Volcanic activity in North America is surprisingly infrequent. In spite of there being no less than 255 volcanoes or volcanic features such as maars or volcanic fields listed for California (20), Oregon (65), Washington (21) and British Columbia (149), situated on or adjacent to the Cascades Range and above the subduction zone where the last dregs of the Farallon Plate is disappearing beneath the main continent and the Juan de Fuca plate knocking on the door, there have only been some 20 eruptions or periods of eruptive activity over the past 1,000 years. This is a surprisingly low number for the Ring of Fire and if we compare with Alaska, Japan, the Philippines or Indonesia, we can only surmise that the entire Cascadian Arc with its associated volcanic fields is in a quiescent phase.
Located at the southern end of the Cascade Range in Siskiyou County, California, the 4,322 metres high (14,179 ft) Mount Shasta is ranked as the second highest peak of the Cascade Range after Mount Rainier. It rises 2,994 metres (9,822 ft) above the surrounding landscape and has an estimated volume in excess of 350 km3 (85 cubic miles) which makes it the largest of all Cascade volcanoes, ahead of the similarly sized Mount Adams and the imposing Mount Rainier which towers above Seattle. It is mainly composed of silicic andesite to dacite.
Mount Shasta is composed of four overlapping cones, five if you count the destroyed cone of ancestral Shasta, which together form the main peak Shasta and the satellite cone Shastina. At 3,760 metres (12,330 ft), Shastina would rank fourth of the Cascade peaks if it were recognised as a separate mountain. Nearby is a 9,500 year old dacite lava dome associated with Mount Shasta, Black Butte. In it’s own right, it could be considered to be a large mountain as it rises more than 600 metres above the plain to the NNW of Shasta, even if it is dwarfed into insignificance by Shasta and Shastina in the background.
Before we move on to have a look of the geological history of Shasta, it is worth noticing that because of the meteorological conditions, quite often a difference of 60C or more between the valley to the north and the mountain peak, Mount Shasta is often accompanied by strange cloud formations. Is it surprising then, located in California as she is, that she is a favourite with UFO-ologists with claims such as the one that there is a portal located on Mount Shasta from which alien spacecraft appear periodically…
Even if there is evidence of much earlier volcanism in the area, activity at Mount Shasta is thought to have begun about 590,000 years ago and is divided into four distinct periods. The first cycle, which lasted about a quarter of a million years, saw the building of the ancestral cone from mainly andesitic lavas and ended with its destruction somewhere between 300 and 380 thousand years ago. The cone of ancestral Shasta was almost completely destroyed by a sector collapse of truly epic proportions. The total volume of the collapsed sector is estimated at 45 km3 and covers an area of about 675 square kilometres with a maximum extent of 49 kilometres to the north. Some of the “boulders” carried down by the avalanche still protrude more than 100 metres in height above the avalanche deposit floor… As a comparison, the debris avalanche from the 1980 sector collapse of Mount St Helens had a total volume of about 2.5 km3 and reached 23 km down a river valley. There is no evidence of contemporary volcanic activity, thus the cause of the avalanche remains unknown.
After the sector collapse that destroyed ancestral Shasta, there seems to have been a hiatus of between 50 to 130 thousand years after which the oldest of the four cones that make up modern Shasta, the Sargents Ridge cone, began to grow less than a quarter of a million years ago. What remains today of this cone that has undergone two periods of glaciation is visble on the south side of the mountain. What today is the main edifice of Shasta, the Misery Hill cone began to grow about 130, 000 years ago and has suffered one period of glaciation. The final two cones are both Holocene. The truly massive satellite cone Shastina was formed over a period of merely 300 years between 9,700 and 9,400 years ago while the present summit, the Hotlum cone is younger even if it may overlap Shastina in age. A small craterlike depression in the summit dome contains several small groups of fumaroles and an acidic hot spring. The extrusion of these domes was accompanied by pyroclastic flows which extended more than 10 kilometers south and 5 kilometers north of the domes.
Shastina is a stratovolcano composed of silicic andesite flows and dacite plugs and has a topographic prominence of over 137 metres (450 ft) as measured from the saddle that connects it with Shasta. This easily exceeds the typical mountaineering standard (91 metres or 300 ft) for a peak to qualify as an independent summit, yet for some peculiar reason, this is not applied. As Shastina towers at 2432 metres (7979 ft) above the surrounding landscape, I personally feel that Shastina is hard done by. In comparison, Vesuvius is no more than a 1,281 metres high midget of a volcano.
The final major feature of the Shasta complex is the parasitic dacite cone complex that makes up the 600 metres high Black Butte, 13 km northwest of Shasta. It was formed right at the end of the period of activity that created Shastina and the Hotlum Cone some 9½ thousand years ago.
Over the last 10,000 years, Shasta has erupted on average once every 800 years. The most recent eruption at the Hotlum cone, the present summit of Shasta proper, may have occurred as late as 1786 A.D. The eruptions appear to have been small, something that cannot be said for the eruptive period of c. 9,500 years ago – andesitic lava flows, dacitic lava dome extrusions accompanied by large pyroclastic flows that reached up to 20 km from the volcano and formed the massive Shastina, the current summit complex and Black Butte.
Suggested further reading:
Shasta geology – http://www.siskiyous.edu/shasta/geo/his.htm
Shasta volcanic hazards – http://vulcan.wr.usgs.gov/Volcanoes/Shasta/Hazards/Bulletin1503/framework.html