Although it’s quieted down in the past several weeks, the Kilauea volcano in Hawaii put on quite a show earlier this year as its lava engulfed local neighborhoods.

While we may think of Hawaii when we think of volcanoes, volcanoes are big in the Pacific Northwest, too.

Mount St. Helens was national news in 1980 when the peak bulged and collapsed, unleashing a gigantic flow of hot ash and debris down its slopes and sending a plume of ash and steam into the air. In the end, the mountain was 1,300’ shorter, the forests along its flanks were demolished, and thick blankets of ash settled downwind, some ash traveling as much as 1,000 miles.

From 2004 to 2008, the 1980s crater inside the exploded top of Mt. St. Helens bulged again, spewing more steam and ash, signaling that molten rock was again pushing up from below. Like her sister High Cascade peaks, Mt. St. Helens is a volcano, a cone built up by layers of lava.

Such volcanic steam is water that is vaporized beneath the surface as it contacts the rising magma. “Magma” is molten rock under the ground; “lava” is molten rock above ground.

Igneous rocks, one of the three main groups of rocks, are solidified magma or lava. Sedimentary rocks are made of sediment. Metamorphic rocks are igneous or sedimentary rocks that have been altered by heat, pressure, or chemistry.

As do the other two types of rock, igneous rocks vary by their chemical composition (minerals) and by the size of their crystals. Generally speaking, the longer magma has to cool, the larger the crystal grow. Lava that cools very quickly can form obsidian glass. Lava that cools suddenly as it is shot into the air can form globs (or “bombs”) of various sizes—pumice or ash, depending on the fineness of the spray.

There are actually two Cascade Mountains in Oregon. The “Western Cascades” are older (40 million-year-old) rocks, the remains of a coastal volcanic arc. The higher cone-shaped peaks of Oregon’s “High Cascades” are a string of younger (10 million-year-old) volcanoes perched on the eastern side of the older range. Mount St. Helens is one of this younger group. Mt. Mazama is also one of the younger group: after blowing its top 7,700 years ago, the crater was filled with rainwater — now called Crater Lake.

The High Cascade peaks, including Mt. St. Helens — the youngest — are layered cones of basalt and andesite; both basalt and andesite are dark, fine-grained rocks that cooled near the surface. Most of the High Cascade peaks are composite volcanoes, built of lava flows interspersed with layers of rubble, cinders, and ash that were ejected from a central vent. That buildup around a central vent gives such peaks their characteristic cone shape.

Those photogenic glacier-cloaked peaks are just one example of volcanics in our state. Basalt of various ages forms sheets that blanket central Oregon and forms seams and plugs that dot western Oregon. Many of Oregon’s most dramatic coastal headlands, from Tillamook Head to Heceta Head, are basalt; all are past lava flows and magma intrusions.

The magma to make those landforms came from below. But what prompts it to emerge from the depts to become flows or volcanoes?

That part of Earth’s crust we call North America has been moving westward for about 400 million years. We don’t notice our movement much since we’re now moving only about an inch a year — about as fast as your fingernails grow. We do notice when part of our continent gets hung up then jars loose, creating a massive, “subduction zone” earthquake.

As we’ve traveled, Oregon’s area of the continent has crumpled up the ocean bottom and collected that material on the continent’s leading edge. That older, mostly metamorphosed ocean bottom is now the Klamath/Siskiyou Mountains; the younger, less-altered volcanic arc is now the Cascades.

Now, as in the past, the west-moving North American plate rides over the ocean plate as it creeps along. The thin ocean bottom, progressing to the east, is “subducted” – pulled — beneath the thicker land mass.

At hotter depths, the water-infused ocean bottom melts and then seeps upwards, melting more rock as it goes. Some of the magma moves up through cracks in the land mass, cooling into new rock or emerging as lava.

The lava and ash that made the High Cascade peaks (and our most durable coastal headlands) is recycled ocean floor.

Even if it doesn’t threaten our homes, such volcanism certainly captures our attention. Past volcanism has shaped our horizons and punctuated our landscapes, framing much of Oregon’s beauty.

For information on how you can arrange an exploration of our fascinating natural history, contact Marty at 541-267-4027, mgiles@wavecrestdiscoveries.com, or www.facebook.com/wavecrestdiscoveries. Gift certificates are available. Questions and comments about local natural history are welcome. www.wavecrestdiscoveries.com

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