Kenozoic Mountain Building

KENOZOIC MOUNTAIN BUILDING Cenozoic is known for its intense mountain building. The surge in mountain growth over the past 5 million years may have triggered the Pleistocene ice age. The Rocky Mountains (Figure 181), stretching from Mexico to Canada, rose upward during the Laramide orogeny from about 80 million to 40 million years ago. During the Miocene, beginning about 25 million years ago, most of western North America was uplifted. The entire Rocky Mountain region rises about a mile above sea level. Large granite blocks soar high above the surrounding terrain. To the west in the Basin and Range Provinces, the crust stretches and thins, in some places falling below sea level. Arizona's Grand Canyon (Figures 182 and 183) is located at the southwestern end of the Colorado plateau, a generally mountain-free expanse that stretches from Arizona north to Utah and east to Colorado and New Mexico. Initially, the area around the canyon was practically flat. Over the past 2 billion years, heat and pressure bound the soil into mountains that were then flattened by erosion. Once again, mountains were formed and eroded, and the region was flooded with shallow seas. The ground was lifted next time during the rise of the Rocky Mountains. Between 10 and 20 million years ago, the Colorado River began eroding layers of sediment, exposing the raw underground rock below. Its current journey is less than 6 million years old. Much of the eastern Grand Canyon is geologically young, having eroded only in the last million years. About 30 million years ago, the North American continent approached the spreading center of the East Pacific Rise, the counterpart of the Mid-Atlantic Ridge. The first part of the continent to override the seafloor spreading axis is the coast of southern California and northwestern Mexico.When rift systems and subduction zones meet, the intervening oceanic plates plunge into deep trenches. Sediments in the trench are compressed and pushed upward to form the California Coast Ranges.A fault system associated with the 650-mile-long San Andreas Fault (Figure 184) crosses the mountain belt. The Sierra Nevada in the east rose about 7,000 feet over the past 10 million years and may be supported by masses of hot rock in the upper mantle. In the Pacific Northwest of the United States and British Columbia, the Juan de Fuca plate plunges into the Cascadia subduction zone located beneath the continent. When crustal plates 50 miles thick are subducted into the mantle, Earth's interior heat melts parts of the descending plates and adjacent lithospheric plates, forming pockets of magma. Magma rose to the surface, forming volcanoes from the Cascade Range (Fig. 185), which erupted in one great abundance after another. India and the rocks that make up the Himalayas broke away from Gondwana at the beginning of the Cretaceous, drove across the ancestral Indian Ocean, and hit southern Asia about 45 million years ago. When the Indian and Asian plates collided, the oceanic lithosphere between them pushed under Tibet, destroying 6,000 miles of subduction plates. The increase in buoyancy lifts the Himalayan Mountains and the vast Tibetan Plateau (Fig. 186), the size of which has not been equaled on the planet for more than 1 billion years. Over the past 5 to 10 million years, the entire region rose more than a mile in elevation. The collision of continents heated up a large amount of carbonate rock, spewing several hundred trillion tons of carbon dioxide into the atmosphere. This may explain why Earth grew so warm during the Eocene epoch from 54 million to 37 million years ago, when temperatures reached their highest in the last 65 million years. According to the fossil record, winters were warm enough for crocodiles to roam as far north as Wyoming, and forests of palms, cycads, and ferns covered Montana. About 50 million years ago, the Tethys Sea separating Eurasia from Africa narrowed as the two continents approached each other, then began to close completely about 20 million years ago. Thick sediments that have accumulated over tens of millions of years are bent into the long belts of mountains on the north and south sides (Fig. 187). Contact between the continents began an episode of great mountain-building that lifted the Alps and other ranges in Europe and squeezed the Tethys Sea. This episode of mountain-building, called the Alpine orogeny, raised the Pyrenees on the border between Spain and France, the Atlas Mountains in Northwest Africa, and the Carpathians in East-Central Europe. The Alps in northern Italy were formed in the same way as the Himalayas, when the Italian branch of the African plate thrust into the European plate. In South America, the spine of mountains comprising the Andes that runs along the western edge of the continent rose in most of the Cenozoic due to increased crustal buoyancy from the subduction of the Nazca plate beneath the South American plate. By the time all continents had wandered to their current positions and all the mountains had risen to their current heights, the world was ripe for the arrival of the Ice Age.