Shastina cone of Mount Shasta

Shastina stands apart from other cones of Mount Shasta

Photograph by R. Christiansen on 24 July 1975

Whereas, the central vents for most of Mount Shasta's individual cones are so close together that they have been recognized separately only through detailed study, the Shastina cone lies distinctly to the west and is a notable stratocone in itself. If it were by itself anywhere else in the Cascades it would be considered a major volcano. We mapped out the distribution of lava flows from Shastina and found them all to be younger than the 10,000-year-old pumice deposits of the Red Banks eruption.

Early eruptions of Shastina cone are explosive

Photograph by B. Christiansen in 2 August 1975

An initial explosion perforated the west flank of the Misery Hill cone and formed the crescent-shaped crater rim we see enclosing a large snowbank near the center of this view. Our mapping showed that Shastina grew quickly by successive eruptions of lava from the new vent, burying the west half of the initial crater and forming the Shastina cone. Dating of charcoal from deposits below and above the Shastina lavas indicates that this cone grew within no more than a few centuries.

Extrusion and explosive disruption of Shastina's lava domes

Photograph by R. Christiansen on 24 July 1975

At the end of Shastina's cone growth, many domes of viscous lava extruded in the summit crater. These extrusions were punctuated by explosive eruptions and collapse of the domes. This activity broke down the western crater rim and spread hot lava fragments over the west flank of Shastina and Mount Shasta below. After each series of explosions, another dome grew in the crater. We estimate that at least 4 or 5 lava extrusions and subsequent explosions occurred at Shastina. In this aerial view toward the northeast, the oldest domes form the north and south crater rims, and remnants of 3 younger domes rise above the crater floor. The broken down west rim of Shastina's crater is the head of Diller Canyon, at the left side of the view.

Pyroclastic flows pour down west flank of Shastina cone
Photograph by R. Christiansen on 30 July 1975	The explosions in Shastina's crater blasted hot lava fragments into the head of Diller Canyon, outlined by a snowbank in the upper left photograph. The debris spread out in destructive pyroclastic flows–high-speed flows of hot volcanic ash, blocks, and gases–and buired the west flank of Shastina and the lowlands beyond. Note large fractured blocks of lava in the foreground. Click on image for larger-sized image. Room-sized blocks were carried in these pyroclastic flows and left behind to litter the surface of the fan of hot debris. Such blocks–one of them being examined by a geologist in the photo at the lower left–contracted upon cooling to form three-dimensional jigsaw puzzles of prismatic fractures perpendicular to each surface of the block.
Photograph by R. Christiansen on 30 July 1975

Black Butte erupts at base of Shastina cone

Photograph by R. Christiansen on 30 July 1975

The last event related to the spectacular volcanism of Shastina was the extrusion of another volcanic dome low on its western flank. This dome, seen in the aerial view on the left, rises steeply beside Interstate Highway 5, the main north-south artery of ground travel among the Pacific Coast States. The dome suffered partial collapse by avalanching of hot debris, spreading flows of hot volcanic ash and blocks to the west and south. The town of Weed and part of the town of Mount Shasta are built on the surfaces of the 9,400-year-old ash pyroclastic flows from Shastina and Black Butte.

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References