The southern portion of the Cascade mountain range in Washington consists predominantly of Cenozoic volcanic rocks and associated deposits. Basalts of the Columbia Basin lap onto the southern Cascades to the east, and the Puget Lowland is situated to the west. To the south, the Cascade Range is severed by the gorge of the Columbia River.
The South Cascades lack the structural complexity that characterizes the range to the north. It is only in the vicinity of Rimrock Lake, along State Highway 12, that a window of highly deformed Cretaceous metasedimentary and metavolcanic rocks are exposed. Some of these rocks contain limestone cobbles with fossils of probable Permian age. Radiolaria suggest that most of these metasedimentary rocks are of late Jurassic to early Cretaceous age.
By the end of the Mesozoic, basement rocks were eroded to a plain on which sediments were deposited during the Eocene. These sediments are now represented by nonmarine shales, siltstones, and sandstones. Coal beds occur in some parts of the section, particularly in Puget Group rocks in the Cascade foothills east of Tacoma and north of Morton. At Morton, these same rocks served as hosts to mercury mineralization.
Basalt and andesite volcanism of the Cascade arc was initiated during the Eocene. During the Oligocene, the Neogene, and through Quaternary time, mountain building in the form of volcanism predominated. Volcaniclastics, lahars, ash beds, and mud flows from volcanic centers filled depressions in the southern Cascades. They interfingered with nearshore sediments to the west. During the middle and late Miocene, Columbia River basalt flowed up against the eastern flanks of the Cascades; these flows were later arched upward with uplift of the range. Tertiary volcaniclastic rocks contain petrified wood, and andesite flows of Eocene, Oligocene, and early Miocene age are widely known for crystallized zeolites.
Locally, stocks were intruded during the Oligocene. However, it is during the Miocene that the greatest number of dikes, sills, stocks, and plutons of granitic to dioritic composition were emplaced in the older rocks. Miocene intrusive rhyolite also occurs. Gold is associated with early Miocene quartz diorite, and copper deposits are found in the Spirit Lake (north of Mount St. Helens) and Silver Star (east of Vancouver) plutons, which have been dated at about 20 Ma. Pliocene and Quaternary dacite domes are associated with volcanic centers.
During the late Miocene, the Cascade mountains began to be uplifted. Coincident with uplift, the ancestral Columbia River began to cut a canyon in the same general area as the present gorge. About 12 million years ago, the Columbia River paleocanyon saw the deposition of fluvial channel deposits followed by intracanyon flows of Columbia River basalt (Pomona Member, Saddle Mountains Basalt). This very fluid basaltic lava, along with earlier Grand Ronde and Frenchman Springs basalts, flowed through the ancestral Columbia River gorge and reached the Pacific Ocean. During the Quaternary, the gorge was the site of gigantic landslides as well as the scene of cataclysmic floods resulting from breaches of lakes dammed by continental glaciers. (For additional details, the reader is referred to the Columbia Basin portion of this article where the great Missoula Floods are discussed.)
Towering over the Cascade Range is majestic Mount Rainier with its peak at 14,410 feet above sea level. It contains the greatest concentration of glaciers of any single mountain in the lower 48 states. Mount Rainier's eruptive history started nearly 1 million years ago with the building of a pyroclastic cone. Then, 700,000 years ago, the volcano began to grow in earnest through eruption of voluminous andesite flows. During the last 120,000 years, the volcano has been subjected to repeated cycles of mountain glaciation. Between 6,600 and 5,700 years ago, destructive eruptions modified the summit. Historical eruptions were noted by settlers between 1820 and 1894. The mountain has been the source of huge mud flows that raced down river systems, nearly reaching Tacoma. Mount Rainier is also known for gigantic rock avalanches--the latest occurring on August 16, 1989, when 2.6 million cubic yards of rock tumbled down the north flank of the mountain. That event was recorded on the Washington Division of Geology and Earth Resources seismograph in Olympia, 50 miles to the west.
Mount Adams is the second highest peak in the Pacific Northwest at 12,276 feet above sea level. Instead of a single cone, Mount Adams was built up of andesite flows emanating from a cluster of vents. Tephras are rare. Mount Adams is a relatively young volcano having developed during late Pleistocene time. There was repeated volcanic activity from 275,000 to 2,500 years ago, interspersed with periods of glaciation. Large debris flows and rock falls have been described for Mount Adams. Thermal activity evidenced by fumaroles has altered the andesite and deposited sulfur near the summit.
Mount St. Helens is the youngest of the Cascade volcanic peaks. The eruptive history of Mount St. Helens started only 40,000 years ago, and intermittent eruptions have occurred to this day. The mountain is characterized by repeated explosive silicic volcanism with pyroclastic flows, widespread tephra deposits, and dacitic dome growth. Pumice and ash falls from eruptions between 2,500 and 1,600 B.C. covered large areas of the Pacific Northwest, and ash from this period has been identified in Banff National Park, Alberta, Canada. Lahars and mudflows are associated with many of the eruptions. For a short period of time, around 400 B.C., silicic volcanism ceased. Instead, very fluid basaltic magma was extruded on the south flank forming numerous lava tubes. Ape Cave, actually a lava tube, is the largest cave in Washington and at 11,215 feet is the longest single lava tube cave in the United States.
Prior to 1980, the most recent major eruptive cycle at Mount St. Helens extended from A.D. 1801 to 1857. Then on March 20, 1980, an earthquake under Mount St. Helens measuring 4.1 on the Richter scale sent a signal that the mountain was waking up. The following months were characterized by crater development, and steam and ash emissions. A pronounced bulge began to form on the north flank as the lava was pushing up from the magma chamber. Initial surface deformation averaged 16 feet per day. As the mountain continued to bulge, a major eruption was predicted, and the Spirit Lake resort area was evacuated. At 8:32 A.M. on May 18, 1980, the mountain exploded. An earthquake registering 5.1 on the Richter scale initiated one of the largest landslides and eruptions in the United States in historic times. The bulge gave way to a giant avalanche of rock, mud, and ice, immediately followed by a lateral and vertical blast of hot gas, ash, and rock. In a instant, the beautiful 9,677-foot summit was reduced to 8,365 feet above sea level. Where once there was a summit, there now existed a huge gaping crater with a breach to the north. Three-quarters of a cubic mile of material moved. Ash was blown 14 miles vertically into the atmosphere and drifted east. At cities in the path of the ash cloud, such as Yakima and Spokane, day turned to night. Over 230 square miles of forest was leveled. Huge mud flows surged west down river valleys toward the Columbia River. There, the navigation channel for ocean-going ships was blocked by mud flows and logs for a distance of 9 1/2 miles. At some locations the channel was only 14 feet deep. There was widespread destruction of natural resources and property. Fifty-seven people lost their lives as a result of the May 18 eruption. The blast was followed by numerous pyroclastic flows and the start of dacite dome growth in the crater. In seven years the dome had grown to a height of 876 feet and had a 3,000-foot-diameter base. It continues to grow to this day and may eventually fill the entire crater. Mount St. Helens National Volcanic Monument provides an unusual opportunity to view geological processes at work.
The above text is modified from the following article: Lasmanis, Raymond, 1991, The geology of Washington: Rocks and Minerals, v. 66, no. 4, p. 262-277. © Copyright Heldref Publications (Helen Dwight Reid Educational Foundation). Used with permission.