North America

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 in the Laramide orogeny, 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 Laramide orogeny 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 surface—the mountains' roots were buried in sediment from the eroded mountain tops. 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 volcanos 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 volcanos 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 today—perhaps 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.