Similar, but much older, layers are found in NPS sites in the Keweenawan Rift of the Lake Superior region, where divergent plate boundary forces tried, unsuccessfully, to rip the North American continent apart 1.1 billion years ago. Lower pressure on the hot asthenosphere also has another important effect. Letters in ovals represnt th 6 Keweenawan Rift parks listed above. The Basin and Range Province has the world’s 8th deepest lake, Lake Tahoe on the California/Nevada border (1,645 feet; 501 meters). The topography of the Basin and Range Province and Rio Grande Rift reveals the full range of characteristics of a continental rift zone.
Above illustrations modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.
These include the world’s deepest, Lake Baikal in Siberia (5,387 feet; 1,642 meters deep) and the 2nd and 4th deepest, Lake Tanganyika (4,323 feet; 1,318 meters) and Lake Malawi (2,316 feet; 706 meters), in the East African Rift. The lowest point on Earth, the Dead Sea, is in a rift valley in the very dry Middle East.
Today, the theory of continental drift has been replaced by the science of plate tectonics. It breaks along normal faults (the blocks fall down), causing earthquakes and blocks of mountains ranges separated by rift valleys. As the plate stretches and thins, the underlying asthenosphere flows upward and expands like a hot-air balloon, lifting the region to higher elevations. Tuscon Basin lies between the Rincon Mountains and Tucson Mountains (ranges). Modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172. And while Craters of the Moon National Monument in southern Idaho lies along the Columbia Plateau–Yellowstone hotspot track, recent basalt fissure eruptions and cinder cones are most likely due to Basin and Range continental rifting.
The drop in pressure on the shallow asthenosphere makes it start to melt. The high elevations (brown shading) and rugged topography of the Basin and Range Province and Rio Grande Rift, which are actively forming can be seen in the shaded relief map, below. The long valleys (“basins”) separating the mountains (“ranges”) are filled with river and lake deposits and lava flows. Thus a second characteristic of continental rifts is that their valleys contain most of the deepest lakes in the world. A Continental rift is the belt or zone of the continental lithosphere where the extensional deformation (rifting) is occurring. Death Valley (basin) and Panamint Mountains (range). At its normal depths beneath the lithosphere, this part of Earth’s mantle is solid because it is under so much pressure. The underlying asthenosphere rises and expands like a hot-air balloon, elevating a broad region.
Magma originating from the mantle must initially melt its way through that crust in order to reach the surface.
Earthquakes, fault-block mountains, and volcanism at Guadalupe Mountains National Park and Bandelier and White Sands national monuments are consequences of the ongoing continental rifting. These landscapes are a result of continental rifting, or places where the continental crust is extending and thinning.
Molten rock (magma) thus melts off the decompressed mantle beneath the Basin and Range Province and Rio Grande Rift. There the whole landscape is moving upward, with the ranges rising a little faster than the adjacent basins.
The theory of continental drift is most associated with the scientist Alfred Wegener.
The process stopped, but not before the area looked something like the present Basin and Range Province or Rio Grande Rift. Continental rifting causes valleys floors to drop down along fault lines in the Basin and Range Province. Like other continents, North America has thick crust, compared to the thin crust beneath the adjacent Atlantic and Pacific oceans.
Continental rifting processes are showcased in the steep fault escarpments and active volcanoes that characterize the western United States. Lakes pond against active fault escarpments where one side of a rift valley moves downward faster than the other.
The continental crust breaks along faults, forming long mountain ranges separated by rift valleys.
Such lakes were important to indigenous Americans. The basin and range topography develops over a few million years as fault lines move gradually, or more abruptly during tens of thousands of earthquakes. The lower crust and underlying asthenosphere are so hot that they stretch in a ductile fashion (like silly putty), without producing earthquakes.
During the waning stages of the latest ice age, between 20,000 and 10,000 years ago, the Basin and Range Province was much cooler and wetter than it is today. (From Lillie, 2005.).
Magma reaching the surface erupts from volcanoes and fissures as lava flows and other volcanic materials, mingling with river and lake sediments to fill rift basins. But the upper part of the crust is relatively cold and breaks in a brittle fashion (like peanut brittle), making earthquakes. Line A-A’ is the line of the geologic cross section shown below. Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893. Some of the lava that initially pours out at continental rift zones is thus thick and pasty, cooling to light-colored rocks (rhyolite and dacite), and steep-sided lava domes and composite volcanoes. Rifting is the process by which the continental lithosphere stretches.
Under the lowered pressure it begins to melt (much like superheated water flashes to steam when the lid is suddenly removed from a pressure cooker).
(Click on arrows and slide left and right to see labels.). These landscapes are a result of continental rifting, or places where the continental crust is extending and thinning. Snake Valley, Tucson Basin and Death Valley are the basins, while the Snake Range, Rincon Mountains, Tucson Mountains and Panamint Mountains are the adjacent ranges. Similar rocks (shown in gray) are buried beneath younger sedimentary layers in Michigan’s lower peninsular, and extend southwestward all the way to Kansas. If the plate is capped by thick continental crust, the resulting continental rift zone rises high above sea level.
The Basin and Range Province is thus a high-elevation region that includes all of Nevada and portions of Wyoming, Utah, California, and Arizona, and extends into southern Oregon, Idaho, and Montana.
Some of the National Parks and Monuments formed by continental rifting are: • Capulin Volcano National Monument, NM• City of Rocks National Reserve, ID• Craters of the Moon National Monument and Preserve, ID• Newberry National Volcanic Monument, OR• Sunset Crater Volcano National Monument, AZ, VW is a higher education, k-12, and public outreach project of the, Craters of the Moon National Monument and Preserve, ID. Sedimentary and volcanic rocks found in Isle Royale National Park and Keweenaw National Historical Park are products of this ancient continental rifting. The underlying asthenosphere rises and expands like a hot-air balloon, elevating a broad region. In the western part of the continent, divergent plate boundary forces are beginning to rip the continent apart, forming the Basin and Range Province and its adjacent eastern arm, known as the Rio Grande Rift. But in the southern part of the province, where rifting is more advanced, elevations are generally lower—much of the floor of Death Valley in eastern California is below sea level! Eventually the asthenosphere upwells so close to the surface that magma that erupts onto the surface. Some of the liquid rock (magma) makes it to the surface and forms lava flows, shield volcanoes, cinder cones, and other volcanic features. As the crust thins, the hot, buoyant upper mantle (the asthenosphere) rises.
Basins thus held much more water than they do today. It is similar to the East African Rift, where volcanic materials and sedimentary layers deposited by rivers and lakes partially fill rift valleys. For example, the western half of Lassen Volcanic National Park in northern California has composite volcanoes and high-silica lava domes characteristic of the Cascadia Subduction Zone, while cinder cones and basalt lava flows in the eastern part of the park are more diagnostic of continental rifting in the Basin and Range Province. The same continental rifting processes that form the high-elevation valleys and mountain ranges of the Basin and Range Province and Rio Grande Rift also result in earthquakes and volcanic activity.
Eventually the asthenosphere upwells so close to the surface that magma that erupts onto the surface. b) The upper part of the plate is cold and brittle. NHP = National Historical Park; NL = National Lakeshore; NM = National Monument; NP = National Park; NSR = National Scenic Riverway.
The bulk of the eruptions at continental rift zones thus produce fluid, dark-colored lava (basalt) that spreads out as low-profile shield volcanoes, or erupts in fountains forming cinder cones. The Rio Grande Rift is an arm of the Basin and Range Province extending across westernmost Texas, New Mexico, and southern Colorado.
Where tectonic plates move away from one another the lithosphere thins. Archaeological sites along ancient shorelines—today more than 300 feet (90 meters) above the levels of modern “pluvial” lakes—reveal much about these earlier cultures. The colder upper crust cracks and breaks along faults (like peanut brittle), causing earthquakes and forming long mountains (ranges) separated by valleys (basins). For example, the floor of a basin in Wyoming, Jackson Hole, is 6,000 feet (1,830 meters) above sea level, while the adjacent Teton Range rises to over 13,000 feet (4,000 meters).
Tule Lake basin is partially filled with sediments and lava flows. Continental crust is thick and rich in silica.