Welcome to Week 2 of Pacific Northwest Geology. The topics of this week's lecture are:
Related Basics Pages: Depositional Environments; Landform Sculpting
Related Focus Pages: #3--Changing Climate, Landscapes, and Life Forms of the Pacific Northwest; #4--Effects of the Lake Missoula Floods on the Pacific Northwest; #5--Continental Ice Sheets in the Pacific Northwest
Holocene Geologic Activity
The Holocene epoch began approximately 10,000 years ago, when the Cordilleran Ice Sheet retreated back to the mountains of western Canada. We are still in the Holocene epoch. The Holocene geology of the Northwest comprises all the geological activity from the retreat of the last ice sheet until today. A lot has happened in that time, including volcanism, major earthquakes, and a major rise in sea level that changed coastline.
Sea-Level Rise and Coastal Processes
Towards the end of the Pleistocene epoch the global climate warmed significantly. At the same time the continental ice sheets wasted away from North America and northern Europe, and the alpine glaciers of the world retreated a great deal as well. The melting of ice and the rise in temperature combined to raise the average level of the sea about 100 m, or about 350-400 feet. This moved the coastlines in and reduced the width of dry land on the edges of the continents. It also drowned the mouths of many rivers, turning them into estuaries.
Puget Sound itself came into existence in its present form during this time, after the sea advanced into the troughs that the Vashon glaciation left behind. As the rise in sea level slowed as it neared its present level, new coastlines were established.
Waves along the shore have proceeded to erode steep coasts into receding, high cliffs. The waves have deposited sandbars, mud, and sand spits in the bays and flat areas of the shoreline. All the beaches and other shoreline features of the Northwest have been established in their present location during the Holocene epoch.
The weight of the glacier on the Puget Sound area caused the continent to sink deeper into the underlying earth. Once the glacier left, the continent slowly rebounded. The extent of post-glacial rebound in the Puget Sound region is uncertain. As a result of uplifting continental crust, beaches become stranded on dry land, higher up and inland from the new beach. Some beaches on Puget Sound show this relationship. Apparently, since the rise of sea level due to glacial melting has slowed, smaller amounts of continental uplift have occurred as the crust finished rebounding from the weight of the glacier.
Earthquakes also cause changes in elevation, relative to sea level, of parts of the coast along the activated fault. Major earthquakes on the coast-subduction earthquakes-have caused such changes on the Pacific Northwest coast. These local elevation changes due to subduction earthquakes are on the scale of a few feet to a few tens of feet at a time.
River and Deltas
In the northern zone and alpine regions of the Pacific Northwest streams have reestablished their valleys and drainages since the ice sheet and alpine glaciers retreated. Some streams have established new floodplains at lower levels within their older floodplains several times, leaving behind a series of benches or stream terraces along the sides of the valley. The Okanogan River in north central Washington shows this stream terrace effect prominently.
Where steep streams empty onto flatter ground they build alluvial fans, deposits of sediment sloping out in a fan-shaped pattern from the mouth of the stream canyon (as seen on a map). Holocene alluvial fans occur in all parts of the Northwest, although they are harder to recognize in the heavily vegetated coastal zone.
Where streams empty into larger, slower-moving bodies of water they tend to build deltas. Where they empty into bays they may gradually fill the bay with sediment. This has happened to some of the bays and river mouths of the Pacific Northwest since the Pleistocene epoch.
On Puget Sound, the Duwamish River in Seattle, the Puyallup River in Tacoma, and the Nisqually River north of Olympia have partly filled in their estuaries with sediments, forming flat ground at sea level. In Seattle and Tacoma the tidelands adjacent to the flat ground were drained and filled. This draining and filling created the main industrial sectors of those two cities-examples of how human activity adds to the Holocene geologic history of the Northwest.
For more information on how streams shape the land see the Basics page on landform sculpting.
Earthquakes
Estuaries along the Pacific Coast provide a record of elevation changes and tsunamis stretching back several thousand years. Major subduction earthquakes occur at intervals of 200 to 600 years, according to tree-ring counting and radiocarbon ages. The last subduction earthquake on the coast was in the winter of 1700-1701.
The Seattle fault last had an earthquake 1000-1100 years BP, according to a variety of evidence. That earthquake appears to have caused an inside-Puget-Sound tsunami, uplifted Alki Point in West Seattle along with the south end of Bainbridge Island in Puget Sound, and caused many large landslides around the Puget Sound region. The Seattle fault is still active. A similarly large earthquake on the Seattle fault today would be devastating to the highly populated Seattle area.
The Puget Sound region in historic time has been subject to two types of earthquakes, shallow (in the continental crust), and deep (in the oceanic plate subducting underneath the continental crust). Both types of earthquakes can be damaging depending on their magnitude and other factors. The largest-magnitude earthquakes at subduction zones tend to be the deep ones, in the subducting plate. The February, 2001 earthquake, which was centered in the Olympia-Tacoma area and knocked down some older brick structures in Pioneer Square in Seattle, was a deep earthquake.
Other parts of the Pacific Northwest are not immune to earthquakes. In 1872 a large earthquake centered near the Entiat Valley in the North Cascade Mountains of Washington caused numerous landslides. One landslide came down the side of the Columbia River valley near the town of Entiat and temporarily dammed the Columbia River. This location is now called Earthquake Point. It is alongside Highway 97 just north of Entiat on the west side of the Columbia River.
Spokane, the Okanogan Highlands, and the Yakima area have all undergone numerous minor earthquakes in recent human history, enough to be felt and shake things up a little bit. In Idaho and Montana much larger earthquakes have occurred in the last century, in association with faults along the borders of mountain ranges in the Rockies. The Borah Peak and Hebgen Lake earthquakes caused fatalities in Idaho and Montana earlier in the twentieth century.
Even though the highest risk of earthquakes is at the leading edge of the Cascadia subduction zone, along the coast, the entire Northwest is at some risk to at least moderate shaking by earthquakes. Parts of the Rocky Mountains and Basin and Range, near active faults on the edges of mountain ranges, are at risk of infrequent but damaging earthquakes.
Volcanism
Nearly all of the tall composite cone volcanoes of the Cascade Range have been active during the Holocene epoch, including Mt. Baker, Glacier Peak, Mt. Rainier, and Mt. St. Helens, Mt. Hood and Mt. Mazama (Crater Lake) in Oregon, and Lassen Peak (also called Mt. Lassen) in northern California. Eruptions have ranged from lava flows and lava dome eruptions to explosive eruptions that blanketed nearly the entire Pacific Northwest with volcanic ash and left a caldera (extra-wide crater) behind. Such eruptions will continue to occur into the foreseeable future, as long as subduction continues beneath the Northwest.
Holocene volcanism has also occurred just east of the Cascade Range at the Newberry Volcano area south of Bend, Oregon; and at the Medicine Lake Volcano area near the Oregon border in north central California. Newberry Volcano and Medicine Lake Volcano are both shield volcanoes.
Holocene volcanic eruptions of small to moderate scale have occurred in the Basin and Range region of southeastern Oregon, and in the Snake River Plain at Craters of the Moon National Monument.
In sum, most of the Holocene volcanic activity has occurred in the Cascade Range. A second zone of Holocene activity has been associated with shield volcanoes just east of the Cascade Range. The rest of Holocene volcanic activity in the Northwest has either occurred in the Basin and Range region, or in the Snake River Plain area.
Lahars and Landslides
Sometimes gravity works with the land to move a large mass of earth down-slope all at once. This can happen on the sides of volcanoes, the sides of steep hills or cliffs, or anywhere the land has a steep enough slope and the material beneath the slope becomes too weak to hold together. If a
Lahars are muddy, water-saturated debris flows that come down from composite cone volcanoes and tend to be channeled along river valleys. They can be set off by a volcanic eruption, or may simply start as a non-eruption landslide that mixes with snow, ice, and water on its way down. Probably all the large composite cone volcanoes of the Cascade Range have spawned lahars in the Holocene, and certainly Mt. Rainier, Mt. St. Helens, Mt. Baker, Mt. Hood and Mt. Shasta have. Mt. Rainier's lahars are considered particularly threatening to humans, because it has previously buried river valleys with mud and boulders as far away as Puget Sound near Tacoma and Seattle. There are now towns and neighborhoods in those river valleys.
The Osceola Mudflow is the largest-volume lahar known to have come from Mt. Rainier. It is associated with collapse of a large portion of the peak of Mt. Rainier about 5600 year BP. There have been other lahars that reached down the river valleys close to Tacoma and south of Seattle, before and after the Osceola mudflow. Very small lahars set off by extensive glacial ice melting have occurred on the slopes of Mt. Rainier as recently as the summer of 2001. Along with the risk of volcanic eruptions, composite cones such as Mt. Rainier continue to pose the risk of lahars.
Landslides, at least on a small scale, occur every year in the Northwest. They tend to be set off by heavy rain, or heavy rain mixed with rapidly melting snow. Parts of neighborhoods in Seattle and Kelso, Washington have had to be abandoned in recent years due to failure of the ground beneath the whole neighborhood.
Glacial Activity
Since the Cordilleran Ice Sheet left, the alpine glaciers of the Pacific Northwest have advanced and retreated several times. However, none of the advances have extended so far or been so thick as the maximum alpine glaciations of the Pleistocene Ice Ages. The most recent time that most Northwest glaciers advanced was during the Little Ice Age of approximately 1300-1870 AD. Terminal and lateral moraines left by the glaciers during this recent "neoglaciation," as it is sometimes called, are still relatively fresh and unweathered, with little sign of vegetation or erosion.
To give an example, the Nisqually Glacier near Paradise on Mt. Rainier has retreated about one mile from the neoglacial maximum that it reached in the mid-1800s. This is a significant glacial retreat. However, it is small in comparison with the 25 miles the glacier retreated from its Pleistocene maximum.
For more information on how glaciers and ice sheets shape the land see the Basics page on landform sculpting.
The Pleistocene Ice Ages
Although large portions of continents were covered by ice sheets in earlier periods of geologic time, the Pleistocene epoch was so recent that the effects of the glaciers are still prominent in many parts of the Pacific Northwest. The Pleistocene epoch is distinguished by the repeated accumulation of ice sheets in the cordillera (mountain ranges) of western Canada. The ice flowed as far south as the vicinities of Olympia, Washington; Chelan, Washington; Coeur d'Alene, Idaho; and Kalispell, Montana. The land adjacent to the ice sheets became the repository of glacial outwash sediments. More distant areas received fine silt that blew out of the glacial sediments and accumulated into loess in places such as the Palouse country of southeastern Washington.
Alpine glaciers, which formed in the local mountain ranges, also grew much thicker and covered more area several times during the Pleistocene. In the Glacier National Park region in the northern Rocky Mountains of Montana an ice cap formed across the crest of the range, and large glaciers extended miles out onto the surrounding plains. Ice caps and large valley glaciers left behind deep glacial troughs and a variety of lakes and other landforms in and around the edges of such mountain ranges as the Wallowa Mountains in Oregon and the North Cascades in Washington.
For more information on the effects of the giant ice sheet that came down from Canada during the Pleistocene epoch, see Focus Page #5.
Where the lobes of the advancing continental ice sheet blocked river valleys, lakes were impounded, creating deposits of lake-bottom sediment.
Some of the lakes broke through their glacial ice dams and flooded downstream valleys and lowlands. The most prominent of the glacial outburst floods were the ones from glacial Lake Missoula, which raced out of the Idaho panhandle and across the Columbia Plateau and created a new landscape called the Channeled Scablands. These are the largest-scale floods that have yet been definitively documented in the geological record of earth. See the Focus Page on the glacial Lake Missoula floods for more information.
Glossary terms that appear on this page: ice sheet; estuary; sand spit; floodplain; stream terrace; alluvial fan; delta; tsunami; subduction zone; composite cone; lava dome; lahar; glacier; terminal moraine; lateral moraine; glacial outwash; loess
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Lecture #2
© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes
updated: 10/14/01