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 g ...

lake bed in north Lake County,

Oregon Oregon ( , ) is a U.S. state, state in the Pacific Northwest region of the United States. It is a part of the Western U.S., with the Columbia River delineating much of Oregon's northern boundary with Washington (state), Washington, while t ...

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. 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

palisade A palisade, sometimes called a stakewall or a paling, is typically a row of closely placed, high vertical standing tree trunks or wooden or iron stakes used as a fence for enclosure or as a defensive wall. Palisades can form a stockade. Etymo ...

s of a fort. The region of Fort Rock–Christmas Lake Valley basin contains about 40 such tuff rings and

maar A maar is a broad, low-relief volcanic crater caused by a phreatomagmatic eruption (an explosion which occurs when groundwater comes into contact with hot lava or magma). A maar characteristically fills with water to form a relatively shallow ...

s and is located in the Brothers Fault Zone of

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 t ...

Great Basin The Great Basin () is the largest area of contiguous endorheic watersheds, those with no outlets to the ocean, in North America. It spans nearly all of Nevada, much of Utah, and portions of California, Idaho, Oregon, Wyoming, and Baja Californi ...

. 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 aphanite, 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 planetary surface, surface of a terrestrial ...

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

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 Natural satellite, moon onto its surface. Lava may be erupted at a volcano or through a Fissure vent, fractu ...

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 water ...

and formed a saucer-shaped ring of

lapilli tuff Lapilli (: lapillus) is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' is Latin for "little stones". By definition lapilli range from in diam ...

and

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

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 Fort Rock Cave was the site of the earliest evidence of human habitation in the US state of Oregon before the excavation of the Paisley Caves. Fort Rock Cave featured numerous well-preserved sagebrush sandals, ranging from 9,000 to 13,000 years ...

, where in 1938 Luther Cressman from the

University of Oregon The University of Oregon (UO, U of O or Oregon) is a Public university, public research university in Eugene, Oregon, United States. Founded in 1876, the university is organized into nine colleges and schools and offers 420 undergraduate and gra ...

discovered

sagebrush Sagebrush is the common name of several woody and herbaceous species of plants in the genus ''Artemisia (plant), Artemisia''. The best-known sagebrush is the shrub ''Artemisia tridentata''. Sagebrush is native to the western half of North Amer ...

sandals and human artifacts dated approximately 9,000 to 10,000 years ago. Hole-in-the-Ground and

Big Hole The Kimberley Mine or Tim Kuilmine () is an open-pit mining, open-pit and underground mine in Kimberley, South Africa. It has been considered the deepest hole excavated by hand, contending the title with Jagersfontein Mine. History and size ...

are two nearby maars nearly one mile in diameter that were formed by steam explosion. They resemble impact

crater A crater is a landform consisting of a hole or depression (geology), depression on a planetary surface, usually caused either by an object hitting the surface, or by geological activity on the planet. A crater has classically been described ...

s formed by

meteorites A meteorite is a rock that originated in outer space and has fallen to the surface of a planet or moon. When the original object enters the atmosphere, various factors such as friction, pressure, and chemical interactions with the atmospheri ...

, but lack the heavy metal signature residues of space objects. Crack in the Ground and Fossil Lake 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 g ...

geological features. South Ice Cave is a lava tube.

Derrick Cave Derrick Cave is a lava tube located in the remote northwest corner of Lake County, Oregon. The cave is approximately long. It is up to wide and high in places. It was named in honor of H.E. Derrick, a pioneer rancher with a homestead southea ...

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 A kīpuka is an area of land surrounded by one or more younger lava flows. A kīpuka forms when lava flows on either side of a hill, ridge, or older lava dome as it moves downslope or spreads from its source. Older and more weathered than their ...

, 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 unde ...

and pāhoehoe flows. Fort Rock Valley Historical Homestead Museum and the small community of

Fort Rock Fort Rock is a tuff ring located on an ice age lake bed in north Lake County, Oregon, Lake County, Oregon, United States. The ring is about in diameter and stands about high above the surrounding plain. Its name is derived from the tall, stra ...

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 Pleistocene volcanoes