Death Valley: The Lowest, Hottest Place
The spectacularly eroded Death Valley is a strange desert in a continental sinkhole that remains one of the most bizarre and forbidding places in the world. Pine-clad mountains harboring the oldest trees on Earth overlook the breathtaking plunge into a valley that ranks as the lowest, hottest, and driest place in North America.
The rocks in and around Death Valley have a much longer history than the valley itself, which is relatively young. Some of the rocks in the surrounding mountains are more than 500 million years old, formed miles beneath the Earth before the first living things crawled out onto dry land. Those deeply buried rocks were thrust upward through miles of rock into their present positions. These Precambrian rocks include gneiss, schist, granite, and some volcanic rocks dating back 1.8 billion years.
In Mesozoic times (245 million to 65 million years ago), the quiet, sea-covered continental margin was replaced by erupting volcanoes, uplifting mountains, and compressional thrust faulting. These changes were brought about by a tectonic collision to the west when the western edge of the North American continent was pushed against the oceanic plate under the Pacific Ocean. As the dinosaurs died out, the mammals flourished, the Ice Ages came and went, and Death Valley and the whole of the Mojave Desert region changed dramatically. During the 1.8-millionyear-long progression of Ice Ages, Death Valley filled with lakes, along with camels, deer, rhinoceroses, titanotheres, and three-toed horses. But the rise of the Sierra Nevadas some 25 million years ago began to transform the valley into a desert. The surge of the Earth provoked numerous volcanic eruptions, sometimes blanketing the entire region in ash and creating a complex network of new faults and fractures.
Death Valley is bounded by long, deep, active earthquake faults. Such faults mark the place where the Earth has been stretched, strained, and cracked. Pressures from beneath cause the crust to crack along such faults, which often allows mountain ranges to rise. When a chunk of the Earth between two such mountain-range-raising faults splits lose and drops down as the land on either side rises, it is called a graben. That makes Death Valley one of the most dramatic grabens on the planet. So although the mountains overlooking the valley rise to 11,000 feet (3,667 m), the floor of Death Valley is actually 282 feet (94 m) below sea level. The valley itself didn't begin to open up along these fault lines until some 4 million years ago, when the faults lifted the mountains and dropped the valley floor. The shift caused dramatic changes all around the valley. For instance, the lake bottom deposits laid down in earlier wet periods were lifted high above the valley floor as the mountains rose and the floor sank, to intrigue future generations of geologists.
Although the mountains tower above the sunken valley, they are really only the shattered remnants of mountains that have eroded away. In fact, the floor of Death Valley is still sinking and the mountains are still rising due to the continued strain and stretch of Earth's crust. The erosion of the surrounding mountains has filled the still-sinking Death Valley with some 10,000 feet (3,300 m) of sediment. Taking that 10,000 feet shaved off the top of the mountains and deposited into the valley into account, the mountains around Death Valley have been pushed upward a total of nearly four miles.
All of the debris carried off the mountains by running water has ended up in Death Valley, which is so low that no streams can run out of it. Such low-lying valleys with “interior drainage” are a hallmark of deserts. In wet cycles such as Ice Ages, they often hold huge, freshwater lakes. In dry cycles when the planet warms, they become scorching deserts. In fact, one key definition of a desert is a region where the evaporation exceeds the annual rainfall. For instance, trapped in the rain shadow of the Sierra Nevadas, Death Valley receives only about 1.7 inches (43 mm) of rain per year. However, it is so low and hot that a lake in the bottom of Death Valley would lose 12.5 feet (4.1 m) of water per year to evaporation.
So even the streams that run down off the imposing surrounding mountains quickly vanish into the thirsty sands of Death Valley. This strange geologic history has made Death Valley the most extreme place in the Mojave Desert, with its mere trace of rain, blazing hot summers, and frigid winters. It has also created a bizarre collection of geological wonders, which includes Badwater, at 282 feet (94 m) below sea level, the lowest place in North America. The deepest, youngest place in the valley, Badwater began gaping open and sinking some 3 million years ago. Nonetheless, the gravel and sand washed out of the surrounding mountains have accumulated in a layer 8,000 feet (2,667 m) deep.
This nearly two-mile-deep (3.2 km) layer of rocks and debris is covered by a thick layer of silt and salt. The water that washes out of the surrounding mountains and comes finally to this lowest place contains dissolved salt and minerals. But the water evaporates quickly from the soil, leaving behind those salts and minerals. The result is the great Death Valley Salt Pan, an amazingly flat expanse that in the winter collects a shallow sheen of water far saltier than the ocean and in the summer hardens to a salt flat.
Northwest of this low point lies Devils' Golf Course, a bed of solid rock salt that is the remains of Lake Manly, which was 90 miles (144 km) long and 600 feet (200 m) deep before drying up as the last Ice Age ended some 8,000 years ago, leaving behind its salt and minerals. The bed of the vanished lake is now covered with a 3–6-foot-thick (1–2 m) layer of salt. Portions of the salt bed that are above the water table and out of reach of the infrequent floods have dissolved into a strange landscape of pinnacles and spires.
Death Valley also contains valuable deposits of a chemical salt called borax, once vital in making ceramic glazes, fertilizer, glass, fiberglass, solder, water softeners, and some drugs. The borax leaches out of volcanic ash in superheated volcanic hot springs. Such hot springs fumed and steamed beneath Death Valley for millions of years as a result of all the geological upheaval, which left layers of borax up to 200 feet (67 m) thick. Fractures and uplift cracked open the layers of borax, and rain dissolved the mineral salts, carried them to the valley floor, and then deposited thick layers as the water evaporated. This layer of mineral salt spurred one of the few viable industries in Death Valley, a borax mining operation that scraped off the surface of the salt pan, purified it, dried it, and shipped it to the waiting world in great wagons hauled by 20-mule teams.
Another oddity on the floor of Death Valley is the Racetrack Playa, which features seemingly impossible wind-pushed rocks that have slid great distances on a perfectly flat plain. Strangely enough, no one has actually seen the rocks move, but their tracks are perfectly preserved in the hardened mud of the flat surface. Geologists have not settled on one explanation for the mystery, but one theory suggests that the occasional combination of ice, a slippery flat surface, and wind are responsible for the movement. The rocks sit on an extremely smooth, nearly flat surface of fine-grained silt. About once in every 10 years, a severe winter storm will cover the Racetrack Playa with about a foot of water. Usually, such winter storms bring a cold front in their wake, resulting in a sharp drop in temperature. In that case, ice can form around the edges of the rock and on the surface of the water collected in the playa. As the desert warms up and this layer of ice thaws, some of the rocks will end up on their own little ice raft. A strong wind can push this ice raft, often with the rock dragging a projecting knob, which cuts a path through the silt of the surface. The result is a rock sitting in the middle of a seemingly perfectly level mud-crack playa, with the track of its passage over the silt plainly visible along its path.
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