The Simcoe Mountains Volcanic Field is a group of lava flows and
extinct
Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...
cinder cones located in the
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 territorie ...
in south-central
Washington
Washington commonly refers to:
* Washington (state), United States
* Washington, D.C., the capital of the United States
** A metonym for the federal government of the United States
** Washington metropolitan area, the metropolitan area centered o ...
state, east of Mount Adams. The Northern and central sections of the Simcoe Mountains are located in
Yakama Indian Reservation
The Yakama Indian Reservation (spelled Yakima until 1994) is a Native American reservation in Washington state of the federally recognized tribe known as the Confederated Tribes and Bands of the Yakama Nation. The tribe is made up of Klikitat, ...
. Although the volcanic field is located near the Cascade Arc of volcanoes, it is a much older intraplate volcanic field, and not a part of the Cascade Arc.
Highest Points
Most of the volcanic cones are 75 m to 250 m taller than the surrounding land, except for Signal Peak Shield, which is 500 m taller than its surroundings. The highest elevation peaks in the Simcoe Mountains are:
* Jennies Butte (6,410 ft; 1,954 m), a
dacite
Dacite () is a volcanic rock formed by rapid solidification of lava that is high in silica and low in alkali metal oxides. It has a fine-grained (aphanitic) to porphyritic texture and is intermediate in composition between andesite and rhyolite. ...
cone
A cone is a three-dimensional geometric shape that tapers smoothly from a flat base (frequently, though not necessarily, circular) to a point called the apex or vertex.
A cone is formed by a set of line segments, half-lines, or lines con ...
* Indian Rock (5,823 ft; 1,775 m), a
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 ...
ic
shield
A shield is a piece of personal armour held in the hand, which may or may not be strapped to the wrist or forearm. Shields are used to intercept specific attacks, whether from close-ranged weaponry or projectiles such as arrows, by means of a ...
* Castle Rock (5,656 ft; 1,724 m), a basaltic shield
* Signal Peak Shield (5,100 ft; 1,555 m), a 5.7 million year old
mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...
shield volcano
Geologic History
The Simcoe Mountains Volcanic Field is located in the
Yakima Fold Belt
The Yakima Fold Belt of south-central Washington, also called the Yakima fold-and-thrust belt, is an area of topographical folds (or wrinkles) raised by tectonic compression. It is a structural-tectonic sub province of the western Columbia Plat ...
in the
Columbia River Basalt Group
The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The basalt grou ...
. The eruptions occurred in three periods during the
Pliocene
The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Quaternary
The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
; the first episode occurred 4.2–3.2 million years ago. the second was from 2.2–1.2 million years ago, and the most recent episode lasted from 1.0 to 0.6 million years ago. The most recent eruption was a
trachybasalt
Trachybasalt is a volcanic rock with a composition between trachyte and basalt. It resembles basalt but has a high content of alkali metal oxides. Minerals in trachybasalt include alkali feldspar, calcic plagioclase, olivine, clinopyroxene and li ...
lava flow dated to 631,000 ± 27,000 years ago called the "Trachybasalt of Pretty Swamp".
Some of the volcanic activity in Simcoe Mountains happened at the same time as eruptions of
Goat Rocks
Goat Rocks is an extinct stratovolcano in the Cascade Range, located between Mount Rainier and Mount Adams in southern Washington, in the United States. Part of the Cascade Volcanoes, it was formed by the subduction of the Juan de Fuca ...
to the northwest. For the last 350,000 years Simcoe was active, there was also activity in the Mount Adams region, but Mount Adams itself had not yet started forming.
Indian Heaven
Indian Heaven is a volcanic field in Skamania County in the state of Washington, in the United States. Midway between Mount St. Helens and Mount Adams, the field dates from the Pleistocene to the early Holocene epoch. It trends north to south ...
, a volcanic field farther to the west, is also generally much younger than Simcoe, becoming active only in the most recent 200,000 years of activity at Simcoe Mountains Volcanic Field. Mount St. Helens, like Mount Adams, entirely postdates activity at Simcoe Mountains.