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 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 plate tectonics.
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 volcanoes 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 volcanoes, 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.
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.
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.
Various parts of North America 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 the 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 10 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 volcanoes 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.
In the late Archean Eon, the plates of Earth's crust may have moved 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 Earthre found imbedded in them.
North America's little Archean continents slammed together in a series of mountain-building collisions. The core of the modern continent was formed 1,850 million years ago when five of these collisions occurred at once around northeastern Canada. This unified piece of ancient continental crust, called a craton, lies exposed at the surface in the Canadian Shield, and forms a solid foundation under much of the rest of the continent.
In the two billion years of the Proterozoic Eon (2,50070 million years ago), North America's geologic setting became more like the world as we know it. The cores of the modern continents were assembled, and the first collections of continents, or supercontinents, appeared. Life, however, was limited to bacteria and algae, and unbreatheable gases filled the atmosphere. Rampant erosion filled the rivers with mud and sand, because no land plants protected Earth's barren surface from the action of rain, wind, heat, and cold.
Rich accumulations of both rare and common metallic elements make Proterozoic rocks a significant source of mineral wealth for North America, as on other continents. Chromium, nickel, copper, tin, titanium, vanadium, and platinum ores are found together in the onion-like layers of crystallized igneous rocks called layered intrusions. Greenstone belts are mined for copper, lead, and zinc, each of which is mixed with sulfur to form a sulfide mineral. Sulfide minerals of lead and zinc are found in limestones formed in shallow seas, while mines in the ancient continental river and delta sediments uncover buried vanadium, copper, and uranium ores.
During the middle to late Proterozoic Eon, continental collisions attached new pieces of continental crust to North America's southern, eastern, and western borders. Between 30% and 40% of North America joined the continent in the Proterozoic. The crust underlying the continental United States east of Nevada joined the craton, as well as the crust underlying the Sierra Madre Occidental of Sonora, Chihuahua, and Durango in Mexico. The Mazatzal Mountains, whose root outcrop is in the Grand Canyon's inner gorge, rose in these mountain-building times in southern and central North America.
North America experienced the sea washing over its boundaries many times during the three-billion-plus years of its Archean and Proterozoic history. Life had flourished in the shallow tidewater. Algae, a long-term resident of North America, was joined later by worms and other soft-bodied animals. Little is known of early soft-bodied organisms, because they left no skeletons to become fossils. Only a handful of good fossils remain from the entire world's immense Precambrian rock record.
Then, about 570 million years ago, several unrelated lineages of shell-bearing sea animals appeared. This was the beginning of the Phanerozoic Eon of earth history, which has lasted from 570 million years ago to the present day. Vast seas covered much of North America in the early Phanerozoic, their shorelines changing from one million-year interval to the next. The seas teemed with creatures whose bones and shells we have come to know in the fossil record. These oceanic events are memorialized in the layers of stone each sea left behind, lying flat in the continent's heartland, and folded and broken in the cordilleras. Geologists have surveyed the stacked sheets of stone left by ancient North American seas and have made maps of the deposits of each continental sea. The stacked layers are divided into sequences, each named for the sea that laid it down. Each sequence consisted of a slow and complex flooding of the continent. Sea level mountain uplift, the growth of deltas, and other factors continually changed the shape of the continental sea.
The eastern coast of North America was once part of an ancient "Ring of Fire" surrounding an ocean that has disappeared forever from Earth. From Greenland to Georgia, and through the Gulf Coast states into Mexico, the collision of continents raised mountains comparable to the Himalayas and Alps of today. Several ranges were raised up on the eastern border of North America between 480 and 230 million years ago.
Another collision about 450 million years ago created the Acadian mountain range, whose roots are exposed today in Newfoundland. These mountains began to be torn down by rain and wind, and by the time they had worn down to nothing, more than 63,000 cubic miles of sediment made from them had been dumped into the shallow continental sea between New York and Virginiabout the same amount of rock as the Sierra Nevadas of today. The bones of amphibians, the first land animals, are found in the rocks laid down by the streams of East Greenland.
The sleepless crust under North America's Pennsylvanian-age borders tossed and turned in complex ways. Three hundred million years ago, North America sat on the equator, its vast inland sea surrounded by rain forests whose fossilized remains are the coal deposits of the eastern United States. Small mountain ranges rose out of the sea that covered the center of the continent in Colorado, Oklahoma, and Texas. The Ouachitas stood in the Gulf Coast states, the last great mountain range to stand there. In the eastern United States, the Allegheny Mountain Range stood where the Acadian and Taconic Ranges had stood before.
The Ouachitas welded South America to the Gulf coast, at roughly the same time as the Alleghenies welded the East coast of North America to West Africa. The Ouachitas and Alleghenies stretched, unbroken, all the way around the eastern and southern coasts of North America. This joining of the world's continents formed Pangaea, the most recent supercontinent in geologic history. Pangaea's 150 million year history ended with the birth of the Atlantic Ocean and the separation of North and South America. As South America and Africa tore away from North America, Florida was left behind, attached to the intersection of the Allegheny and Ouachita Mountains. Another legacy of this cracking of Earth's crust is the New Madrid Fault, which runs through the North American Plate under the Mississippi Valley.
Around 340 million years ago, an offshore island arc, called the Antler Arc, struck the shores about where Nevada and Idaho now are (then the westernmost part of the continent), extending the shoreline of North America a hundred miles westward.
By 245 million years ago, the beginning of the age of dinosaurs, another island arc had run into the American West. The Golconda Arc added a Sumatra-sized piece of land to North America, and the continent bulged out to present-day northern California.
After the Golconda Arc piled onto the West Coast of that time, the crust broke beneath the continent's border, and the ocean's plate ran under North America's west coast. A continental arc was born around 230 million years ago in western North America, and its volcanoes have been erupting frequently from the dawn of the age of dinosaurs (the Mesozoic Era) until today.
Several more island arcs struck western North America since the middle Jurassic Period. The granite mountains of the Sierra Nevada are the roots of one of these island arcs. Landmasses created on the Pacific Plate have been scraped from it. This mechanism is the origin of the West Coast's ranges, the Cascades, and much of British Columbia and Alaska's southern coast.
A range of fault block mountains rose far inland as the continent was squeezed from west to east. The Sevier Mountains stood west of the Cretaceous Period's interior seaway, in what is now Montana, Idaho, Nevada, and Utah. The dinosaurs of that time (8030 million years ago) left their tracks and remains in the mud and sand worn off these mountains.
In the same manner as large island chains were carried to North America on moving plates of oceanic crust, small pieces of land came to the coasts in this way as well. Numerous "exotic terrains," impacting on the western coasts during the Mesozoic and Cenozoic Eras, added large areas now covered by British Columbia, Washington, Oregon, California, and Mexico. These little rafts of continental crust were formed far from their present location, for the fossils in them are of creatures that lived halfway around the worldut never in North America. A sizeable piece of continental crustouthern Mexico as far south as the Isthmus of Tehuantepecoined northern Mexico between 18040 million years ago.
Starting 80 million years ago, new forces began to act on the inland west. Geologists do not know exactly what happened beneath the crust to cause these changes, but the features created on the surface by tectonic action underneath the crust are well known.
At the same time as the Sevier Mountains ceased to rise, a similar range, facing the opposite direction, began to move upward. Earth's upper crust beneath the Rocky Mountain states was shoved westward as the Laramides were forming, lifting the Rocky Mountains for the first time. These first modern Rocky Mountains drained the continent's last great shallow sea of inland North America as they rose. Huge mountains now stood in places where seas had rolled over Colorado, Wyoming, Utah, Idaho, Montana, and Alberta. In Mexico, the Laramides raised the Sierra Madre Occidental, and formed the mineral deposits that enrich Sonora, Chihuahua, Durango, and Zacatecas. In Colorado, Wyoming, and neighboring states, the Rocky Mountains began to erode away, and by 55 million years ago, the first Rockies had disappeared from the surfacehe mountains' roots were buried in sediment from the eroded mountaintops. More recent uplift again exposed the Rockies, and Ice Age glaciers sculpted their tops into today's sharp peaks.
Twenty-five million years ago, after a quiet interlude, North America's western continental arc awoke, and its abundant volcanoes again added new rock to the continent from British Columbia to Texas and down the mountainous spine of Mexico. The only area in the Southwest in which volcanoes were uncommon was the Colorado Plateau, whose immunity to the tectonic forces around it is still a mystery. Around the borders of the Colorado Plateau's remarkably thick crust, one volcanic catastrophe after another covered the land. In this time, the San Juan Mountains were formed in Colorado. The Rocky Mountains began to slide westward and rose again on the thrust faults beneath them.
Ten million years ago, the Great Basin area of the United States was much shorter when measured east to west than it is today. It was then a mountainous highland. Some geologists propose that Nevada was an alpine plateau like Tibet is todayerhaps more than 10,000 ft (3,048 m) high. Starting then and continuing for five million years, this area began to be pulled apart. Long faults opened in the crust, and mountain-sized wedges slowly fell between ridges that were still standing on the unbroken basement rock miles below. Sediment from the erosion of these new ridges filled the valleys, enabling the valleys to become reservoirs of underground water, or aquifers. The low parts got so low that the area is indeed a basin; water does not flow out of it. Some geologists believe that the Basin and Range province stretches around the Colorado Plateau, into Texas, and extends down the Sierra Madre Occidental as far south as Oaxaca.
Another kind of pulling-apart of the continent happened in New Mexico's Rio Grande Rift. As at the Keewenaw Rift a billion years before, tectonic forces from beneath Earth's crust began pulling the surface apart just as east Africa is being pulled apart today. The broad rift's mountainous walls eroded, and the sediment from that erosion piled up in the ever-widening valley. A new ocean was about to be formed in the southwest. Lava poured from fissures in the crust near Taos, New Mexico, filling the valley floor. Also like the Keewenaw Rift, the Rio Grande Rift stopped growing after a few million years, as the tectonic processes ceased pulling the continent apart. The modern Rio Grande was born as a consequence of this rift, and still runs through the rift valley.
A cataclysmic volcanic event happened in Oregon and Washington 17 million years ago. For an unknown reason, perhaps a disturbance deep in Earth's mantle, or a meteor impact, lava began pouring out of cracks in Earth. So much lava poured onto the surface at once that it ran from southeastern Oregon down the Columbia River valley to the Pacific Ocean. Huge cracks in the ground called fissures flooded broad areas with basalt lava over about 500,000 years. This flood of basalt is called the Columbia River Plateau. A hot spot, or an upwelling of molten rock from Earth's mantle, appears to have caused the Columbia River Plateau. As the North American Plate moved westward between then and now, the hot spot stayed in one place, scorching holes in Earth's crust under Idaho and erupting the lava that makes the Snake River Plain a fertile farmland. This hot spot is assumed to be the heat source that powers the geysers of Yellowstone National Park.
In the early Jurassic Period, 200 million years ago, the northernmost edge of North America tore away from the continent and began rotating counterclockwise. This part of the continent came to rest to the northwest of North America, forming the original piece of Alaskats northernmost mountains, the Brooks Range. In the late Cretaceous Period, the farthest part of this landmass from North America struck the edge of Siberia, and became the Chukotsk Peninsula. The remaining landmass of Alaska joined North America bit by bit, in the form of exotic terrains. The Aleutians, a classic island arc, formed in the Tertiary Period. The about 40 active volcanoes of the Aleutians have erupted numerous times in the twentieth century, including several eruptions in the last decade from Mt. Augustine, Pavlov, Shishaldin, and Mt. Redoubt.
For reasons that are not yet fully understood, Earth periodically enters a time of planet-wide cooling. Large areas of the land and seas are covered in ice sheets thousands of feet thick, which remain unmelted for thousands or hundreds of thousands of years. Today, only Greenland and Antarctica lie beneath continent-sized glaciers. But in the very recent geologic past, North America's northern regions, including the entire landmass of Canada, were ground and polished by an oceanic amount of water frozen into a single mass of ice. This ice began to accumulate as the planet's weather cooled, and began to stay frozen all year round. As it built up higher and higher, it began to move out from the piled-high center, flowing while still solid.
Vast amounts of Canadian soil and rock, called glacial till, rode on the ice sheets as they moved, or surfed slowly before the front of the ice wall. Some of the richest farmland in the United States Midwest and northeast arrived in its present location in this way-as well as boulders that must be removed from fields before plowing. In the unusual geographic conditions following the retreat of the ice sheets, barren soil lay on the landscape, no longer held down by the glacier. Windstorms moved tremendous amounts of this soil far from where the glacier left it, to settle out of the sky as a layer of fertile soil, called loess in German and English. Loess soils settled in the Mississippi and Missouri Valleys, and also Washington, Oregon, Oklahoma, and Texas.
This continental glaciation happened seven times over the last 2.2 million years. Warm intervals, some of them hundreds of thousands of years long, stretched between these planetary deep-freezes. Geologists do not agree whether the ice will return or not. Even if the present day is in a warm period between glaciations, tens or hundreds of thousands of years may elapse before the next advance of the ice sheets.
California lies between two different kinds of plate boundaries. To the south, the crust under California is growing; to the north it is shrinking. The part of California that sits on the Pacific Plate between these two forces is moved northward in sudden increments of a few feet which are felt as earthquakes. A few feet at a time, in earthquakes that happen every few decades, the part of California west of the San Andreas Fault will move northward along the coast.
Active faults also exist elsewhere in the United States, in the Midwest and in South Carolina. The last sizeable earthquakes in these regions occurred more than a hundred years ago, and geologists assume that earthquakes will probably occur within the next hundred years. The Pacific Northwest and Alaska, sitting atop active tectonic environments, will certainly be shaken by earthquakes for millions of years to come. The Great Basin, the western Rocky Mountains, and the United States northeast are all considered tectonically active enough for earthquakes to be considered possible.
North America's volcanic mountain ranges, the Cascades, and the relatively recent Mexican Volcanic Belt, have erupted often in the recent geologic past. These mountains will certainly continue to erupt in the near geologic future.
See also Continental drift theory; Continental shelf; Earth (planet); Faults and fractures; Fossils and fossilization; Ice ages; Orogeny
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