Granite outcrops above Holcomb Creek (Section 11)
There have been many theories about how continental drift causes the formation of volcanoes. In the Pacific North-West it is thought that the Pacific Plate is descending beneath the Continental Plate. Water and gases from the porous oceanic crust are carried down and superheated, melting the surrounding rock to produce magma (the term given to molten rock, or lava, while it remains beneath the Earth’s surface). A combination of magma and highly pressurised gas can cause explosive volcanic eruptions, such as that seen when Mount St Helens exploded in 1980.
The mountains of Southern California and the Sierra Nevada are primarily composed of granitic rocks, formed about 80–240 million years ago when magma cooled and solidified below the Earth’s surface. Insulating layers of rock meant that that cooling process took place very slowly, allowing coarse-grained crystals to form. Continental drift has caused the rock to be lifted to heights above 20,000ft while the covering rocks have been eroded away, leaving granite as the predominant surface rock today. When granite is eroded, the large crystals tend to form the sandy and gravely soils that predominate in Southern California and the Sierra Nevada.
Over the last two million years, major glacial erosion has produced the fantastic rock scenery and the multitude of lakes we see today in the Sierra Nevada. Granite is an ideal rock for the formation of lakes; most of those you will see in Northern California and Oregon are actually on outcrops of granite in a primarily volcanic landscape.
The situation is further complicated by the San Andreas Fault System, which developed about 30 million years ago and resulted in some rocks being transported as much as 200 miles to the north-west. Hot magma is still present near the surface. Evidence appears in the form of hot springs, such as Deep Creek Hot Springs in San Bernardino National Forest, passed on the PCT. Water flowing underground is heated by the hot magma before coming to the surface.
North of Sonora Pass, in Central California, the rocks become predominantly volcanic although there are outcrops of granite and some of metamorphic rock, such as the limestone in Marble Mountain Wilderness. The northern end of the Sierra Nevada is further complicated in that much of it was buried in volcanic ash about 30 million years ago. Then, ten million years ago, massive lava flows caused metamorphosis of existing rocks, after which erosion left a very complicated geological story.
The Cascade Mountains start with Lassen Peak in Northern California. They continue through Oregon and Washington and into Canada with a succession of major volcanoes and associated lava flows and ashes. You will see many different types of volcano in the Pacific North-West. These developed over the past two million years and remain active today. The last major eruption was in 1980 when Mount St Helens blew its top off. About 7000 years ago, Mount Mazama, in what is now Oregon, exploded with about 40 times as much force, resulting in the formation of Crater Lake. Some of the large lava flows you will see in Oregon are only 200 years old. Volcanic activity can be seen on a smaller scale on Lassen Peak, which suffered a major eruption in 1915. The PCT also passes Terminal Geyser and Boiling Springs Lake, which are evidence of magma very close to the surface there. The next eruption could occur at any time.
Polygonal basalt lava columns, Devil’s Postpile National Monument
Four main types of volcano can be seen on the PCT.
Stratovolcanoes, such as Mount Shasta, are tall conical mountains composed of alternating layers of lava flows and ejected material.
Lava domes, such as Lassen Peak, are built up by slow eruptions of highly viscous lavas.
Cinder cones result from the eruption of small pieces of scoria or pyroclastics, which resemble cinders. These are often relatively short-lived eruptions and build up cones of between 100ft and 1000ft high. Most cinder cones erupt only once and often form as flank vents on the sides of larger volcanoes.
Shield volcanoes are formed by the eruption of low viscosity lavas which can flow a great distance from the vent. They don’t usually explode catastrophically or form volcanic cones but they can produce massive lava fields.
There is no agreed distinction between an active and a dormant volcano but the Smithsonian Institute defines a volcano as active if it has erupted within the last 10,000 years and many of the volcanoes in the Pacific North-West fall into this category. Volcanoes can be dormant for thousands of years and become eroded and worn down but magma remains close to the surface, which means they could erupt again. Often these eruptions will be violent, as pressure will have built up beneath the plug that has prevented lava escaping. Volcanoes are only considered extinct when they no longer have a lava supply.
Pumice is formed when frothy, escaping lava solidifies, trapping lots of bubbles. Pumice is very light and will float on water until it becomes waterlogged. In violent volcanic eruptions, escaping steam often tears magma and the solid rock surrounding the vent into small particles, resulting in volcanic ash.
In places along the PCT you will see polygonal basalt lava columns, the best-known being the Devil’s Postpile at Reds Meadow. The Postpile was created about 100,000 years ago when a lava flow was impounded by a moraine and reached a thickness of 400ft. Because of its great thickness, much of the pooled lava mass cooled slowly and evenly, producing long, symmetrical columns. The mainly hexagonal joints developed when the lava contracted during the cooling process.
More recently, ice ages have eroded the volcanoes of the Cascade Mountains. Glaciers remain on many volcanoes, particularly Mount Rainier and Glacier Peak. You can gain an idea of the comparative age of volcanoes from their appearance. Younger ones, including many of the small volcanoes that you see, tend to have the classical conic shape, whereas older volcanoes have been eroded during the ice ages, leaving only the towering crags of their more resistant cores.
There are three main types of glacial erosion.
Plucking: glacial melt water freezes around lumps of cracked and broken rock. When the ice moves downhill, the rock is plucked from the back wall.
Abrasion: rock frozen to the base and back of the glacier scrapes the bedrock.
Freeze-thaw: melt water or rain penetrates cracks in the bedrock. At night it freezes, expands and enlarges the crack, eventually breaking the rock away.
Volcanic rocks are easily eroded but granite is very resistant. Only where it is highly fractured or has been subjected to deep weathering is it easily eroded. Such weakened rock is easily excavated by glaciers, which leave basins of resistant granite that fill with water as the glaciers retreat. These are the corrie lakes that make granite mountain landscapes so attractive. Volcanic mountains lack the resistant rocks that allow lakes to form but in places glaciers have peeled them right down to the granite beneath, allowing the formation of lakes.
There are very few sedimentary rocks in the mountains through which the PCT passes.
Weather patterns
The mountains of Southern California, the Sierra Nevada and the Cascades form a highly effective rain shadow. Very little rain falls to the east of the mountains and desert conditions predominate as a result.
The PCT generally keeps to the crests or western slopes of the mountains to avoid the desert areas. This is epitomised by the long detour west of Mount Shasta, in Northern California, to avoid the dry, waterless hills east of the massive volcano. The main exception is the crossing of the western corner of the Mojave Desert, which has very little rainfall.
Rain and snow come either from frontal storms (often tropical storms