This is the mountain range I’m most familiar with, my home range. I’ve climbed all of the high Oregon Cascades and many of the bigger Washington ones as well. So I have personal experience and knowledge of these peaks. Named for the many waterfalls that tumble over their volcanic cliffs, the Cascades are essentially a northern analogue of the Andes in South America.
The Cascades are volcanoes that still erupt from time to time, but with the exception of a single mountain are not the most active volcanic chain in the world by any means. More on the exception below. The Cascade Range, which stretches for 700 miles (1100 km.) in a north-south direction from Mount Garibaldi in Canada to Mount Lassen in California, is part of the Pacific Ring of Fire (see below). This whole region of the western Pacific Northwest is often called Cascadia.
The dramatic and beautiful mountains that make up the Cascades in most cases exceed 10,000 feet (3000 meters). The high peaks are generally well-spaced, with many miles of forested lower mountains and hills between each snow-capped peak. Oregon’s Three Sisters area (which actually includes 5 big volcanoes) is an exception to this wide spacing. The bunched-up and much more rugged North Cascades in Washington are a whole different range geologically, one that happens to coincide in space (but not time) with the volcanoes of the Cascades.
The highest peaks in the Cascades are quite young, most less than 100,000 years old – a moment in the earth’s 4.5 billion-year history. They are built upon a much older eroded volcanic range, arranged along an axis situated slightly to the west of the present locus of volcanic activity. These older volcanoes began erupting some 37 million years ago. It’s lucky for life (including us) that these older, heavily eroded volcanoes are around. It’s the reason we have those lush forests, those cold streams that nourish the region’s great fish runs, and the habitat for the region’s other wildlife. And let’s not forget the many waterfalls!
The older ancestral Cascades are also responsible for the region’s enormous timber resources plus the very rich soils that drew settlers west along the Oregon Trail. Volcanoes combine with ample rainfall to make rich soil for farming. By the way, many often wonder why so many people, worldwide, live near dangerous volcanoes. It’s simple: the rich soils around volcanoes, the productive farmland. There is really not much choice. We must eat, and so we must live near volcanoes.
While the Western Cascades are responsible for many of the Northwest’s assets, let’s not totally dismiss the younger High Cascades. Their snowpack, lasting well into summer, gives farmers and ranchers (especially those to the east) water for their crops through typically dry summers.
The Cascades are stratovolcanoes (aka composite cones). These are the steep-sided, conical volcanoes you drew as a kid in school. They are made of alternating layers of lava-rock and pyroclastic (ash) deposits. The volcanic rock is characteristically lighter colored than the basalt which covers the region to the east of the Cascades. A typical volcanic rock for the Cascades is andesite (named for the Andes), which flows over the ground in a somewhat stickier manner than more fluid basalt (Hawaiian volcanoes erupt basalt). The Cascades do have their share of basalt too, along with dacite and a few other types of volcanic rock.
An uncommon volcanic rock of the Cascades is obsidian. It is very rich in silica (SiO2), which is also what quartz is made of. In liquid lava, dissolved silica acts to make it stickier, more viscous. Water does the opposite, makes lava less viscous – more fluid. Obsidian is so rich in silica and erupts so dry that it literally squeezes out of the ground like thick toothpaste, heaping up into mounds and ridges. Once cooled, obsidian is a beautiful natural glass, normally black, that can be sharp enough to serve as surgical instruments. Obsidian arrowheads left along old American Indian trails and hunting grounds can still be found throughout the Northwest.
THE RING OF FIRE AND PLATE TECTONICS
The Pacific Ring of Fire is that huge circle of volcanoes and earthquake activity that circles the Pacific ocean basin. Some of the world’s most spectacular eruptions and devastating earthquakes happen along the Ring of Fire. Truly an enormous geologic feature, it exists because the earth’s tectonic plates rub against and collide with each other (see addendum below if you don’t know about plate tectonics already). Although they act slowly, the forces are gargantuan. And something has to occasionally give.
One example of the power and beauty of the Ring of Fire lies in the remote Aleutian Islands and Russia’s Kamchatka Peninsula. Here the huge Pacific Plate dives under the North American continental plate (plus a smaller one called the Okhotsk Plate) along a so-called subduction zone. The plate partially melts as it descends, because of the heat of course – but also because of it is loaded with water (which acts as a flux). Plumes of magma rising from the descending and melting plate eventually erupt into some of the world’s most active (and thankfully remote) volcanoes. In the Southern Hemisphere on the opposite side of the Ring of Fire, the oceanic Nazca Plate subducts under the South American plate to form the longest volcanic range in the world, the Andes.
All throughout the Ring of Fire there are earthquakes. Some of the largest happen as a result of subduction and are called megathrust quakes (how’s that for a name!). The earthquake that caused the destructive Japanese tsunami of 2011 was of the megathrust variety. This enormous earthquake happened where the Pacific Plate subducts beneath Japan’s Honshu Island. The Pacific Plate moved as much as 20 meters (66 feet) west during the minutes-long quake. Honshu drew closer to America by about 2.5 meters (8 feet). The equally destructive Indian Ocean tsunami of 2004 was also generated by a megathrust quake along a subduction zone.
Other earthquakes happen when two tectonic plates slide past each other. The San Andreas in California is the most famous example of this so-called transform boundary. Because these earthquakes happen on land and have fairly shallow epicenters, they can be very destructive. This is despite the quakes being generally smaller than subduction-zone, megathrust earthquakes.
ADDENDUM: PLATE TECTONICS
The crust of the earth (plus some extra beneath it) is broken into enormous semi-rigid plates. Over time, the plates move across the planet’s surface, on average about as fast as your fingernails grow. That’s an average; during big quakes they can move up to a hundred feet! But overall it’s a very slow process. It can take over a million years for a plate to move 50 miles. They ride atop enormous convection currents in the semi-molten part of the upper mantle. The mantle is that layer that lies directly beneath the earth’s crust. The weight of tectonic plates as they descend into the mantle along subduction zones (like the one that lies just off the Pacific Northwest coast) helps to pull the oceanic plates along.
Why do we have tectonics while the other planets don’t seem to? For one thing the energy that drives the convection currents comes from heat given off by the still cooling interior of the earth. Mars is too small to have much heat left. For Earth, much of the core is still molten, and our fast spin sets up complex circulation patterns (which cause our magnetic field). Combined with heat from the decay of radioactive elements, this gives rise to huge, slowly rising zones of heat. When they hit the colder, more rigid upper parts of the earth, the crust, the currents spread outward horizontally.
But there’s another reason for plate tectonics. It is because we are a water planet that all this partly molten rock is around. Venus is much too dry for plate tectonics to get going. Without water the pressures deep below would not allow enough melting. Water essentially lubricates the earth’s tectonic system. And without plate tectonics complex life would most likely not be possible, yet another way water is crucial to a living earth.
This series will continue. If you are interested in any of the images, just click on them. They are copyrighted and not available for download without my permission. Please contact me if you have any questions. Thanks for reading!