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thumb|right|upright=1.25|Shaded relief map of the United States, showing 10 geological provinces
The richly textured landscape of the United States is a product of the dueling forces of plate tectonics, weathering and erosion. Over the 4.5 billion-year history of the Earth, tectonic upheavals and colliding plates have raised great mountain ranges while the forces of erosion and weathering worked to tear them down. Even after many millions of years, records of Earth's great upheavals remain imprinted as textural variations and surface patterns that define distinctive landscapes or provinces.
The diversity of the landscapes of the United States can be easily seen on the shaded relief image to the right. The stark contrast between the 'rough' texture of the western US and the 'smooth' central and eastern regions is immediately apparent. Differences in roughness (topographic relief) result from a variety of processes acting on the underlying rock. The plate tectonic history of a region strongly influences the rock type and structure exposed at the surface, but differing rates of erosion that accompany changing climates can also have profound impacts on the land.
The Pacific Province straddles the boundaries between several of Earth's moving plates:the source of the monumental forces required to build the sweeping arc of mountains that extends from Alaska to the southern reaches of South America. This province includes the active and sometimes deadly volcanoes of the Cascade Range and the young, steep mountains of the Pacific Border and the Sierra Nevada.
Although the largest volcanoes like Mount St. Helens get the most attention, the Cascades is really made up of a band of thousands of very small, short-lived volcanoes that have built a platform of lava and volcanic debris. Rising above this volcanic platform are a few strikingly large volcanoes that dominate the landscape.
thumb|right|The Cascade Range is formed by an active continental margin
A slice of the Earth from the Pacific Ocean through the Pacific Northwest might look something like the adjacent image. Beneath the Cascades, a dense oceanic plate plunges beneath the North American plate; a process known as subduction. As the oceanic slab sinks deep into the Earth's interior beneath the continental plate, high temperatures and pressures allow water molecules locked in the minerals of solid rock to escape. The water vapor rises into the pliable mantle above the subducting plate, causing some of the mantle to melt. This newly formed magma rises toward the Earth's surface to erupt, forming a chain of volcanoes (the Cascade Range) above the subduction zone.
Over of basaltic lava, known as the Columbia River basalts, covers the western part of the province. These tremendous flows erupted between 17 and 6 million years ago. Most of the lava flooded out in the first 1.5 million years: an extraordinarily short time for such an outpouring of molten rock.
The Basin and Range province has a characteristic topography that is familiar to anyone who ventures across it. Steep climbs up elongate mountain ranges alternate with long treks across flat, dry deserts. This basic topographic pattern extends from eastern California to central Utah, and from southern Idaho into the state of Sonora in Mexico. The forces which created this distinct topography lie deep beneath the surface.
thumb|right|The Basin and Range province, in central Nevada, as seen from space.
Within the Basin and Range Province, the Earth's crust (and upper mantle) has been stretched up to 100% of its original width. The entire region has been subjected to extension that thinned and cracked the crust as it was pulled apart, creating large faults. Along these roughly north–south-trending faults mountains were uplifted and valleys down-dropped, producing the distinctive alternating pattern of linear mountain ranges and valleys of the Basin and Range province. The Basin and Range province should not be confused with The Great Basin, which is a sub-section of the greater Basin and Range physiographic region defined by its unique hydrological characteristics (internal drainage).
Great Basin
thumb|right|upright=1.2|Map of the Great Basin
The Great Basin is the geographical and hydrological region comprising most of Nevada, southern Oregon and Idaho, western Utah, and a little of eastern California. Characterized by internal drainage, this region's surface water sources evaporate or percolate before they can flow to the ocean.
The sculptured beauty and brilliant colors of the Colorado Plateau's sedimentary rock layers have captured the imaginations of countless geologists. This is a vast region of plateaus, mesas, and deep canyons whose walls expose rocks ranging in age from billions to just a few hundred years old.
Ancient Precambrian rocks, exposed only in the deepest canyons, make up the basement of the Colorado Plateau. Most are metamorphic rocks formed deep within the Earth while continental collision on a grand scale produced the nucleus of the North American continent well over a billion years ago. Igneous rocks injected millions of years later form a marbled network through parts of the Colorado Plateau's darker metamorphic basement.
The rocks making up the mountains were formed before the mountains were raised. The cores of the mountain ranges are in most places formed of pieces of continental crust that are over one billion years old. In the south, an older mountain range was formed 300 million years ago, then eroded away. The rocks of that older range were reformed into the Rocky Mountains.
The Rocky Mountains took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States. Three major mountain-building episodes reshaped the west from about 170 to 40 million years ago (Jurassic to Cenozoic Periods). The last mountain building event, the Laramide orogeny, (about 70-40 million years ago) the last of the three episodes, is responsible for raising the Rocky Mountains. Periods of glaciation occurred from the Pleistocene Epoch (1.8 million–70,000 years ago) to the Holocene Epoch (fewer than 11,000 years ago). The ice ages left their mark on the Rockies, forming extensive glacial landforms, such as U-shaped valleys and cirques.
Laurentian Upland
Every continent has a core of very ancient metamorphic rocks. The Superior Upland Province is the southern extension of the Laurentian Upland Province, part of the nucleus of North America called the Canadian Shield. The basement rocks of the Laurentian Upland Province were metamorphosed about 2500 million years ago in a mountain-building collision of tectonic plates called the Kenoran Orogeny.
The rocks of the Superior Upland are mostly Precambrian metamorphic rocks and overlying Paleozoic rocks (Cambrian) covered by a thin veneer of glacial deposits left behind when glaciers melted at the end of the Pleistocene Ice Age. If we could strip away all of the younger rocks deposited on top of buried Precambrian basement, you would see a landscape of low relief. The topography of the Precambrian rocks is very subdued, with barely 500 feet difference between the highest point and the lowest. Clearly, this region was exposed to a very long period of erosion in the very distant past which beveled the original mountainous surface to a gently undulating surface. The present surface is not much different. Hills rise just a few hundred feet above the surrounding countryside. The highest of these, such as Rib Hill, Wisconsin, are made up mostly of resistant quartzite or granite.
Throughout the Paleozoic and Mesozoic Eras the mostly low-lying Interior Plains region remained relatively unaffected by the mountain-building tectonic collisions suffered by the western and eastern margins of the continent.]]
The rocks of the Appalachian, Ouachita, Ozark Mountains are old and share a common origin. They largely consist of sedimentary rocks of Paleozoic age that were deposited on the sea floor and are presently folded and faulted. The Appalachians also have volcanic rocks and slivers of ancient sea floor. These mountains were once part of a mighty uplifted mountain range that stretch from the Appalachian Highlands through Texas.
During the earliest Paleozoic Era, the continent that would later become North America straddled the equator. The Appalachian region was a passive plate margin, not unlike today's Atlantic Coastal Plain Province. During this interval, the region was periodically submerged beneath shallows seas. Thick layers of sediment and carbonate rock was deposited on the shallow sea bottom when the region was submerged. When seas receded, terrestrial sedimentary deposits and erosion dominated.
During the Late Triassic, Pangea began to be torn apart when a three-pronged fissure grew between Africa, South America, and North America. Rifting began as magma welled up through the weakness in the crust, creating a volcanic rift zone. Volcanic eruptions spewed ash and volcanic debris across the landscape as these severed continent-sized fragments of Pangea diverged. These terranes were caused by the subduction of the Farallon, Kula, and Pacific plates sequentially.
Hawaii
thumb|right|Fissure eruption of [[Kīlauea in 2018]]
The State of Hawaii consists of a chain of islands, or archipelago. The archipelago developed as the Pacific plate moved slowly northwestward over a hotspot in the Earth's mantle at a rate of approximately per million years. Thus, the southeast island (Hawaii) is volcanically active whereas the islands on the northwest end of the archipelago are older and typically smaller, due to longer exposure to erosion. The age of the archipelago has been estimated using potassium-argon dating methods. From this study and others, it is estimated that the northwesternmost island, the Kure Atoll, is the oldest at approximately 28 million years (Ma); while Hawaii, is approximately 0.4 Ma (400,000 years). The only active volcanism in the last 200 years has been on Hawaii and on the submerged but growing volcano to the extreme southeast, Kamaʻehuakanaloa (formerly Loihi).
Almost all of the magma of the hotspot has the composition of basalt, and so the Hawaiian volcanoes are composed almost entirely of this igneous rock. There is very little coarser-grained gabbro and diabase. Nephelinite is exposed on the islands but is extremely rare. The majority of eruptions in Hawaii are Hawaiian-type eruptions because basaltic magma is relatively fluid compared with magmas typically involved in more explosive eruptions, such as the andesitic magmas that produce some of the spectacular and dangerous eruptions around the margins of the Pacific basin.