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North America

North America In The Proterozoic Eon, Phanerozoic Time, Eastern And Southern Borders Of North AmericaGeologic history

The landmass occupied by the present-day countries of Canada, the United States, and the Republic of Mexico make up North America. Greenland (Kalaallit Nunaat), an island landmass to the northeast of Canada, is also included in North America, for it has been attached to Canada for almost two billion years.

Plate tectonics is the main force of nature responsible for the geologic history of North America. Over ages of geologic time, the plates have come together to form the continents, including North America. Other processes, such as sedimentation and erosion, modify the shape of the land that has been forged by platetectonics.

North American geologic history includes several types of mountain ranges as a result of plate tectonics. When the edge of a plate of Earth's crust runs over another plate, forcing the lower plate deep into Earth's elastic interior, a long, curved mountain chain of volcanos usually forms on the forward-moving edge of the upper plate. When this border between two plates forms in the middle of the ocean, the volcanic mountains form a string of islands, or archipelago, such as the Antilles and the Aleutians. This phenomenon is called an island arc.

When the upper plate is carrying a continent on its forward edge, a mountain chain, like the Cascades, forms right on the forward edge. This edge, heavily populated with volcanos, is called a continental arc. The volcanic mountains on the plate border described above can run into a continent, shatter the collision area and stack up the pieces into a mountain range. This is how the Appalachians were formed. Imagine how high your school would reach if it were squeezed by bulldozers so it remained the same length east to west as it is now, but from north to south measured the width of a convenience store restroom. The result would be a tall wall of fractured rubble, and that is just what a collisional mountain belt is.

When a continent-sized "layer cake" of rock is pushed, the upper layers move more readily than the lower layers. The layers separate from each other, and the upper few miles of rock move on ahead, floating on fluid pressure between the upper and lower sections of the crust like a fully loaded tractor trailer gliding effortlessly along an icy road. The flat surface where moving layers of crust slide along the top of the layers beneath it is called a thrust fault, and the mountains that are heaved up where the thrust fault reaches the surface are one kind of fault block mountains. The mountains of Glacier National Park slid along the Lewis thrust fault over younger rocks, and out onto the Great Plains.

Another kind of fault block mountains comes from stretching of Earth's crust. A model of this kind of mountains could be made by compacting a 6-in (15-cm) thick layer of moist sand on top of a rubber (not rubberized) sheet. When the sheet stretches, mimicking the elastic properties of the lower crust, the sand will crack along lines perpendicular to the direction the sheet is being pulled. Some of the surface will remain the same height, and some blocks will slide down the sides of the blocks which remain stable. This is particularly noticeable if the top surface of the compacted sand has been dusted with powder. This is a model of the process that formed the mountains in the Basin and Range province.

Mountain ranges start being torn down by physical and chemical forces while they are still rising. North America has been criss-crossed by one immense range of mountains after another throughout its almost four-billion-year history.

A range of mountains may persist for hundreds of millions of years, like the Appalachians. On repeated occasions, the warped, folded rocks of the Appalachians were brought up out of the continent's basement and raised thousands of feet by tectonic forces. If mountains are not continuously uplifted, they are worn down by erosion in a few million years. In North America's geologic past, eroded particles from its mountains were carried by streams and dumped into the continent's inland seas, some of which were as large as the present-day Mediterranean. Those rivers and seas are gone from the continent, but the sediments that filled them remain, like dirt in a bathtub when the water is drained. The roots of all the mountain ranges that have ever stood in North America all still exist, and much of the sand and clay into which the mountains were transformed still exists also, as rock or soil formations.

North America in the Archean eon

North America was not formed in one piece, or at one time, the way a cake is baked from batter. Various parts of it were formed all over the world, at various times over four billion years, and were brought together and assembled into one continent by the endless process of plate tectonics. What is now called North America began to form in the first two and one-half billion years of Earth's history, a period of time called the Archean eon.

Some geologists speculate that Earth that created the oldest parts of North America barely resembled the middle-aged planet on which we live. The planet of four billion years ago had cooled enough to have a solid crust, and oceans of liquid water. But the crust may have included hundreds of small tectonic plates, moving perhaps ten times faster than plates move today. These small plates, carrying what are now the most ancient rocks, scudded across the oceans of a frantic crazy-quilt planet. Active volcanos and rifts played a role in rock formation on the Archean Earth. The oldest regions in North America were formed in this hyperactive world. These regions are in Greenland, Labrador, Minnesota, and Wyoming.

Earth changed over the next billion years. A sudden surge of continental construction created much of North America. Between three and four billion years ago, great basalt plateaus gradually built up under the oceans. As the planet cooled, the rock on the undersides of these plateaus changed from basalt to eclogite. Basalt floats on Earth's mantle, but the heavier eclogite sinks into it. All over the world, the eclogite tore away from the basalt plateaus and sank into Earth's hot mantle. Vast amounts of magma liquefied from the eclogite slabs as they sank into the hot mantle. This phenomenon is called partial melting, and it resembles what happens to cheese in a microwave oven. Solid cheese separates into melted fat and hard milk solids. The eclogite is the leftover solid cheese; the liquid magma is the melted fat. This magma rose through the basalt and formed 50% of what would become North America's continental crust. But in the Archean eon, these pieces were still widely scattered on the planet.

In the late Archean eon, the plates of Earth's crust may have continued to move at a relatively high speed. Evidence of these wild times can be found in the ancient core of North America. The scars of tectonic events appear as rock outcrops throughout the part of northern North America called the Canadian Shield. One example of this kind of scar, a greenstone belt, may be the mangled remains of ancient island arcs or rifts within continents. Gold and chromium are found in the greenstone belts, and deposits of copper, zinc, and nickel. Formations of iron ore also began to form in the Archean eon, and fossils of microscopic cyanobacteria-the first life on Earth—are found imbedded in them.

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