TsuInfo Alert is a bi-monthly newsletter that links scientists, emergency responders, and community planners to the latest tsunami research. This newsletter is published by the Washington Department of Natural Resources, Division of Geology and Earth Resources on behalf of the National Tsunami Hazard Mitigation Program, a state/federal partnership funded through the National Oceanic and Atmospheric Administration. It is made possible by a grant from the Federal Emergency Management Agency via the Washington Military Department, Division of Emergency Management.
DGER geologists Trevor Contreras, Michael Polenz, Annette Patton and Harley Gordon are collaborating with the University of Washington’s Burke Museum to better date Tertiary rock formations along Hood Canal. Just as with macrofossil assemblages, microscopic fossils can be used to correlate an unidentified rock with known units. These tiny fossils pack quite a punch, despite their small size. The species of fossils in a given sample can indicate the specific age of the rock and allow researchers to deduce key information about environmental conditions at the time the microfossils and their accompanying sediments were deposited. Such knowledge provides valuable insight into our understanding of the age and stratigraphy of Tertiary sandstones and mudstones exposed in the current mapping area between Discovery Bay and Quilcene.
Foraminifera, like those pictured here, are members of a group of single-celled protozoans that primarily live in marine environments. The living organisms construct chambered shells in a variety of shapes and sizes, either by secreting a solid calcite shell or by cementing together grains that they pick up from the ocean floor. These organisms are particularly valuable microfossils because they are abundant, diverse, and morphologically distinct. Foraminifera also make useful biostratigraphic markers because most species are benthic (they live on the ocean floor). Unlike many planktonic organisms that float along ocean currents, such species remain geographically confined and form unique and local assemblages of fossils.
There have been some important changes to our landslide hazard website since we last blogged about it on November 1, 2012. We have improved the resolution of the map by adding the National Weather Service (NWS) forecast zones. A shaded relief of elevation, along with the addition of salt and fresh water features, improves the look of the map, and makes it easier to locate the place(s) you may be interested in. Behind the scenes, an improved algorithm and the use of NWS precipitation forecasts makes the hazard level indicated on the map more closely relate to what is happening on the ground.
As was true before, the webpage is a test project of the DNR and NWS intended to raise awareness of shallow landslide hazards caused by periods of prolonged rainfall. The map is not intended to predict landslides at any particular time or location; it only rates the overall risk that one might occur, based on rainfall measurements over the past week and forecasts for the coming 48 hours.
Heavy rain this weekend can cause more than just localized flooding and high rivers. Prolonged or intense rainfall increases the chances of shallow landslides on steep slopes. During these rain events some rain will flow on the surface to streams and rivers, some is captured by vegetation, and some rain infiltrates into the ground. The rain that infiltrates the ground may saturate the soil to the point the soil strength decreases, especially during prolonged or intense rain events.
Think of building sand castles with buckets on the beach–the right amount of water and the sand binds together to form a near-perfect cast of the bucket, but if too much water is added, the sand cannot hold its form and the sand collapses under its own weight. The saturation of soil is a similar concern on steep slopes. If the rain intensity or duration is sufficient to saturate the soil to a point where the soil begins to lose strength, the likelihood of a landslide increases. This is further accentuated by our steep slopes and geology in western Washington and can be intensified when drainage systems fail or when development increases surface water runoff near steep slopes.
Warning Signs of an Impending Landslide
If you live on or near a steep slope, here are some warning signs of potential slope instability:
- Formation of cracks in your yard, driveway, sidewalk, foundation or other structures.
- Tilting of trees, especially evergreens, on slopes.
- Sudden difficulty in opening or closing doors and/or windows.
- A hillside that has increasing springs, seeps, or saturated ground, especially if it has been dry.
If you observe one or more of these signs, you should immediately contact your city or county.
Our talented cartographers have put together a great animation showing the evolution of Washington geology. Based on the previous work of Jack Powell and John Figge, the cartoon shows the accretion of terranes through geologic time from the Neoproterozoic (~750 million years ago) to the present. It demonstrates how the breakup and reconstitution of ancient supercontinents, and subsequent volcanism and sedimentation resulted in the complex geology we see in Washington today. The final slide (present-day terrane map) also derived from work presented on the DGER Geologic map of Washington-Northwest quadrant (Dragovich and others, 2002) and the USGS Geologic map of the North Cascade Range, Washington map (Haugerud and Tabor, 2009), as well as more detailed data from a number of other DGER and USGS geologists.
The general story of the Evolution of Washington:
The geology of Washington is the result of a complex history of tectonic events. It is an amalgam of various terranes, accreted through geologic time. A terrane is a discrete block of oceanic or continental material of analogous geologic history compared with the surrounding bodies of rock. The terranes of Washington resulted from continental evolution whereby pieces of ancient continents have broken off and reattached to form different continents.
The oldest rocks in Washington can be traced back to the supercontinent of Rodinia, which is thought to have formed 1.1 billion years ago. About 750 million years ago (Neoproterozoic Era), Rodinia broke apart and the area just west of Spokane became the shoreline for the continent of Laurentia. This new coast transitioned into an active subduction zone (a subduction zone is a plate boundary where one plate is sinking underneath the other, usually with an ocean plate being crushed beneath continental crust). The Laurentia subduction zone existed from about 350 to 170 million years ago (Paleozoic Era). The Kootenay terrane formed during that time period.
Eventually a series of large volcanic island arcs were transported towards the continent through the subduction process. As the oceanic plate they were riding on subducted beneath primordial Washington, the islands were accreted as they smashed against the coastline. This extended the coastline further westward. The first of these island terranes was the Intermontane superterrane that formed about 190 to 170 million years ago (during the Jurassic Period); the Intermontane terrane is located in the modern Okanogan Highlands.
The accretionary process was repeated several more times with a group of archipelagos that docked against coastline as their oceanic plate was overridden. This Insular superterrane extended from the present-day North Cascades up to Vancouver Island.
Between 55 and 20 million years ago (Eocene to Miocene Epochs), intrusives (magma bodies that have cooled and solidified underground) spread throughout the older terranes, and volcanics covered much of them with lava as magma centers migrated westward with the subduction zone. Contemporaneously, during the Eocene to Miocene Epochs, (from about 37 to 25 million years ago), the Siletz-Crescent superterrane accreted onto the west side of the state. The Siletz-Crescent is composed of sea floor basalts and sedimentary rocks. It currently ranges from the Olympic Peninsula region, southward though western Oregon.
About 17 to 6 million years ago (Miocene Epoch), a series of flood basalts, called the Columbia River Basalt Group, erupted out of fissures in southeast Washington and northeast Oregon. These flows covered the eastern half of Washington State and northern Oregon with lava hundreds to thousands of feet thick, deeply burying portions of the accreted terranes.
The Pleistocene (2.6 million to 12,000 years ago) ice age brought ice cover to the northern half of Washington State. Ice dams formed, blocking large waterways and creating reservoirs that eventually sourced huge floods when the dams burst. The massive volumes of floodwater scoured eastern Washington, forming dramatic scablands still visible today. Finally, modern volcanism continues to reshape the Cascades and the geology of Washington State today.
From all of us at the Division of Geology and Earth Resources to all of you, happy holidays! We wish you good tidings for a happy, healthy, and prosperous New Year.