GENERAL GEOLOGIC DESCRIPTIONS OF KEY FEATURES IN CENTRAL AND EASTERN OREGON

by John Stockham

General Description of Mt Bachelor.

Mt. Bachelor (9,065 feet) is a symmetrical shield volcano standing about 3,500 feet above its base.  The mountain is the tallest component of a chain of overlapping shield volcanoes and scoria cones typical of those that make up most of Oregon’s High Cascade platform.  Most of the eruptive sequences occurred between 18,000 and 11,000 years ago. The most recent volcanic activity in the Mt. Bachelor chain occurred with the formation of a scoria cone, located adjacent and slightly northwest of Mt. Bachelor, approximately 8,000 years ago, slightly before the eruption of Mt. Mazama, which formed Crater Lake.  This scoria cone adjacent to Mt. Bachelor is popularly referred to by area skiers as “The Cone.”

The composition of Mt. Bachelor is primarily basalt and andesite flows covered with ash deposited by the large eruption of Mt. Mazama 7,700 years ago and more recent eruptions near South Sister.  There is no crater at the top of Mt. Bachelor. Rather, there are numerous vents marked by low blocky domes and circular depressions that formed as magma sunk back into the feeding conduits. Some of the protruding volcanic conduits on the top of the bowl were the result of glacial deformation which eroded the surrounding eruptive material but did not erode the harder unerupted material in the conduits.  These protruding conduit towers are referred to by local skiers as the “The Pinnacles.”

“The Cone” and other nearby scoria cones consist of basaltic ejecta formed by eruptive volcanic vents.  The scoria clasts range widely in size and have light frothy texture full of vesticles or gas pockets. The eruptive material is generally light to medium gray in color, which turns a deep red color as it oxidizes.

Mt Bachelor experienced mild glacial deformation as evidenced by the bowl and terminal moraine on the north side of the mountain.  The small glacier that occupied the bowl was likely formed in a mini-glacial period occurring in the 1700’s. The glacier has fully receded and there is no remaining glacial ice on the mountain.

Typical of much of the High Cascades, there are no named streams or creeks on the flanks of Mt. Bachelor.  Nearly all the rain water and snow melt on Mt. Bachelor flows into the porous volcanic rock and emerges in lower areas in springs or exposed groundwater.

General Geologic Description of the Three Sisters

The Three Sisters are part of the High Cascades which formed between 5,000,000 years ago and the present. Each of the Three Sisters are subduction volcanos resulting from subduction of the Juan du Fuca plate in the Pacific under the westerly-moving coastal Oregon and adjoining parts of California and Washington.  The more prominent peaks in the High Cascades formed within the past 450,000 years.

The Three Sisters represent several types of volcanoes, including shield volcanos, highly explosive stratovolcanoes and a variety of domes and flows.  The Three Sisters has also been altered by several glacial advances, the most recent being the Suttle Lake Advance which peaked about 22,000 years ago.

In the past the Three Sisters volcanoes were referred to as Faith, Hope and Charity and they were familiar landmarks for early pioneers arriving from the east.

General Geologic Description of Broken Top

Broken Top (9,175 feet) is one of the most dramatic and picturesque mountains in the High Cascades.  Most of the building of Broken Top occurred 300,000 to 150,000 years ago. The mountain is a classic stratovolcano that developed as a series of pyroclastic eruptions.  The stratovolcano developed on top of older glaciated basaltic andesite lava flows.

Repeated periods of glaciation have eroded much of original mass of the volcano, leaving serval deep cirques and exposing numerous dikes and volcanic conduits, which stand as protruding towers among the surrounding more erodible strata consisting of red purple and black scoria and alternating layers of welded pumice and ash flows.  

A prominent terminal moraine occupies the base of the large south-facing bowl. A small lake, referred to locally as No Name Lake, occupies an east-facing bowl.

The bowls and ridges extending around Broken Top are popular hiking destinations.

General Geologic Description of Mt. Jefferson

Mt Jefferson (10,497 feet) is Oregon’s second tallest mountain and is a visible landmark from either side of the High Cascades.  The mountain is one of the least studied mountains in the High Cascades.

Like the other High Cascades, Mt. Jefferson is a subduction-related volcano that has developed over the past 5,000,000 years.  Beginning about 280,000 years ago, Jefferson erupted in a series of alternating andesite and dacite flows. Beginning about 70,000 years ago the modern mountain erupted with a series of high magma eruptions, some of which surfaced through thick glacial ice cover.  The most recent eruptions on Mt. Jefferson occurred about 20,000 years ago.

Many glacial advances have eroded the flanks of the mountain and lowered the summit by up to 1,000 feet. Today the Whitewater Glacier on the east side and the smaller Mill Creek Glaciers on the west continue to carve into the mountain.  Numerous debris flows caused primarily impounded lakes breaching moraines continually alter the terrain on the flanks of the mountain.

General Description of the Deschutes River

The Deschutes River is the primary river receiving water from east side of the High Cascades.  The headwaters are Lava Lake and several other smaller springs and lakes located south and west of Mt. Bachelor.  Major tributaries are the Crooked, Metolius and White Rivers. The river drains into the Columbia River near The Dalles.

The river is quite unique in that most of its flow originates from springs and groundwater released from porous volcanic materials, as opposed to surface runoff.  Prior to construction of man-made impoundments, the river flowed at relatively consistent level throughout the year. Impoundments and with draws of water for irrigation and other uses has a severely altered the natural flows.

General Description of Newberry Volcano and Paulina Peak

Newberry is a broad shield volcano that stretches for approximately 40 miles in a north-south direction and contains approximately 1200 square miles.  Many of its lava flows extend well beyond the volcano boundaries, extending northward to the Billy Chinook Reservoir north of Redmond. The Badlands, east of Bend, are composed of Newberry volcano lave that traveled northward in lava tubes, as are Lava Butte and numerous other cones in vicinity.

The central feature of Newberry is a large central caldera that contains Paulina Lake and East Lake.  The current caldera dates from 75,000 years ago, but there were at least two earlier caldera forming episodes.  The caldera is 15 square miles. The age of the overall Newberry caldera is approximately 400,000 years ago, which is relatively young in geologic terms.

The volcano consists of numerous basalt flows that extend both north and south of the caldera.  The caldera itself is rimmed by numerous rhyolitic domes and vents. There are up to 400 eruptive cinder cones and vents on the flanks of the volcano.

The most recent explosive activity on Newberry Volcano is the 1,300-year-old Big Obsidian Flow which is a rhyolitic eruption on the south side of the caldera.  Obsidian which was collected and traded by Native Americans, is abundant in the Big Obsidian Flow other obsidian deposits around the caldera. The isthmus that separates Paulina Lake and East Lake within the caldera was formed by eruptions occurring approximately 7,000 years ago.  There is a small crater-topped volcano and a large pumice cone within the isthmus.

Paulina Peak (7,984 feet) is the highest point on the south perimeter of the caldera.  Paulina Peak consists of rhyolite. The area around Paulina Peak is blanked with rock fragments, pumice and ash.

General Description of Smith Rock

Smith Rock is a volcanic tuff of the John Day Formation.  The primary eruption, which occured over 29,000,000 years ago, was part of a large explosive event that formed the Crooked River Caldera. The caldera, which is about 25 miles wide at its widest part, extends south and eastward from Smith Rock and includes Prineville and sections of the Crooked River Canyon further upstream.  It is one of the largest calderas in North America, aside from the Yellowstone Caldera.

The rock material forming Smith Rock is predominantly tuff, which is volcanic rock made mostly of consolidated ash and pumice derived from ash falls and pyroclastic flows. The more resistant rocks are welded tuff that when the heat of the particles causes them to bland together.  The tuff also contains lens-shaped layers of rock fragments from the older terrane. The basement rock includes ancient Permian limestone which has been brought up by the erupting magma.

The towers and unusual formations are formed by differential erosion.  Parts of the tuff are more resistant than others and form distinctive promontories while the surrounding material erodes more rapidly.  In other places, more resistant rhyolite dikes have intruded into the tuff to form spikes and towers. In some places at Smith Rock a weathering process known as case hardening has made some of the rock more resistant.

The Crooked River separates the 29-million-year-old tuff from the 400,000-year-old basalts that flowed from the Newberry Volcano, located approximately 40 miles to the south.  Smith Rock formed a barrier to the basalt flows causing the lava from Newberry to flow westward and then northward in the Crooked River Canyon that flows into Billy Chinook Reservoir.

General Description of Abert Rim

Abert Rim is the western end of a large tilted block that slopes to the east toward Hart Mountain.  The rim is one of the highest fault scarps in the United States. Abert Rim rises 2,490 feet above the valley floor.  The rim is capped by an 850-foot sheer-sided basalt cap, formed during the time that Columbia River Basalts flooded most of eastern Oregon approximately 17 to 14.5 million years ago.  The specific basalts capping Abert Rim originated from flows coming from the Steens Mountain area, approximately 100 miles to the east, which is the oldest formation of the Columbia River Basalt Group.  In some areas, the basaltic cap has been overlain with Rattlesnake Tuff and other more recent ash and tuff deposits.

The base of Abert Rim consists of basaltic boulders formed by rock falls off of the basalt caps.  Many of the boulders contain elongated crystals of plagioclase, which is typical of the Steen Mountain basalts

Lake Abert Basin is a remnant of a pluvial lake that inundated a large area in south central Oregon.   The lake originated from glacial runoff in the Pleistocene period (2 million to 0.01 million years ago). Because Lake Abert has no outlet, its waters are extremely alkaline, and it precipitates mineral along its shoreline at the base of Abert Rim.