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.
There are twelve main geological provinces in the United States: Pacific, Columbia Plateau, Basin and Range, Colorado Plateau, Rocky Mountains, Laurentian Upland, Interior Plains, Interior Highlands, Appalachian Highlands, Atlantic Plain, Alaskan, and Hawaiian. Each province has its own geologic history and unique features. This article will describe each province in turn.
Pacific Province This region is one of the most geologically young and tectonically active in North America. The generally rugged, mountainous landscape of this province provides evidence of ongoing mountain-building.
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.
Sierra Nevada
Although the Sierra Nevada and Cascade Range form a nearly continuous barrier along the western edge of the United States, the two ranges really have very little in common. They have been and continue to be formed by quite different geological forces and processes.
The rocks that form the backbone of the Sierra Nevada are mostly granitic rocks that formed during the Mesozoic Era, when non-avian dinosaurs roamed the Earth. At that time, an arc-shaped chain of volcanoes, similar to the present-day Cascade volcanic arc, erupted where the Sierra Nevada now stands. Rising through older Paleozoic rock, molten rock erupted at the surface as lava, but most solidified deep within the earth, forming the gray granitic rocks familiar to any Sierra traveler.
Although from a distance the Sierran rock looks quite similar, it is actually made up of many individual rock bodies that formed from repeated intrusions of magma over many millions of years.
Even as they grew, erosion was wearing away these Mesozoic Era volcanoes. By Late Cretaceous time, about 70 million years ago, the once-deep granitic rocks began to be exposed at the Earth's surface. By a few tens of millions of years ago, so much of the upper part had worn away that the surface of the ancient range had a low relief of just a few thousand feet.
It wasn't until quite recently, geologically speaking, that the Sierra Nevada range as we know it today began to grow. During the Miocene Epoch, less than 20 million years ago, the continental crust east of the Sierra Nevada began to stretch in an east–west direction. The crust broke into a series of north–south-trending valleys and mountain ranges: the beginning of the Basin and Range province.
Less than five million years ago, the range that we now know as the Sierra Nevada began to rise along its eastern margin. Through a combination of uplift of the Sierran block and down-dropping of the area to the east, the Sierra rose upward. Rising far more steeply to the east than the west, the entire Sierra Nevada can be thought of as an enormous tilted fault block with a long, gentle slope westward to California's Central Valley and steep eastern slope.
Not long after the Sierra uplift began, the Earth cooled, marking the beginning of the Pleistocene (Ice Age) Epoch. Glaciers grew in the Sierra highlands and made their way down former stream channels, carving U-shaped valleys. The sheer walls and hanging valleys of Yosemite National Park are a product of this chilly past.
Cascades Volcanic Province Where the Sierra Nevada ends, a chain of explosive volcanic centers, the Cascade volcanoes, begins. The Cascades Province forms an arc-shaped band extending from British Columbia to Northern California, roughly parallel to the Pacific coastline. Within this region, 13 major volcanic centers lie in sequence like a string of explosive pearls.
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.
The Cascades volcanoes define the Pacific Northwest section of the Ring of Fire, an array of volcanoes that rim the Pacific Ocean. As if volcanic hazards were not enough, the Ring of Fire is also infamous for its frequent earthquakes. In order to understand the origins of this concentrated band of Earth hazards, we have to slice deep into the Earth.
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.
A close-up look at the Cascades reveals a more complicated picture than the simple subduction zone shown in the image on the left. Not far off the coast of the North Pacific lies a spreading ridge; a divergent plate boundary made up of a series of breaks in the oceanic crust where new ocean crust is created. On one side of the spreading ridge new Pacific plate crust is made, then moves away from the ridge. On the other side of the spreading ridge the Juan de Fuca plate and Gorda plate move eastward.
There are some unusual features at the Cascade subduction zone. Where the Juan de Fuca plate sinks beneath the North American plate there is no deep trench, seismicity (earthquakes) are fewer than expected, and there is evidence of a decline in volcanic activity over the past few million years. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American plates. These two plates converge at 3–4 centimeters per year at present. This is only about half the rate of convergence of 7 million years ago.
The small Juan de Fuca plate and two platelets, the Explorer plate and Gorda plate are the meager remnants of the much larger Farallon oceanic plate. The Explorer plate broke away from the Juan de Fuca about 4 million years ago and shows no evidence that it is still being subducted. The Gorda platelet split away between 18 and 5 million years ago and continues to sink beneath North America.
The Cascade Range made its first appearance 36 million years ago, but the major peaks that rise up from today's volcanic centers were born within the last 1.6 million years (during the Pleistocene). More than 3000 vents erupted during the most recent volcanic episode that began 5 million years ago. As long as subduction continues, new Cascade volcanoes will continue to rise.
Columbia Plateau
The Columbia Plateau province is enveloped by one of the world's largest accumulations of lava. Over of the Earth's surface is covered by it, an area roughly the size of the country of Spain. The topography here is dominated by geologically young lava flows that inundated the countryside with amazing speed, all within the last 17 million years.
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 Snake River Plain stretches across Oregon, through northern Nevada, southern Idaho, and ends at the Yellowstone Plateau in Wyoming. Looking like a great spoon scooped out the Earth surface, the smooth topography of this province forms a striking contrast with the strong mountainous fabric around it.
The Snake River Plain lies in a distinct depression. At the western end, the base has dropped down along normal faults, forming a graben structure. Although there is extensive faulting at the eastern end, the structure is not as clear.
Like the Columbia River region, volcanic eruptions dominate the story of the Snake River Plain in the eastern part of the Columbia Plateau Province. The earliest Snake River Plain eruptions began about 15 million years ago, just as the tremendous early eruptions of Columbia River Basalt were ending. But most of the Snake River Plain volcanic rock is less than a few million years old, Pliocene age (5–1.6 million years ago) and younger.
In the west, the Columbia River Basalts are just that-almost exclusively black basalt. Not so in the Snake River Plain, where relatively quiet eruptions of soupy black basalt lava flows alternated with tremendous explosive eruptions of rhyolite, a light-colored volcanic rock.
Cinder cones dot the landscape of the Snake River Plain. Some are aligned along vents, the fissures that fed flows and cone-building eruptions. Calderas, great pits formed by explosive volcanism, and low shield volcanoes, and rhyolite hills are also part of the landscape here, but many are obscured by later lava flows.
Geologists usually associate volcanic eruptions with the boundaries between colliding or diverging plates. However, the focus of volcanism at Yellowstone in the Columbia Plateau Province is far inland from the subduction zone that lies along the Oregon and Washington coast. Evidence suggests that some concentrated heat source is melting rock beneath the Columbia Plateau Province, at the base of the lithosphere (the layer of crust and upper mantle that forms Earth's moving tectonic plates). In an effort to figure out why this area, far from a plate boundary, had such an enormous outpouring of lava, scientists established hardening dates for many of the individual lava flows. They found that the youngest volcanic rocks were clustered near the Yellowstone Plateau, and that the farther west they went, the older the lavas.
A probable explanation is that a hot spot, an extremely hot plume of deep mantle material, is rising to the surface beneath the Columbia Plateau Province. We know that beneath Hawaii and Iceland, a temperature instability develops (for reasons not yet well understood) at the boundary between the core and mantle. The concentrated heat triggers a plume hundreds of kilometers in diameter that ascends directly through to the surface of the Earth.
When the hot plume arrives at the base of the lithosphere, some of the lighter rock of the lithosphere rapidly melts. It is this molten lithosphere that becomes the basalt lavas that gush onto the surface to form the Columbia River and Snake River Plain basalts.