Intermontane Plateaus facts for kids
The Intermontane Plateaus are a huge area in the Western United States. It is one of eight main land regions in the contiguous United States. This region is made up of plateaus (flat, high lands) and mountain ranges that are found between larger mountain ranges. It is divided into smaller parts called provinces, and those are divided even further.
What are the Main Plateau Provinces?
The Columbia Plateau Province
The Columbia Plateau is a very large area of flood basalts. These are huge amounts of lava that flowed out of the ground a long, long time ago, during the Miocene and early Pliocene epochs. This happened across parts of Washington, Oregon, Idaho, Nevada, and California.
The Colorado Plateau Province
The Colorado Plateau is a vast region that has been shaped by amazing natural forces over millions of years. It was lifted up and cracked into huge blocks by faults. Then, wind and water slowly wore down the land, creating unique shapes. Later, it was lifted up again, and the deep canyons we see today were carved out.
How Erosion Shaped the Land
Imagine layers of rock, like a giant cake. The Colorado Plateau has many layers. Over time, softer layers wore away faster than harder ones. This created huge, step-like cliffs called escarpments. Each cliff stands tall where a strong rock layer sits on top of a weaker one.
In southern Utah, you can see a fantastic series of these cliffs. As you travel north, you climb up through different colored cliffs, each made of different types of rock from different time periods:
- The Chocolate Cliffs (Triassic sandstones)
- The Vermilion and White Cliffs (Jurassic sandstones)
- The Gray Cliffs (Cretaceous sandstones, which look like ancient sand dunes!)
- The Pink Cliffs (Eocene rocks from old rivers and lakes)
Another cool feature is the Mogollon Rim. This is a huge cliff that marks the edge between the high Colorado Plateau and the lower Sonoran Desert.
Cliffs from Faults
When the land was lifted up again, new cracks (faults) appeared. These created new cliffs that are straighter than the older, worn-down ones. The most important of these new cliffs is along the western and southern edges of the Plateau Province.
The rivers in this area, like the Virgin River and the famous Colorado River, became very powerful when the land lifted. They cut deep canyons into the rock. The Grand Canyon in Arizona is the most famous example, carved by the Colorado River through ancient limestone.
Sometimes, old faults that were hidden by erosion become visible again. This happens when softer rock along one side of the fault wears away, leaving the harder rock on the other side standing tall. These are called fault-line scarps.
Volcanic Features
The Plateau Province also has many volcanoes and volcanic features. Some high plateaus are covered with old lava flows. Near the Grand Canyon, you can find a group of large volcanoes, including Humphreys Peak. This mountain is a volcano that has been worn down a bit by erosion.
In western New Mexico, Mount Taylor is another old volcano. Over time, erosion has worn away its outer layers, exposing the hard rock "pipes" that once carried lava to the surface. The Henry Mountains in southwestern Utah are special because their hills were formed when lava pushed up the ground from underneath, creating dome-like shapes.
The Basin and Range Province
The Basin and Range Province is a huge area where the Earth's crust has stretched and pulled apart. This stretching created a unique landscape of many long, disconnected mountain ranges separated by wide, flat desert plains or basins.
These mountain ranges are usually 30 to 100 miles (48 to 160 km) long and can reach heights of 8,000 to 10,000 feet (2,400 to 3,000 meters). The flat basins between them can be as low as sea level or as high as 4,000 to 5,000 feet (1,200 to 1,500 meters). This whole region is very dry.
Many of the flat plains are filled with rock and dirt that has washed down from the mountains. When the land broke into blocks and tilted, one side of each block was a smooth, old surface, and the other side was a fresh, broken surface. Over time, erosion has changed these shapes, and some lower blocks have been buried by material from the higher ones.
Northern Areas
In the northern parts, like southeastern Oregon and northeastern California, the land blocks are tilted. The erosion here is not as extreme, meaning these land changes happened more recently.
Central Areas
In the Great Basin of western Utah and most of Nevada, the mountains were formed by ancient folding and faulting. These old mountains were worn down to lowlands before the land broke into blocks. Now, these blocks have been reshaped so much by erosion that their original tilted shapes are hard to see.
The lower areas are now covered with huge alluvial fans. These are fan-shaped piles of rock and dirt carried down from the mountains by occasional floods. Each fan starts at a ravine in the mountains and spreads out, joining with other fans to form a wide, gently sloping plain.
Southern Areas
In the southern part of the Basin and Range Province, including the Mojave Desert in California and the Sonoran Desert in California, Arizona, and Mexico, the mountain ranges are well-eroded. The flat areas between them often have rock floors that gently slope towards the center. This area also has huge alluvial fans, similar to those in the central areas.
The Basin and Range Province stretches far south into Mexico, along the Mexican Plateau.
River Systems and Basins
Only a small part of the Basin and Range Province drains to the sea. Some northern areas flow west through the Klamath River. The Sacramento River and San Joaquin River from the Sierra Nevada mountains flow through the Central Valley to San Francisco Bay. A few southern basins drain to the Rio Grande and then to the Gulf of Mexico.
The Colorado River's Path
A larger part of the region is drained by the Colorado River, which flows southwest to the Gulf of California. This powerful river has created a huge Colorado River Delta. North of this delta, a part of the old gulf is now cut off from the sea and has dried up, forming a plain below sea level.
In the early 1900s, a project to bring water from the Colorado River to the Imperial Valley led to a flood. The river broke through a canal and flowed into the Salton Sink, creating the Salton Sea. This lake has no outlet to the ocean and is now fed by water from irrigated farms.
Seasonal Streams and Lakes
Many streams that flow down from the mountains simply disappear into the desert floor before reaching a larger river. Others flow only in the winter when there is less evaporation.
Some large streams, like the Mojave River, can spread out into a temporary shallow lake on a flat, clay area called a playa during floods. But in the warm season, the water vanishes, and the stream shrinks. The dry clay floor of the playa becomes firm enough to cross.
A few basins have permanent lakes with no outlet, meaning water leaves only by evaporation. Great Salt Lake in Utah is the largest example. Other examples include Owens Lake and Mono Lake in California.
During the Pleistocene Ice Age, these lakes were much deeper and larger. You can still see the old shorelines of ancient lakes like Lake Bonneville and Lake Lahontan hundreds of feet above the current lake surfaces. These old shorelines tell us a lot about past climate changes.
The Intermediate Province
The Intermediate Province is found in parts of Washington, Oregon, and Idaho.
Vast Lava Plains
The lava fields of the Columbia River Basin are some of the biggest volcanic areas in the world. They cover over 81,000 square miles (210,000 square kilometers) and can be as deep as 4,000 feet (1,200 meters) in some river canyons.
This lava completely buried the old landforms. Some lava flows are so new that they still have their rough, bubbly surfaces. Other parts are much older and have been changed by erosion. For example, the old lava sheets in Yellowstone National Park are connected to these Columbia plains.
Deep River Canyons
The Columbia River has carved a deep, canyon-like valley around the northern and western edges of these lava plains. The Snake River has cut an even deeper canyon farther southeast. This canyon shows the many layers of lava that make up the plains. Sometimes, it even reveals an old buried mountain where the river has cut an even narrower canyon.
One amazing feature of this province is the Grande Coulee. During the Ice Age, glaciers blocked the Columbia River's usual path. The river had to find a new, temporary route across the plains. It flowed there long enough to carve this deep gorge.
The lava plains are mostly treeless and too dry for farming. However, they support wildlife and are used for raising cattle and horses. Along their eastern edge, where there is a bit more rainfall, you can find very deep, fine soil. This soil is thought to be dust blown in from the drier parts of the plains to the west.
