High on rocks and a 'pot of porridge'

The magnificent High Cascade volcanoes are relative youngsters, built on the surface of older, slightly tilted and deeply eroded Western Cascade volcanoes. In our area, the oldest High Cascade lavas are represented by the 7-million-year-old Table Rocks.

The High Cascades, forming the drainage divide between Western and Central Oregon, consist of awe-inspiring stratovolcanoes such as Mount Shasta, Mount McLoughlin and Crater Lake (Mount Mazama).

Mount Shasta rises 14,162 feet above sea level and 11,000 feet above the Shasta valley, a truly imposing sight. Mount Mazama may have reached as high as 11,000 feet before its collapse about 7,700 years ago.

These stratovolcanoes, while impressive, are vastly outnumbered by smaller volcanoes with lower slope profiles. Mount McLoughlin dominates the skyline from our valley, yet Brown Mountain, south of McLoughlin, is a "shield volcano" — its gentle profile looks like a shield — formed by dark, fluid lava. When traveling along Interstate 5, if you look at the ridge east of the Shasta Valley you can see three major shield volcanoes with lower profiles than Mount Shasta.

Stratovolcanoes such as Mount Shasta are built on the gently rising platforms of older shield volcanoes. The change in slope forms as the magma chambers beneath the stratovolcanoes "mature." Think of a pot of porridge on a hot stove. The longer it's left, the thicker the porridge becomes. Similarly, the first lavas erupting from the magma chamber are fluid due to high iron and low silica (quartz) content. As the magma evolves through time, iron content decreases as silica increases, resulting in less fluid ("sticky") lava that doesn't flow far from the vent. That generates progressively steeper slopes that form "stratovolcanoes."

Most stratovolcanoes span time intervals of around 1 million years, and all experience multiple eruptive episodes. For example, Mount Shasta has undergone at least four major episodes, each beginning with a magmatic belch of high-iron magma ("basalt") that, through time, evolved to "sticky" high-silica magma ("dacite").

Not all stratovolcanoes are perfectly symmetrical like Japan's Mount Fuji. Many different eruptive vents may exist on a single volcano. Mount Shasta, with its Quasimodo-like hunchback, Shastina, is one obvious example. Before its eruption and collapse, Mount Mazama with numerous vent areas likely looked like a teenager with a terminal case of acne.

For the curious, High Cascade rocks lie east of Howard Prairie Reservoir, Green Springs Inn on Highway 66 and on the plateau east of Lost Creek Reservoir off Highway 140.

Jad D'Allura is emeritus professor of the former Southern Oregon University Geology Department. Reach him at rockit@dishmail.net.

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