Southern Cascades
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The South Cascades Province in Washington extends from the Columbia River to the south and Interstate 90 to the north. The southern Cascade Range skyline is dominated by three massive stratovolcanoes: Mount Rainier, a towering giant with the highest elevation in Washington of 14,410 feet, Mount Adams, the second highest peak in the Pacific Northwest with an elevation of 12,276 feet, and Mount St. Helens, the youngest volcano in the Cascades with an elevation of 8,366 feet. The volcanoes we see today are the most recent installments of a 40 million year old volcanic complex called the Cascades Volcanic Arc.

Terrane Accretion

The early geologic history of the Southern Cascade Range is much the same as the North Cascades. About 200 million years ago, the dense oceanic Farallon plate began subducting beneath the thicker, more buoyant continent of North America. As the oceanic slab sank and subducted, ocean floor sediments, volcanic island chains, and basalt that was erupted from underwater volcanoes were all scraped off of the oceanic plate onto the continent during a period of accretion (addition of material). The subducting oceanic plate went under the continent where it melted the surrounding mantle. The melted material above the subducting oceanic plate eventually erupted as a chain of ancient volcanoes and plutons.

Between 55 and 43 million years ago, before the formation of the Cascades, basalt from the subducting oceanic slab was being accreted. The Siletzia and Crescent terranes are two packages of rock that represent this period of accretion and can be found in the southern Cascade Range today.

Eocene Basins and Early Cascade Volcanism

During the Eocene, between about 55 and 43 million years ago, the ocean shoreline was located near Interstate 5, and ancient granitic mountains were located east of the present-day Cascade Range. Rivers drained these ancient mountains, and massive amounts of sediment were carried along these rivers and deposited in shallow marine basins. These sediments are now preserved as the sedimentary Cowlitz Formation and Puget Group. Much of western Washington was a coastal lowland, or swamp, where an enormous amount of vegetation grew and was later buried by the sediment that came from the rivers draining the mountains. As the vegetation was buried, high pressures and temperatures converted it to coal. Some leaves, wood, and plants from this swampy environment are preserved as fossils.

Paleogeographic map of the coastal plain of western Washington during Eocene time. Modified from Brownfield (2011).

The earliest Cascade Range volcanoes began erupting around 43-37 million years ago in the coastal plain environment. Many of these volcanoes erupted mafic (dark-colored, low-silica) lavas such as basalt and andesite. Some volcanoes erupted felsic (light-colored, silica-rich) lava and ash. The Rockies, a small mountain range northeast of Morton, Washington are remnants of the volcanoes that erupted in this interval and produced the Northcraft Formation.

After the eruption of the Northcraft volcanoes, the early Cascade volcanic arc began erupting east of the Rockies from around 37 to 17 million years ago. This was a large volcanic arc, spanning from southern British Columbia down into California. These volcanoes erupted at a fast pace and covered the land with a massive outpouring of lava, ash and various rock fragments. The volcanic rocks of the Ohanapecosh Formation, the Hatchet Mountain Formation, and the Goble volcanics were erupted during this time. Together this pile of volcanic rock is around 6 miles (10 km) thick! These old cascade volcanoes were eventually eroded and buried by the next episode of volcanism, and today only the deposits left by the oldest Cascade volcanoes tell of their existence. From ~27 to ~22 million years ago, the ancient Cascade volcanic arc experienced many more volcanic eruptions and pluton intrusions which covered the Cascades with lava, ash, mudflows, and plutonic rocks (granite type rocks). The Fifes Peak volcanics, Bumping Lake granite batholith, and the Spirit Lake Pluton are a few of the main rocks from this episode of volcanism.

Columbia River Basalt Group

Following the last phase of volcanic activity there was a time of erosion and uplift. Gentle folding and tilting of the older volcanic and plutonic rocks occurred from ~21 to 18 million years ago. This lull in volcanism didn’t last long, because starting around 17.5 million years ago huge amounts of basalt flooded the southern Cascades, issuing from linear vents, called dikes. These extensive basalt flows are called the Columbia River Basalt Group and they cover much of southern Washington and northern Oregon. Numerous dikes and sills intruded the volcanic rocks from ~12 to 10 million years ago after the initial outpouring of flood basalts.

Interactive diagram showing volume and area of all of the Columbia River Basalt Group formations and the major members.

Modern Cascade Volcanism

For the last 40 million years, the subduction and melting of the oceanic Juan de Fuca Plate beneath the continental North American Plate has generated the magma that spurs volcanic eruptions in the Cascades Volcanic Arc. The most recent chapter of volcano building began ~500,000 years ago; Mount Rainier, Mount St. Helens, and Mount Adams are products of this period. Evidence of repeated eruptions from these volcanoes exists in the geologic record.

  • Mount Adams
  • Mount Rainier
  • Mount St. Helens
  • Mount Adams is the largest volcano, volumetrically, in the Pacific Northwest. It is actually a cluster of volcanic vents that erupted andesitic lava from the vent cluster rather than a single vent. The Mount Adams system is one of the youngest in the Cascade Range. The volcanic center first erupted 520 to 500,000 years ago, and continued up to about 1,000 years ago.

    There have been no historical eruptions in the Mount Adams volcanic field. However, there were a series of debris avalanches and lahars between ~600 and 300 years ago.

    Hydrothermal alteration is present on the main cone as well as at numerous locations along the slope. Fumarole activity was reported at the summit from miners trying to extract sulfur from the crater in the 1930’s, but later reconnaissance trips did not see any fumaroles, only the faint smell of sulfur.

  • Mount Rainier is the tallest peak in the Cascade Range and provides a stunning backdrop to the Puget Sound region. It contains the most glaciers of any single mountain in the lower 48 states. The first eruption at Mount Rainier was ~1 million years ago, but the modern Mount Rainier started erupting only 500,000 years ago with intermittent eruptions and mudflows thereafter.

    Diagrammatic cross section of Mount Rainier and select basement rocks. Modified from Fiske and others (1963).

    5,600 years ago, a massive debris avalanche, called the Osceola Mudflow, poured down from the summit of Mount Rainier, picking up sediment and anything else in its path as it traveled down the White River valley and into the Puget Sound. The mudflow filled valleys with ~400 feet of sediment and moved at speeds of 40 to 50 miles an hour. Following the Osceola Mudflow, many smaller volcanic eruptions and lahars occurred as the volcano continued to show signs of unrest. The last major mudflow, called the Electron Mudflow, began as a part of a crater collapse and traveled down the Puyallup River into Sumner in ~1502.

    Mount Rainier still issues steam and gases from fumaroles near the summit crater, which melt the snow and ice at the crater, as well as melt the summit icecap, forming caves beneath the ice.

    Researchers study earthquake activity in the Mount Rainier area to learn about the background seismicity, or the small day-to-day earthquakes, that occur in the crust as magma below the volcano shifts and faults in the area move to accommodate the fluids and gasses produced by the magma.

    By studying the earthquakes geologists can hopefully tell if there is an increase in seismicity (earthquakes) and they might be able to tell if the volcano is about to erupt. Geologists are particularly interested in a large north-trending fault zone west of Mount Rainier, called the Western Rainier Seismic Zone, which is an area of dense and shallow earthquakes.

  • Mount St. Helens is the youngest of the Cascade volcanic peaks with the most recent eruption occurring in 1980. Mount St. Helens was formed from four eruptive stages starting ~275,000 years ago, and intermittent eruptions occur to this day. During one such eruption around 2,000 years ago, lava flowed down the side of the volcano in streams, one stream formed the “Ape Caves”, a spectacular 13,042-foot-long lava tube on the southeastern flank of the volcano. Miners in 1924 claim to have fought Bigfoot near Ape Cave.

    Pre-eruption profile of Mount St. Helens.
    Pre-eruption crater development on the summit of Mount St. Helens.
    Bulge in north flank at the summit of Mount St. Helens
    May 18, 1980.
    Ash cloud
    Dome building within summit crater of Mount St. Helens, 1981.
    Mount St. Helens breach zone viewed from the Pumice Plain, 2012.

    The modern eruptive period at Mount St. Helens is most notably known for the catastrophic May 18, 1980 eruption. Earthquakes began on March 20, 1980 that signaled movement of molten magma beneath the volcano. The earthquakes were followed by a small steam and rock eruption that created a summit crater on March 27, 1980. By mid-April of 1980, a large bulge of new volcanic material had formed on the north flank of the mountain, and moved outward at an average rate of ~5 feet per day. On May 18th, the cataclysmic eruption was triggered by a magnitude 5.1 earthquake. The bulge collapsed in a series of three massive slide blocks. This bulge collapse generated a chain reaction, starting with the largest avalanche in recorded history (0.6 cubic miles of material, reaching speeds of 60 miles per hour). The removal of this material decreased the pressure holding back the magma and caused the release of gas, large rocks, and smaller particles moving across the landscape and destroying most vegetation at an astounding speed of 650 miles per hour. This initial blast caused a chain reaction of major lahar flows, pyroclastic flows, and an ash eruption that formed a eruption column that grew to 12 miles high and 45 miles across. In only 10 minutes, the ash spread all the way to Montana.

    After the May 18, 1980 eruptions, five smaller explosive eruptions occurred in 1980, as well as a few small explosions in the late 1980’s. A revival of intense earthquake activity in September of 2004, accompanied by four short explosions signaled the next vent-clearing phase. For the next 3+ years, lava continued to build up in the crater and generated a lava dome that is 1,500 feet high.

    Mount St. Helens is located along a 65-mile-long zone of intense earthquake activity called the St. Helens seismic zone. Many small and moderate (up to 5.5) magnitude earthquakes occur in this area. Geologists monitor the earthquake activity very closely to looks for signals of another eruption.

Mineral Deposits

The two main mining areas in the Southern Cascades are the St. Helens and Washougal Mining Districts. Metal deposits are typically found in small fractures and veins within the rocks and contain predominantly: pyrite, magnetite, chalcopyrite, bornite, galena, and sphalerite; with lesser amounts of: copper, zinc, lead, molybdenum, gold, and silver. Minerals that can be found in the area include: azurite, malachite, tourmaline and chrysocolla.

The Bonneville Landslide

The Bonneville landslide is a massive landslide in the Columbia River Gorge about 30 miles east of Portland, Oregon. The slide covered an area over 6 square miles and temporarily dammed the river, creating a large lake. Native American legend refers to the slide as “The Bridge of the Gods” referring to the massive amount of land that slid from the hills to the north. The land blocked the river, creating a temporary land bridge. Lewis and Clark noticed the slide on their journey in 1805, stating that the river was “obstructed by the projection of large rocks, which seem to have fallen promiscuously from the mountains into the bed of the river". The Columbia River has since reclaimed its path, and the Bridge of the Gods is now made of steel.

The area around the Bonnevile landslide is especially prone to sliding. Weathered, clay-rich volcanic sedimentary rocks of the Eagle Creek and Weigle formations. Beds in these formations dip toward the Columbia River and are overlain by basalt flows at the top of the section. The basalts periodically slip toward the river upon the clay-rich beds. Geologists and dendrochronologists (scientists who study tree rings) are still figuring out exactly when the slide happened by examining the trees that were demolished during the slide. The best estimate is that the slide happened around 400 to 250 years ago.


The Bonneville landslide is actually part of a larger complex of landslides, called the Cascade Landslide Complex.