DESCRIPTION:
Volcanic Fields and Centers near Mount Adams, Washington

During the past one million years, numerous volcanic vents were active throughout south-central Washington, from Vancouver to Goldendale. Most were probably active for relatively short times ranging from days to tens of years. Unlike Mount Adams, which has erupted repeatedly for hundreds of thousands of years, these vents typically did not erupt more than once. Rather, each erupting vent built a separate, small volcano, and over time a field of numerous overlapping volcanoes was created. Clusters of these vents define the Mount Adams, Indian Heaven, and Simcoe Mountains volcanic fields. In addition, the Goat Rocks volcanic center lies 30 kilometers (18 miles) north of Mount Adams. The Mount Adams and Indian Heaven fields have been the most active recently; the Simcoe field and the Goat Rocks center have not erupted for hundreds of thousands of years.

Mount Adams, one of the largest volcanoes in the Cascade Range, dominates the Mount Adams volcanic field in Skamania, Yakima, Klickitat, and Lewis counties and the Yakima Indian Reservation of south-central Washington. The nearby Indian Heaven and Simcoe Mountains volcanic fields lie west and southeast, respectively, of the 1,250 square kilometers (500 square miles) Adams field.

Mount Adams Volcano Menu

During the past one million years, numerous volcanic vents were active throughout south-central Washington, from Vancouver to Goldendale. Most were probably active for relatively short times ranging from days to tens of years. Unlike Mount Adams, which has erupted repeatedly for hundreds of thousands of years, these vents typically did not erupt more than once. Rather, each erupting vent built a separate, small volcano, and over time a field of numerous overlapping volcanoes was created. Clusters of these vents define the Mount Adams, Indian Heaven, and Simcoe Mountains volcanic fields. In addition, the Goat Rocks volcanic center lies 30 kilometers (18 miles) north of Mount Adams. The Mount Adams and Indian Heaven fields have been the most active recently; the Simcoe field and the Goat Rocks center have not erupted for hundreds of thousands of years.

The Goat Rocks volcano is a deeply eroded, glaciated volcanic center in an area of wide-spread Pliocene and Pleistocene volcanism along the Cascade crest in southern Washington. Volcanism began approximately 3.2 million years ago with eruption of at least 650 meters of high-silica rhyolite tuff (perhaps a caldera fill), which is exposed on the east flank of the subsequent Goat Rocks volcano. The silicic volcanism ended approximately 3 million years ago, and olivine basalt was locally erupted onto the rhyolitic rocks. Soon thereafter lava flows of high-K2O andesite, dominantly pyroxene phyric but including flows with significant hornblende, formed the Goat Rocks volcano. The volcano was probably build between approximately 2.5 and 0.5 million years ago. Some large volume lava flows moved many kilometers downvalley away from the volcano. The most notable such flow is the 1.0 million-year-old Tieton Andesite, which advanced approximately 80 kilometers eastward down the ancestral Tieton and Naches rivers, and is the longest known andesite flow on Earth. Thick sections of flows with radial dips of 10-20 degrees surround the hydrothermally altered core of the volcano; the flows are cut by numerous dikes that define several sectors of a radial swarm. The Cispus Pass pluton occupies the southern part of the altered core and is questionable as young as 1 million years.

Following extensive erosion, lava flows of hornblende andesite erupted onto the dissected flanks of Goat Rocks volcano. Gilbert Peak (2,494 meters), the highest point in the area, is capped with hornblende andesite. Old Snowy Mountain, along the Cascade crest, erupted flows of hornblende andesite that poured westward into the glaciated Cispus River valley. These young flows themselves were glaciated and so are middle to late Pleistocene, no Holocene, in age. Whether the hornblende andesite records rejuvenation of the Goat Rocks volcano or independent volcanism is not clear.

Coeval and younger volcanism occurred north and south of Goat Rocks volcano. Eruption of more than 200 olivine basalt and basaltic andesite lava flows formed the 700-meter-high Hogback Mountain shield volcano just south of White Pass during the late Pliocene and early Pleistocene. These lava flows intertongue with those from Goat Rocks volcano. At least five late Pleistocene andesitic volcanoes formed on the Tumac Plateau north of Goat Rocks. Young olivine basalt erupted from several vents between the Goat Rocks area and the north base of Mount Adams, 25 kilometers to the south.

The Goat Rocks volcano is 70 kilometers west of Yakima and 15 kilometers south of White Pass. Access is by foot along the Pacific Crest trail system from White Pass of several feeder trails east and west of the crest.

Goat Rocks Volcanic Field Menu

During the past one million years, numerous volcanic vents were active throughout south-central Washington, from Vancouver to Goldendale. Most were probably active for relatively short times ranging from days to tens of years. Unlike Mount Adams, which has erupted repeatedly for hundreds of thousands of years, these vents typically did not erupt more than once. Rather, each erupting vent built a separate, small volcano, and over time a field of numerous overlapping volcanoes was created. Clusters of these vents define the Mount Adams, Indian Heaven, and Simcoe Mountains volcanic fields. In addition, the Goat Rocks volcanic center lies 30 kilometers (18 miles) north of Mount Adams. The Mount Adams and Indian Heaven fields have been the most active recently; the Simcoe field and the Goat Rocks center have not erupted for hundreds of thousands of years.

The Indian Heaven volcanic field is midway between Mount St. Helens and Mount Adams, its principal feature is a 30 kilometer long, N10degreesEast-trending linear zone of coalescing, polygenetic shield volcanoes, cinder cones, and flows, with a volume of 100 cubic kilometers.

The center of the field lies around 60 kilometers east of Vancouver, Washington, and around 35 kilometers north of the Columbia River, in the Gifford Pinchot National Forest.

Indian Heaven Volcanic Field Menu

During the past one million years, numerous volcanic vents were active throughout south-central Washington, from Vancouver to Goldendale. Most were probably active for relatively short times ranging from days to tens of years. Unlike Mount Adams, which has erupted repeatedly for hundreds of thousands of years, these vents typically did not erupt more than once. Rather, each erupting vent built a separate, small volcano, and over time a field of numerous overlapping volcanoes was created. Clusters of these vents define the Mount Adams, Indian Heaven, and Simcoe Mountains volcanic fields. In addition, the Goat Rocks volcanic center lies 30 kilometers (18 miles) north of Mount Adams. The Mount Adams and Indian Heaven fields have been the most active recently; the Simcoe field and the Goat Rocks center have not erupted for hundreds of thousands of years.

East and southeast of Mount Adams, extending south to the town of Goldendale, is a broad expanse of folded and faulted basalt that is little known. A north-south line of around 24 cinder cones crosses the center of the field, and south of the 46th parallel another dozen cones extend the line to the southeast. The oldest units are thick flows of rhyolite in the eastern Simcoe Mountains. These are contemporaneous with pyroxene-olivine basalt and olivine basalt of the Simcoe Mountains. The pyroxene-olivine basalt forms short, 6-10-meter-thick flows, whereas the olivine basalts are 1-6-meter-thick flows, extruded from small shields and cinder cones. A few of the above units are dated at 4.5 to 0.9 million years ago.

East of Mount Adams are a shield volcano and the lava flows forming Lincoln Plateau; all are olivine basalt probably younger than 0.9 million years. Perhaps the youngest unit of this volcanic field is the dacite that forms Signal Peak, east of Mount Adams, that is believed to be 1.0-0.5 million years.

Simcoe Mountains Menu

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