Fort Rock is a

tuff ring Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions cont ...

located on an

ice age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...

lake bed in north Lake County,

Oregon Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. T ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territori ...

. The ring is about in diameter and stands about high above the surrounding plain. Its name is derived from the tall, straight sides that resemble the palisades of a fort. The region of Fort Rock–Christmas Lake Valley basin contains about 40 such tuff rings and maars and is located in the

Brothers Fault Zone The Brothers Fault Zone (BFZ) is the most notable of a set of northwest-trending fault zones including the Eugene–Denio, McLoughlin, and Vale zones that dominate the Geology of Oregon, geological structure of most of Oregon. These are also ...

central Oregon Central Oregon is a geographic region in the U.S. state of Oregon and is traditionally considered to be made up of Deschutes, Jefferson, and Crook counties. Other definitions include larger areas, often encompassing areas to the north towards ...

's Great Basin. William Sullivan, an early settler in the area, named Fort Rock in 1873 while searching for lost cattle.

Geology

Fort Rock was created when

basalt Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

rose to the surface and encountered the wet muds of a lake bottom. Powered by a jet of steam, molten basalt was blown into the air, creating a fountain of hot

lava Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

particles and frothy ash. The pieces and blobs of hot lava and ash rained down around the

vent Vent or vents may refer to: Science and technology Biology *Vent, the cloaca region of an animal * Vent DNA polymerase, a thermostable DNA polymerase Geology *Hydrothermal vent, a fissure in a planet's surface from which geothermally heated wate ...

and formed a saucer-shaped ring of lapilli

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock ...

and

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

sitting like an island in the lake waters. Steam explosions also loosened angular chunks of black and red lava rock comprising the valley floor. These blocky inclusions are incorporated into the fine-grained tuff layers at Fort Rock. Waves from the lake waters eroded the outside of the ring, cutting the steep cliffs into terraces above the floor of Fort Rock Valley. The wave-cut terraces on the south side of the ring mark former lake levels of this now-dry lakebed. Southerly winds, which are still predominant in this region, apparently drove waves against the south side of the ring, eroding the soft ash layers, breaching it, and creating a large opening on the south side.

Age estimates

Previous age estimates of Fort Rock ranged upwards to 1.8 million years. Recently, the age of Fort Rock has been estimated at 50,000 to 100,000 years. This coincides with a period of time when large

pluvial In geology and climatology, a pluvial is either a modern climate characterized by relatively high precipitation or an interval of time of variable length, decades to thousands of years, during which a climate is characterized by relatively high ...

lakes filled the valleys of central Oregon and much of the Great Basin of the western United States. At its maximum, the water in Fort Rock Lake was estimated to cover nearly and was about deep where the Fort Rock tuff ring formed. The extensive terrace on the side of Fort Rock marks one lakeshore about 14,000 years ago. Even higher water levels are recorded on the tuff cliffs and at one point only the tops of the tuff ring were exposed as rocky islands in this inland sea. An age of about 21,000 years ago has been found for this highest lake level.

Fort Rock State Natural Area

Fort Rock is designated as ''Fort Rock State Natural Area'' (formerly Fort Rock State Park).

Other geological features

A nearby tuff ring has a water-formed cave, called Fort Rock Cave, where in 1938 Luther Cressman from the

University of Oregon The University of Oregon (UO, U of O or Oregon) is a public research university in Eugene, Oregon. Founded in 1876, the institution is well known for its strong ties to the sports apparel and marketing firm Nike, Inc, and its co-founder, billion ...

discovered

sagebrush Sagebrush is the common name of several woody and herbaceous species of plants in the genus '' Artemisia''. The best known sagebrush is the shrub '' Artemisia tridentata''. Sagebrushes are native to the North American west. Following is an al ...

sandals and human artifacts dated approximately 9,000 to 10,000 years ago. Hole-in-the-Ground and Big Hole are two nearby maars nearly one mile in diameter that were formed by steam explosion. They resemble impact craters formed by meteorites, but lack the heavy metal signature residues of space objects. Crack in the Ground and

Fossil Lake Fossil Butte National Monument is a United States National Monument managed by the National Park Service, located west of Kemmerer, Wyoming, United States. It centers on an assemblage of Eocene Epoch (56 to 34 million years ago) animal and pl ...

are two more nearby

Ice Age An ice age is a long period of reduction in the temperature of Earth's surface and atmosphere, resulting in the presence or expansion of continental and polar ice sheets and alpine glaciers. Earth's climate alternates between ice ages and gre ...

geological features. South Ice Cave is a lava tube. Derrick Cave is estimated to be long. Devil's Garden Lava Field and East Lava Field are other geological oddities northeast of Fort Rock a few miles. Inflated lava, kīpuka, and lava ponds are found here, plus both

ʻaʻā Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...

and

pāhoehoe Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or un ...

flows. Fort Rock Valley Historical Homestead Museum and the small community of Fort Rock are one mile south of Fort Rock State Natural Area.

References

External links

"Fort Rock State Natural Area"
Oregon State Parks. *
"Picture of Fort Rock in 1911"
Library of Congress. * {{authority control Volcanoes of Oregon Maars of Oregon Tuff cones State parks of Oregon National Natural Landmarks in Oregon Mountains of Lake County, Oregon Mountains of Oregon Parks in Lake County, Oregon Landforms of Lake County, Oregon