Exploring Volcanic California

Yesterday morning, we left Lake Tahoe and drove north on Route 89, undulating across the east flank of the Sierra Nevada; initially, all streams (including the scenic Little Truckee River) rumbled eastward, flowing into the Great Basin.  Then, about an hour into our journey, we crossed a fork of the Feather River; its flow was to the west, indicating that we had reached the gap between the Sierra Nevada Range and the Cascades.  After lunch in Quincy, California, we climbed along Indian Creek and then Wolf Creek, eventually reaching Lake Almanor, east of Lassen Peak.

Though we had planned to hike at Lassen National Park, we soon learned that the Park road was closed due to a heavy snowpack.  We thus visited McArthur-Burney Falls Memorial State Park, off Route 89; there we enjoyed the scenic falls and hiked through a gorge of volcanic basalt.  Another detour, resulting from a snow-blocked National Forest road, sent us westward, where we circled the majestic cone of Mt. Shasta.  Angling northeast on Route 97, we then crossed tule marshes and the Butte Valley Grasslands, passed through Klamath Falls, Oregon, and dipped back into California, spending the night in Tulelake.

This morning, we visited the fabulous Tule Lake NWR, where we encountered American white pelicans, western grebes, eared grebes, bald eagles, a few sandhill cranes, cormorants, various terns and a wide variety of waterfowl (including cinnamon teal).  We then visited Lava Beds National Monument, just northeast of the dormant Medicine Lake volcano, where we explored basalt flows and lava tubes.  Finally, we passed through Lower Klamath NWR (where avocets were especially abundant) and drove west to Medford, Oregon, dropping through the scenic Siskiyou Mountains en route.  Tomorrow we head for the California Coast.

The California Cascades

According to the USGS, there are more than 500 volcanic vents in California.  The great majority of these are located in the northernmost portion of the State where the southern end of the Cascade Range extends across the Oregon border.  Seven hundred miles long, the Cascade Volcanic Arch began to form 37 million years ago as the Juan de Fuca Plate and its associated microplates (all remnants of the Farallon Plate) were subducting beneath the North American Plate; this process continues today as demonstrated by the spectacular eruption of Mt. St. Helens in 1980.

The Cascades of California result from the subduction of the Gorda Plate, a southern fragment of the Juan de Fuca Plate.  Mt. Shasta (14,179 feet) is second in size among modern Cascade volcanoes (exceeded only by Mt. Rainier) and, over the past 4000 years, has been the second most active of the major Cascade volcanoes (exceeded only by Mt. St. Helens).  Lassen Peak (10,461 feet), which last erupted from 1914-1917, sits among 30 volcanic domes and peaks within its own National Park and Medicine Lake Volcano, a large shield volcano northeast of Mt. Shasta, hosts Lava Beds National Monument on its northeast flank.

Until the Juan de Fuca and associated Plates have been completely recycled (shutting off the fuel for volcanism), the Cascades will continue to form and erupt.  In addition, the entire coast of the Pacific Northwest (from British Columbia to Northern California) will remain susceptible to earthquakes and tsunamis, generated along the subduction trench.

Ecuador's Subduction Earthquake

A massive earthquake struck the Ecuador coast, northwest of Quito, yesterday evening.  Triggered by pressure release between the South American and Nazca Plates, the magnitude 7.8 quake was 20 times more powerful than the most recent 7.0 earthquake on Kyushu Island, Japan.

The Nazca Plate, a remnant of the Farallon oceanic Plate, is subducting beneath the South American Plate along the western coast of that Continent; the volcanic Andes Mountain Range formed (and continues to form) as the leading edge of the Nazca Plate melts near the outer surface of the Earth's mantle.  As the Nazca Plate advances (about 2 inches per year), friction builds between the Plates; this may release as a series of small quakes or as a mega-thrust event as occurred this weekend.

Numerous aftershocks typically follow such mega-thrust subduction earthquakes and have done so in this case.  To date, more than 260 persons have been killed and more than 2500 have been injured; considering the power and extent of this earthquake, one can expect both figures to rise significantly.  As with the recent Japan quakes, this earthquake was centered over land (16 miles SSE of Muisne) and a destructive tsunami did not develop.

The Kyushu Earthquake

The islands of Japan sit at the convergence of four major tectonic plates: the Eurasian, North American, Pacific and Philippine Plates.  Throughout most of their history, the Pacific Plate was moving northwestward, bringing in terranes and producing volcanic island arcs as it subducted beneath the North American and Eurasian Plates.

About 45 million years ago, the movement of the Pacific Plate shifted to a westward direction (as evidenced by the angle change of the Hawaiian Ridge), creating the Philippine Plate as the Pacific Plate began to subduct along the Izu Bonin Trench.  Surrounded by subduction trenches, the Philippine Plate now subducts beneath the southern half of Japan (which lies on the Eurasian Plate), including Kyushu Island, fueling volcanic activity and triggering earthquakes.

This week's earthquake on Kyushu Island, which killed at least 9 and injured hundreds, was a magnitude 6.2 quake, centered beneath the Island just west of Kumamoto.  While subduction forces initiated the earthquake, it occurred due to slippage along a fault that bisects the Island, separating older and younger terranes.  Shifting pressure along this and associated faults led to numerous aftershocks that may continue for weeks.  Since the earthquake developed on land (not at the subduction trench itself), there was no risk of a tsunami; on the other hand, the quake could have triggered a volcanic eruption, a complication that, to date, has fortunately not occurred.

Addendum:  A second, far more powerful (magnitude 7.0) earthquake has struck near Kumamoto.  Initial reports indicate widespread damage, including landslides, and at least 26 deaths (in addition to those killed by the quake earlier this week).

Volcanic Rocks

Volcanic rocks are those that form from compacted volcanic ash (tephra) or from cooled volcanic lava.  Tuff, a light weight, porous rock, forms from layers of tephra that are subjected to heat and compression over millions of years.

Lava rocks are grouped into those that form from mafic magma (rich in iron and magnesium) or from felsic magma (rich in silica).  Basalt, which has a silica content near 50%, is the primary mafic magma rock while andesite, dacite and rhyolite have a silica composition of 60%, 65% and 70%, respectively.  The higher the silica content, the more viscous the lava; basalt generally forms extensive surface flows or shield volcanoes above volcanic hotspots or mid oceanic ridges while the felsic magmas, most common along subduction zones, produce more explosive stratovolcanoes.  Pumice is a porous, spongiform rock that forms during stratovolcano eruptions when the felsic magma contains a large amount of water and gas.

Unlike granite, which cools slowly within the Earth's crust and is thus rich in crystals, the extruded felsic magmas cool rapidly and possess smaller and fewer crystals; the higher the silica content of the magma, the more finely grained the volcanic rock and the less its crystalline structure.  Obsidian (rhyolite devoid of crystals) is essentially volcanic glass.