Welcome to Week 2 of Pacific Northwest Geology. The topics of this week's lecture are:
- Holocene Geologic Activity
- The Pleistocene Ice Ages
- Glossary Terms
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
The Holocene epoch began 11,700 years ago, when the Cordilleran Ice Sheet had retreated back to the mountains of western Canada.
We are still in the Holocene epoch. Some geologists have suggested we draw a new time boundary and call our epoch the Anthropocene, beginning when global changes due to anthropic (human) activities became pervasive in the geological record. However, the idea of an Anthropocene epoch is still under discussion and debate, and in the meantime we officially continue to live 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 coastlines. Geological activities like these occurred in the past, occur today and will occur in the future. Such dynamic geological activities, especially those that occur abruptly, can pose hazards to humans.
Towards the end of the Pleistocene epoch the global climate warmed significantly. The continental ice sheets wasted away from North America and northern Europe. At the same time, alpine glaciers around the world retreated a great deal. The melting of ice and the rise in temperature combined to raise the average level of the sea over 100 m, around 350-400 ft. The sea-level rise shifted coastlines inland and drowned the mouths of many rivers, turning them into estuaries.
Puget Sound came into existence in its present form in the early Holocene, after the sea advanced into the troughs that the Vashon glaciation left behind.
All the beaches and other shoreline features of the Northwest were established in their present location during the Holocene epoch. Steep coasts have been eroded by wave action along the shore creating receding, high cliffs. Wave action has deposited beaches of sand and gravel, sandbars, mud, and sand spits in the bays and flat areas of the shoreline.
The weight of the Cordilleran ice sheet on the North American continent pushed it deeper into the underlying earth. Once the glacier left, the continent slowly rebounded. Estimates of the amount of post-glacial rebound in the Puget Sound region are complicated because two geologic events were occurring at the same time. As the earth’s surface was uplifting due to post-glacial rebound, sea level was rising due to the addition of water melting from the retreating glaciers. In addition, tectonic effects, including vertical movement of the earth’s crust along faults, have been significant in the Puget Sound basin during Holocene time. Taking into consideration all the complications, geologists think that post-glacial rebound may exceed 10 m (35 feet) in northern Puget Sound, including northern Whidbey Island.
Great earthquakes, called subduction earthquakes, have caused abrupt changes on the Pacific Northwest coast during the Holocene epoch. The great Cascadia earthquake of 1700 caused vertical elevation changes along the ocean coast of as much as about 2 m (6 ft). In addition to sudden changes in elevation, gradual uplift of the coastal region occurs between major earthquakes.
Within the Puget Sound basin, land south of the shallow Seattle-Bremerton fault has undergone significant uplift prior to and during the Quaternary period. A striking difference in bedrock exposure north and south of the fault is evidence of the significant amount of vertical offset. You can find exposed bedrock in parts of Seattle and its suburbs south of the fault, but north of the fault, on the down-dropped side, the bedrock is buried beneath over a hundred meters (hundreds of feet) of glacial drift.
Today and tomorrow along the shores of the Pacific Northwest coast, the tides will continue to rise and fall and the waves will continue to break. As a result, the beaches, bays, estuaries, sea cliffs, and sea stacks will continue to be subject to erosion, transport and deposition of sediments. Landslides will continue to be common along steeper parts of shorelines. Tsunamis and abrupt changes in land level will continue to occur during great earthquakes. Sea level in general is expected to rise another 0.25-1.0 m (0.8-3.2 ft) in the next hundred years, which will submerge some land near sea level along the coast, and expose adjacent land to greater risk from storm waves. Every day, grains of sand and gravel will continue to move in the beaches and tide zones, and every storm will move cobbles and boulders as well.
Most valleys in the Pacific Northwest are the result of erosion of uplifted areas of the earth's crust by streams. Geologists use the word stream to refer to a body of flowing water that runs downhill and is confined in a channel between stream banks most of the time. The Columbia River is a stream, and so is Thornton Creek. As a stream works towards a balance between erosion, transport, and deposition of sediment, it tends to form an alluvium-filled floodplain, a zone of relatively flat ground between the stream banks and the steeper sides of the valley beyond the floodplain.
Many streams and rivers in the Pacific Northwest have a series of benches or stream terraces along the sides of the valley that mark previous floodplain levels. A stream terrace is formed when a stream downcuts into its own floodplain and establishes a new floodplain at a lower level. A stream may downcut into its own floodplain for several reasons, inluding from steepening of the stream gradient, change in amount of sediment carried by the stream, or change in amount of water flowing through the stream.
An explosive volcanic eruption may overload a stream with volcanic debris, temporarily raising the floodplain level. If the stream subsequently erodes into the sediments and forms a lower floodplain, remnants of the higher-level floodplain, left as topographic benches on the sides of the valley, will be stream terraces. The level of the Toutle River floodplain was raised by volcanic sediments from the eruption of Mount St. Helens in 1980. Since then, the river has been cutting downward into the sediments. If you drive up the Toutle River into Mount Saint Helens National Volcanic Monument, look around the river and its tributaries to see if new stream terraces are forming.
If a glacier fills a stream valley then melts away, a sequence of kames of glacial outwash that accumulate alongside the shrinking glacier may be left behind as stream terraces. The prominent terraces that run along the sides of the Okanogan River valley and Columbia River valley from Chelan Falls to the Canadian border are thought to be kame terraces.
As the glaciers melted away at the end of the Pleistocene epoch, they shed large amounts of sediment that traveled down stream valleys in melt-water-rich currents. This large volume of sediment filled floodplains. During the Holocene epoch, some streams, such as the Wenatchee River below Leavenworth, have repeatedly established new floodplains at lower levels within the higher, older floodplains.
Where streams disgorge from steep, narrow mountain valleys 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 land for the main industrial sectors of those two cities--an example of how human activity has added 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 occur in the Pacific Northwest with the greatest power and frequency at the coast. This is because the Northwest’s ocean coastline is the leading edge of the Cascadia subduction zone, where great subduction earthquakes occur. Great subduction earthquakes are the most powerful type of earthquake, and also the most likely to produce a tsunami.
Much of the evidence for great subduction earthquakes and tsunamis has come from estuaries along the Pacific Coast. An estuary is a bay where fresh water from a river mixes with salt water of an ocean. Geologists have discovered layers of sediment deposited by tsunamis that surged up estuaries from the ocean during the Holocene epoch. Many of the tsunami deposits buried sea-level marshes in in the coastal estuaries. The sudden lowering of the land during the subduction earthquake caused the death of marsh grasses and nearby trees when their roots were drowned in salt water. The resulting record of earthquakes and tsunamis can be age-dated by counting annual growth rings in the trunks of the trees and by radiocarbon-dating both the trees and the dead marsh plants.
The record shows that along the Pacific Northwest coast great subduction earthquakes occur at intervals of 300 to 600 years. The last subduction earthquake on the coast was in the winter of 1700. With a magnitude estimated as close to 9.0, the 1700 Cascadia subduction zone earthquake set off a great tsunami that destroyed numerous Indian villages on the Pacific Northwest coast and washed ashore in Japan on the other side of the Pacific Ocean, damaging some structures in harbors there.
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 cause serious damage. The February, 2001 earthquake centered in the Olympia-Tacoma area was a deep earthquake. It knocked down some older brick structures in Pioneer Square in Seattle.
The shallow Seattle-Bremerton fault last had a major earthquake 1000-1100 years before present (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 fault today would be devastating to the highly populated Seattle area.
Further inland, along the volcanic arc of the Cascadia subduction zone, The Cascade Mountains have active fault zones in the shallow crust associated with the major volcanoes, and non-volcanic faults and earthquakes caused by tectonic stresses in the crust.
East of the Cascades, in the interior of the Pacific Northwest beyond the subduction zone, faults and earthquakes are not necessarily due to the interaction of tectonic plates. In the Basin and Range and Rocky Mountains major earthquakes occur on normal faults associated with tension (stretching) of the crust and the growth of block mountain ranges. The Borah Peak and Hebgen Lake earthquakes caused fatalities in Idaho and Montana earlier in the twentieth century.
Earthquakes also occur in the Columbia Plateau and Okanogan Highlands, though the underlying tectonic processes in those regions are not yet well resolved. In 1872 a large earthquake centered near the town of Entiat at the border between the Cascade Mountains and Columbia Plateau of Washington caused numerous landslides. One landslide 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.
Modern seismographs, instruments that detect earthquakes, have identified a seismic zone that runs from about Entiat to Chelan in north central Washington, which produces small earthquakes every year, and every few years one large enough to be felt by local residents. It may be that the Chelan-Entiat seismic zone is related to the major earthquake of 1872.
Even though the highest risk of earthquakes is at the leading edge of the Cascadia subduction zone, and even though that is the only place in the Pacific Northwest thought capable of producing earthquakes of magnitude 9.0 or higher, the entire Pacific Northwest is at risk from earthquakes.
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 (which contains 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 almost 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 along the Northwest coast and hotspot volcanism occurs in the Yellowstone region.
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 in the Pacific Northwest 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 leading up to Yellowstone National Park, which is an active and potentially dangerous volcanic zone.
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.
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 been abandoned in recent years due to failure of the ground beneath the neighborhood.
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, including Mt. Rainier, Mt. St. Helens, Mt. Baker, Mt. Hood and Mt. Shasta. Mt. Rainier's lahars are considered particularly threatening to humans, because they have buried river valleys with mud and boulders in the past, extending as far as Puget Sound near Tacoma and a few miles south of Seattle. There are now towns and neighborhoods full of people 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. Other lahars have 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.
Alpine glaciers of the Pacific Northwest have advanced and retreated several times since the Cordilleran Ice Sheet retreated. However, none of the advances have extended so far or been as thick as the maximum alpine glaciations of the Pleistocene Ice Ages. The most recent advance of Pacific Northwest glaciers occurred 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.
The Nisqually Glacier near Paradise on the south side of Mt. Rainier has retreated over a 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 between 20,000 and 10,000 years ago.
For more information on how glaciers and ice sheets shape the land see the Basics page on landform sculpting.
The Pleistocene epoch is characterized by cold climates and the growth of glaciers. During cold climate stages of the Pleistocene, ice accumulated in the mountains of western Canada and formed a giant glacier known as the Cordilleran ice sheet. At its maximum advances, lobes of this glacier reached carved out the basin of Puget Sound, reached into Lake Chelan, covered the Okanogan Highlands, and filled other parts of northern Washington, northern Idaho, and northwestern Montana.
Also during the cold climate intervals of the Pleistocene epoch, land around Hudson Bay in northeastern Canada became the source of a separate continental glacier, the Laurentide ice sheet, which reached down into parts of the United States, including the great plains of eastern Montana where the glacier diverted the flow of the Missouri River southward from its previous course.
For more information on the effects of the giant ice sheet that came down from Canada during the Pleistocene epoch, see Focus Page #5
Land adjacent to the ice sheets became the repository of glacial outwash sediments. More distant areas received fine silt that winds picked up and carried from the glacial sediments. This wind-blown silt accumulated into loess in places such as the Palouse country of southeastern Washington.
Another distinguishing feature of the Pleistocene epoch in the Pacific Northwest is the greater amount of alpine glaciation than is present now. Alpine glaciers form in local mountain ranges and most are confined to valleys within the mountains. 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 down the valleys and 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 higher mountain ranges of the Rockies in Idaho, Montana, and Wyoming.
The northern part of the North Cascades mountains in Washington state were covered by the Cordilleran ice sheet at its maxiumum advance, while the rest of the high Cascade range in Washington, Oregon, and California had much larger valley-filling alpine glaciers and local ice caps.
The Olympic Mountains have a few, relatively small glaciers remaining today, including Blue Glacier on Mt. Olympus. During maximum glaciations of the Pleistocene epoch, there was much more glacial ice on the Olympic Mountains and some glaciers extended far down the valleys to the coastal plain.
During the times of cold climate in the Pleistocene, unglaciated parts of the interior of the western United States received much more annual snow and rain than they do now. This led to large lakes forming in interior basins, such as glacial Lake Bonneville in Utah, of which the Great Salt Lake is but a small remnant.
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 glacial outburst floods were those 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; gradient; kame; glacial outwash; alluvial fan; delta; subduction zone; subduction earthquake; tsunami; composite cone; lava dome; lahar; glacier; terminal moraine; lateral moraine; loess
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© 2001 Ralph L. Dawes, Ph.D. and Cheryl D. Dawes