by postulating that land bridges, now sunken, had once connected far-flung continents. But Wegener noticed the close fit between the coastlines of Africa and South America. Might the similarities among organisms be due, not to land bridges, but to the continents having been joined together at one time?

Such an insight, to be accepted, would require large amounts of supporting evidence. Wegener found that large-scale geological features on separated continents often matched very closely when the continents were brought together. For example, the Appalachian mountains of eastern North America matched with the Scottish Highlands, and the distinctive rock strata of the Karroo system of South Africa were identical to those of the Santa Catarina system in Brazil. Wegener also found that the fossils found in a certain place often indicated a climate utterly different from the climate of today: for example, fossils of tropical plants, such as ferns and cycads, are found today on the Arctic island of Spitsbergen. All of these facts supported Wegener's theory of "continental drift." In 1915 the first edition of The Origin of Continents and Oceans, a book outlining Wegener's theory, was published; expanded editions were published in 1920, 1922, and 1929. About 300 million years ago, claimed Wegener, the continents had formed a single mass, called Pangaea (from the Greek for "all the Earth"). Pangaea had rifted, or split, and its pieces had been moving away from each other ever since. Wegener was not the first to suggest that the continents had once been connected, but he was the first to present extensive evidence from several fields.

Reaction to Wegener's theory was almost uniformly hostile, and often exceptionally harsh and scathing; Dr. Rollin T. Chamberlin of the University of Chicago said, "Wegener's hypothesis in general is of the footloose type, in that it takes considerable liberty with our globe, and is less bound by restrictions or tied down by awkward, ugly facts than most of its rival theories." Part of the problem was that Wegener had no convincing mechanism for how the continents might move. Wegener thought that the continents were moving through the earth's crust, like icebreakers plowing through ice sheets, and that centrifugal and tidal forces were responsible for moving the continents. Opponents of continental drift noted that plowing through oceanic crust would distort continents beyond recognition, and that centrifugal and tidal forces were far too weak to move continents -- one scientist calculated that a tidal force strong enough to move continents would cause the Earth to stop rotating in less than one year. Another problem was that flaws in Wegener's original data caused him to make some incorrect and outlandish predictions: he suggested that North America and Europe were moving apart at over 250 cm per year (about ten times the fastest rates seen today, and about a hundred times faster than the measured rate for North America and Europe). There were scientists who supported Wegener: the South African geologist Alexander Du Toit supported it as an explanation for the close similarity of strata and fossils between Africa and South America, and the Swiss geologist Émile Argand saw continental collisions as the best explanation for the folded and buckled strata that he observed in the Swiss Alps. Wegener's theory found more scattered support after his death, but the majority of geologists continued to believe in static continents and land bridges."


Convection Currents

"Heat from the core causes convection currents in the mantle. These currents slowly move the crust around. In some places the crust is destroyed. In other places new crust is formed."
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Plate Tectonics

"Trenches are formed as a result of plate tectonics, or the movement of the Earth’s crust."

The mountains created by this can provide homes for animals and wonderful landmarks. Also, islands can provide new land for our growing population.

Earthquakes can cause buildings to fall and people to die. Volcanoes can kill people by covering them in lava and ash.

"Studies of rocks found in ancient areas of North America have revealed that the oldest known pieces of the continents began to form nearly 4 billion years ago, soon after the Earth itself formed."

The rocks off the mid-Atlantic ridge are newer because they're made by the mantle cooling off because there is a divergent boundary there, and that is where the plates pull away from each other.