Shasta Valley Avalanche Gulch on Mt Shasta Black Butte from Weed, California Diller Canyon on Mt Shastina from Weed, California

About 593,000 years ago andesitic lavas erupted in what is now Mount Shasta's western flank near McBride Spring. Over time an ancestral Shasta stratovolcano was built to an unknown height but sometime between 300,000 to 360,000 the entire north side of the volcano collapsed, creating an enormous landslide, 6.5 mile³ (27 km³) in volume. The slide flowed northwestward into Shasta Valley where the Shasta River now cuts through the 28 mile (45 km) long flow.

The remains of the oldest of Shasta's four cones is now exposed at Seageants Ridge on the south side of the mountain. Lavas from the Sargeants Ridge vent cover the Everitt Hill shield at Shasta's southern foot. The last lavas to erupt from the vent were hornblende-pyroxene andesites with a hornblende dacite dome at its summit. Glacial erosion has since modified its shape.

The next cone to form is presently exposed south of Shasta's current summit and is called Misery Hill. It was formed 15,000 to 20,000 years ago from pyroxene andesite flows and has since been intruded by a hornblende dacite dome.

Since then the Shastina cone has been built by mostly pyroxene andesite lava flows. 9500 years ago these flows reached some 6.8 miles (11 km) south and three miles north of the area now occupied by nearby Black Butte (see below). The last eruptions formed Shastina's present summit about a hundred years later. But before that, Shastina, along with the then forming Black Butte dacite plug dome complex to the west, created numerous pyroclastic flows that covered 43 mile&sup2 (110 km&sup2), including large parts of what is now Mt. Shasta, California and Weed, California. 400 ft (120 m) deep and quarter-mile (400 m) wide Diller Canyon is an avalanche chute that was probably carved into Shastina's western face by these flows.

The last to form and highest cone, the Hotlum Cone, formed sometime before 8000 years ago. It is named after the Hotlum glacier on its northern face and its longest lava flow, the 500 ft (150 m) thick Military Pass flow, extends 5.5 miles (9 km) down its northwest face. Since its creation a dacite dome intruded the cone and now forms the summit. The rock at the 600 ft (180 m) wide summit crater has been extensively hydrothermally altered by sulfurous hot springs and fumaroles there (only a few examples still remain).

In the last 8000 years, the Hotlum Cone has erupted at least eight or nine times. About 200 years ago the last significant Shasta eruption came from this cone and created a pyroclastic flow, a hot lahar (mudflow), and three cold lahars, which streamed 7.5 miles (12 km) down Shasta's east flank via Ash Creek. A separate hot lahar went 12 miles (19 km) down Mud Creek.

Reference

• Fire Mountains of the West: The Cascade and Mono Lake Volcanoes, Stephen L. Harris, (Mountain Press Publishing Company, Missoula; 1988) ISBN 0-87842-220-X

External link

• http://www.siskiyous.edu/shasta/geo/