Trimble Knob, located southwest of

Monterey Monterey ( ; ) is a city situated on the southern edge of Monterey Bay, on the Central Coast of California. Located in Monterey County, the city occupies a land area of and recorded a population of 30,218 in the 2020 census. The city was fou ...

in Highland County,

Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...

, is a

conical hill A conical hill (also cone or conical mountain) is a landform with a distinctly conical shape. It is usually isolated or rises above other surrounding foothills, and is often of volcanic origin. Conical hills or mountains occur in different sha ...

composed of

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 ...

, a

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

rock, of

Eocene The Eocene ( ) is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (Ma). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene'' comes ...

(early

Tertiary Tertiary (from Latin, meaning 'third' or 'of the third degree/order..') may refer to: * Tertiary period, an obsolete geologic period spanning from 66 to 2.6 million years ago * Tertiary (chemistry), a term describing bonding patterns in organic ch ...

) age. It is the eroded remnant of what was an active

volcano A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most oft ...

diatreme A diatreme, sometimes known as a maar-diatreme volcano, is a volcanic pipe associated with a gaseous explosion. When magma rises up through a crack in Earth's crust and makes contact with a shallow body of groundwater, rapid expansion of heated ...

that last erupted approximately 35 million years ago, making it one of the youngest volcanos on the east coast of

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

Description

Trimble Knob is an isolated conical hill in an otherwise relatively flat valley, surrounded by farmland. The peak of the hill has an elevation of 3123 ft (952m).

U.S. Route 220 U.S. Route 220 (US 220) is a spur route of US 20. It runs in a north–south layout in the eastern United States, unlike its parent route as well as conventionally even-numbered highways, which run east-west. US 220 extends for ...

lies along the southeast flank of the hill. Trimble Knob is the most obvious of many igneous intrusions in the area. The central part of the hill is composed of

with a diameter of approximately . The basalt intrudes through the gently dipping

Devonian The Devonian ( ) is a period (geology), geologic period and system (stratigraphy), system of the Paleozoic era (geology), era during the Phanerozoic eon (geology), eon, spanning 60.3 million years from the end of the preceding Silurian per ...

Needmore Formation (

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

iferous shale and calcareous mudstone), and is near the axis of a

syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimposed ...

in the center of the valley.

Age

The basalt at Trimble Knob (and other igneous rock

dikes Dyke or dike may refer to: General uses * Dyke (slang), a slang word meaning "lesbian" * Dike (geology), formations of magma or sediment that cut through and across the layering of adjacent rocks * Dike (mythology), ''Dikē'', the Greek goddess ...

in the area) were originally thought to be of

Paleozoic The Paleozoic ( , , ; or Palaeozoic) Era is the first of three Era (geology), geological eras of the Phanerozoic Eon. Beginning 538.8 million years ago (Ma), it succeeds the Neoproterozoic (the last era of the Proterozoic Eon) and ends 251.9 Ma a ...

age by relative age dating using

cross-cutting relationships Cross-cutting relationships is a principle of geology that states that the geologic feature which cuts another is the younger of the two features. It is a relative dating technique in geology. It was first developed by Danish geological pioneer ...

. In 1993, Southworth and others give a date of 35.0 ± 0.5 Ma for basalt of Trimble Knob In 2012, Bulas and others studied the other igneous rocks and sampled them to obtain the mineral separates and perform age dating. The amphibole minerals composing the volcanic rock Trachydacite were dated utilizing the Ar-Ar age dating method and found them to be 48.86±.37 million years of age. The age of the volcanic eruption is not explained by the age of the formation of amphibole minerals which formed at a time before the volcanic eruption occurred. There may have been a bi-model or two-stage eruption mechanism responsible for producing the different varieties of mafic and felsic volcanic rock that have been discovered and associated with this volcanism. Both age dates place the earliest age of the eruption to be during the

epoch and further research is needed to better understand the timing and eruption mechanisms responsible for these rock types. Mole Hill, located in Rockingham County, is geologically similar to Trimble Knob and thought to be contemporaneous with it, along with other intrusive

igneous Igneous rock ( ), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rocks are formed through the cooling and solidification of magma or lava. The magma can be derived from partial ...

rocks near

Ugly Mountain Ugly Mountain is a summit in Pendleton County, West Virginia, in the United States. With an elevation of , Ugly Mountain is the 507th highest summit in the state of West Virginia. Ugly Mountain is a volcanic neck with a diatreme breccia Bre ...

in Pendleton County,

West Virginia West Virginia is a mountainous U.S. state, state in the Southern United States, Southern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States.The United States Census Bureau, Census Bureau and the Association of American ...

References

{{coord, 38, 24, 17, N, 79, 35, 16, W, source:enwiki-plaintext-parser, display=title Landforms of Highland County, Virginia Diatremes of the United States Volcanoes of Virginia Eocene volcanoes Cinder cones of the United States