GENERAL GEOLOGIC SETTING OF THE CASCADE REGION
Before a discussion of the general geologic setting of the area is undertaken, it might be well to delineate the span of time we will be considering. In the minds of most people, geological events involve thousands or millions of years. But just how many thousands or millions? And how long is a million years? Such vast quantities of time are not easily fathomed by the human mind.
Geologists have divided all the time that has passed since the earth was created into units called eons, eras, periods, and epochs, among others. The divisions separating these units are based on pronounced changes in life on earth as revealed in rocks of the earth's crust. A simplified listing of these units appears below.
Each of the geologic eras has been further subdivided. Of interest to the student of Cascade geology is the Cenozoic Era and its subdivisions. It is divided as follows:
In order to impress upon the reader the concept of geologic time, let us assume the 4-1/2 billion years that have passed since the earth was formed may be compressed into one calendar year. We would find that Precambrian time, which would have begun with the formation of the earth on January 1, would extend until November 6. By this time the only forms of life on earth were simple organisms similar to algae. The Paleozoic Era would continue from November 6 to December 13. During this time fishes, reptiles, amphibians, and numerous plants evolved. The Mesozoic Era, dominated by the dinosaurs and other large animals, would extend from December 13 to December 25. The Cenozoic Era (which includes the Tertiary and Quaternary) would extend from December 25 to midnight December 31. The quaternary, spanning the last 1 million years, would comprise only the last 2-1/2 hours in our compressed year. Modern man, who has been on earth only some 25,000 years, would appear at 11:57 p.m., December 31. Mt. Mazama, which collapsed only 6,600 years ago to produce the caldera Crater Lake now occupies, would erupt just 45 seconds before the stroke of midnight.
The oldest rocks exposed in the Cascade Region were formed during the Eocene Epoch, and are therefore 60 million years old or younger. The older rocks lying beneath those exposed in the Cascade Range are assumed to be marine sedimentary rocks of Mesozoic age. These rocks are believed to have been formed by precipitation from sea water in a broad depression between the Klamath, the Blue, and the Sierra Nevada Mountains. At the close of the Mesozoic, some 60 million years ago, widespread upheavals in the earth's crust caused the seas to retreat from the area and volcanic activity began in the Cascade Region. Evidence of this activity is preserved in the Umpqua Formation to the west of the Cascades. This formation, which is Eocene in age and sedimentary in nature, contains beds of tuffaceous material furnished by explosive volcanic activity in the Cascade Region.
The Cascade Region is divided, physiographically, into the Western Cascades and the High Cascades. Volcanic activity began in the Western Cascades at the close of the Mesozoic Era, or the beginning of the Eocene Period. Thousands of feet of volcanic rocks piled up and flowed east and west to interfinger with different types of rock in those areas. The landscape appeared in those times as a low, rolling plain, dotted with volcanoes of various sizes, cinder cones, and broad sheets of lava. For approximately 48 million years the volcanic outpourings continued, and in some areas the accumulation of lava and ashes reached almost 10,000 feet in thickness by the close of the Miocene Period. By this time a broad plateau of volcanic rocks had been built, but there was no mountain range such as the High Cascades comprise today. The moisture-laden winds from the Pacific Ocean swept eastward across the plateau to lose their water in central and eastern Oregon. During this time many lakes existed in the John Day country. The fossilized remains of the fauna and flora of the Miocene age are preserved in the ash and flood deposits in that area. Meanwhile, in eastern Oregon and Washington, copious sheets of fluid lava spread over the country. These "plateau basalts" eventually covered more than 200,000 square miles.
At the close of the Miocene Period, about 12 million years ago, a disturbance in the earth's crust uplifted, bent, and tilted the thick sequence of rocks making up the Cascade plateau. This was the origin of the present Cascade Mountains. The Western Cascade rocks were tipped so that they dipped toward the east or northeast. The slope of the beds decreased toward the east and approached the horizontal in the vicinity of what are now the High Cascades. The Western Cascades also were intruded by a north-south chain of molten igneous masses. These did not pour out on the surface as lava, but came to rest and cooled far below the surface. Hence they are coarse-grained igneous rocks, unlike the fine-grained lavas that enclose them.
Also associated with the crustal disturbance at the close of the Miocene was a large, north-south fracture which opened along the east edge of the Cascades. Actually, the fracture probably consisted of numerous parallel cracks of shorter lengths which overlapped to form a fracture zone along the margin of the Cascades. Apparently the rocks on the east side of the fissure dropped down some 1,000 to 2,000 feet below those on the west side. Although younger lavas obscure the fissure, the older lavas beneath them crop out at lower elevations to the east of the younger lavas.
This change in the form of the Cascade area did not happen within the span of a few years. Most geologic changes occur slowly, and the uplift and faulting of the Cascade Region at the close of the Miocene period undoubtedly took place over a span of a few million years.
From the fractures produced by the crustal disturbance at the close of the Miocene Period, lava began to erupt. Throughout the next 10 million years a chain of high cones built themselves along the crest of the Cascade Range. Their eruptions were not violent at first, but rather quiet outpourings of fluid basaltic lava (olivine basalt and basaltic andesite). High Cascade peaks such as Union Peak, Mt. Thielsen, and Pig Iron Mountain grew in this fashion throughout the Pliocene Period, which ended about one million years ago. Toward the close of the Pliocene Period the shapes of some of the typical shield volcanoes were modified by the development of cinder cones atop their summits. The fragmental material was generally more silicic than the basalt making up the broad bases of the volcanoes. Other volcanoes, however, have continued to erupt basaltic material even up to the present. In other words, basaltic shield volcanoes and more silicic cinder cones were forming at the same time, and often in the same general area.
While the Cascade peaks were being built during the Pliocene Period, the older volcanic rocks of the Western Cascades were either overlapped by younger High Cascades lavas or subjected to severe erosion. On the plateau to the east of the Cascades, basaltic lavas erupted throughout the Pliocene Period. As the Pliocene Period drew to a close the Cascade Range appeared as a high plateau surmounted by broad, basaltic shield cones. During the close of the Pliocene and throughout the Pleistocene Period, a series of large cinder cones, more silicic than basaltic in composition, grew atop the basaltic shield cones. These cinder cones were arranged along the north-south crest of the Cascades and include such peaks as Mt. Mazama, Mt. Shasta, Mt. Baker, Mt. Rainier, and Mt. Hood. Some of these peaks continued to grow even into the present century. Mt. Lassen erupted as late as 1917.
The Pleistocene has been called the Great Ice Age, for during that span of about one million years, four great advances of glaciers and ice sheets crept across the continent from the north. Glaciers first form on mountain tops where snowfall is heaviest due to heavier precipitation and cooler temperatures. As the snows pile up, the layers on the bottom are compacted to granular ice which flows downhill very slowly under the weight of overlying snow. Thus, glaciers flow down the flanks of the mountains, down existing drainages, and eventually, if the quantity of ice and snow is sufficient, may emerge to cover broad expanses of land.
Glaciers have a very erosive effect on mountain peaks. Loose rock is frozen into the bottom of the glacier and dragged across the solid bedrock. Both surfaces are literally ground to rock flour, the source of glacial clay deposits. Large blocks of bedrock may freeze to the glacier and be torn loose as the glacier plucks at the mountain slopes, producing large, scooped-out hollows called cirques.
The Cascade peaks that ceased to grow at the close of the Pliocene PeriodMt. Thielsen, Howlock Mtn., and Union Peak among themwere deeply dissected by glacial erosion during the Pleistocene. Their flanks presently exhibit hollow cirques and occasionally their hard, resistant plugs protrude above the cirques as matterhorns. Those High Cascade peaks that continued to grow throughout the Pleistocene, including Mt. Mazama, Mt. Rainier, Mt. Hood, and others, fought the destructive force of glaciation by adding new material to their slopes through eruptions from their summits or flanks. Many of them continued to grow in elevation and bulk despite the ravages of glaciation. Several rock outcrops within the caldera of Mt. Mazama (Crater Lake) reveal scratched and polished slabs of rock produced by glaciers as they dragged boulders over the bedrock. Therefore, the degree to which the Cascade volcanoes retained their conical forms is a function of their age relative to that of the age of glaciation.
Several examples of volcanism in the Cascades postdate the glacial age, which ended about 10,000 years ago. These include Belknap Crater, Trout Creek Butte, and the latest eruptions of Mt. Mazama. The lava flows along the highway at McKenzie Pass are less than 1,000 years old. The rocks are untouched by glaciation.
In his book "Crater Lake: The Story of its Origin", Howel Williams has tabulated in chronological order, the geologic events in the Cascade Region. His chart, with minor modifications, follows:
Last Updated: 01-Jul-2008