An earthquake occurs when rock inside the Earth moves or breaks. This movement happens because stress builds up as tectonic plates move. The process of breaking and moving rock releases a large amount of energy that travels through the Earth as seismic waves.

More information can be found on our Earthquakes and Faults webpage.

For WGS, active means that a fault or fold has evidence for movement within the Holocene time period (since about 12,000 years ago). It usually also means that there are earthquakes (even small ones) documented to have occurred on the fault. Other faults that have evidence for movement within the Quaternary (last 2 million years or so) are also of concern.

Washington has dozens of faults, fault zones, and folds with evidence of movement in the Quaternary. Some of these features are in remote areas. Others, like the Seattle Fault and southern Whidbey Island fault zone, cross under major cities and pose a significant hazard.

More information can be found on our Earthquakes and Faults webpage.

Earthquakes cause damage by moving and shaking the ground, sometimes for several minutes. The shaking can damage or destroy buildings and other infrastructure.

Soils can be classified by the National Earthquake Hazard Reduction Program (NEHRP) seismic site class of an area. Site class is a simplified method for characterizing the ground-motion amplifying effects of soft soils during an earthquake by evaluating the relation of average shear-wave velocity in the upper 100 feet of the soil-rock column to the amplification of shaking at ground surface. Shear waves are the earthquake waves that create the strongest horizontal shaking and are the most damaging to buildings and structures. Site class provides some measure of the potential for strong shaking in a particular area during an earthquake.

Site class B represents a soft rock condition, where earthquake shaking is neither amplified or reduced by the near-surface geology. Site classes C, D, and E represent increasingly softer soil conditions which result in a progressively increasing amplification of ground shaking. Site class F is reserved for unusual soil conditions where prediction of the amplification of earthquake shaking can only be determined by a site-specific evaluation. On this map we delineate areas of peat soil as site class F. This information is useful when deciding where and how to build.

For more information about ground shaking, go to our Faults and Earthquakes webpage.

A volcano is an opening in the surface of a planet (or moon) that allows hot material to escape from the magma chamber below the surface. When the hot material, such as lava, ash, and gas make their way to the surface, the volcano erupts. The style of eruption depends on the type of volcano. Eruptions can be explosive, sending hot mixtures of ash and gas high into the sky; or they can be calm, only sending small amounts of lava down the slope.

There are over 500 volcanic vents in Washington State. A volcanic vent is an opening on the Earth's surface where magma and gases are emitted. All volcanoes have a central vent which is connected to the subsurface magma chamber that supplies volcanic materials. Different types of volcanoes produce different kinds of lavas. The volcanoes in Washington produce lavas mainly composed of dacite or andesite.

For more information about volcanoes, visit our volcanoes page.

Geochronology is the analytical determination of age of a sample of rock to establish its age of formation or to provide limits on a geological event. Absolute methods are usually radiometric, meaning they use the predictable decay of radiogenic isotopes with known half-lives to calculate the age of the sample. However, other methods use other properties, such as observable light emittance or tree rings to better determine age. Relative dating techniques bracket the time frame of formation, and include fossil assemblage identification, paleomagnetism, magnetostratigraphy, chemostratigraphy, or cosmogenic nuclide geochronology.

Argon-argon radiometric analyses use irradiation of a sample to convert the stable ³⁹K to the radioactive ³⁹Ar. During subsequent incremental heating, each step releases argon having a specific ⁴⁰Ar/³⁹Ar ratio. Using a half life for ⁴⁰K of 1.25 billion years with the measured ⁴⁰Ar/³⁹Ar ratio and irradiation parameter values, the age of the sample can be calculated.

Potassium-argon dating relies on the relative proportions of ⁴⁰K to ⁴⁰Ar within a sample; this method has been largely superseded by the ⁴⁰Ar/³⁹Ar method, because the K-Ar results may be too old due to excess argon or too young due to argon loss during weathering and alteration. In both cases, the age of the sample represents the age in which closure temperatures for specific minerals within the rock were met, and may therefore represent a crystallization age, a metamorphic age, or an age of faulting.

This method can be used on many rock types as young as a few thousand years old.

Uranium-thorium-lead radiogenic isotope age determinations are made by measuring the abundances of two uranium isotopes (²³⁸U and ²³⁵U) and one thorium isotope (²³²Th) relative to the abundances of their daughter lead isotopes ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb, respectively, using their respective decay constants. The ratio of ²⁰⁷Pb to ²⁰⁶Pb is also measured, which may yield the closest age determination of crystallization, because this measurement is not affected by U or Pb loss as with uranium-dependent analyses. Measurements are typically performed on uranium-enriched mineral separates, primarily zircon and apatite. Ages for these analyses are determined through use of concordia or isochron diagrams. Recent improvements in the technique allow individual grains or fragments of grains to be analyzed on mass spectrometers, rather than bulk analysis of dissolved mixtures of detrital minerals. This reduces the chance for anomalously old age determinations. Thus, recent analyses are considered to be generally more reliable than older measurements. Pb-alpha analyses have been compiled into WGS datasets, but the method has been largely superseded by the isotopic U-Th-Pb method.

Uranium-lead-based analyses are typically used on igneous rocks older than about 1 millions years. Uranium-thorium-based analyses can be used on carbonates and fossils over 700,000 years old.

Radiocarbon analysis measures the decay of the radioactive carbon-14 isotope (¹⁴C) to estimate the age of organic material. This works because when organic matter forms, it absorbs ¹⁴C in proportion to the environmental abundance of ¹⁴C, and once absorbed, the ¹⁴C decays at a predictable rate. The 5,730 year half-life of ¹⁴C, the assumed initial abundance of ¹⁴C in the sample, and the remaining ¹⁴C decay activity are used to calculate the age.

The results of this method are refined by correcting for known variations in the radiocarbon content in the atmosphere in the past. Additionally, the date may be further refined by correcting for isotope fractionation based upon the abundances of ¹²C and ¹³C in the sample.

Radiocarbon analysis is performed on organic material younger than 45,000 years old. Age determinations of tree samples may at times be refined by dendrochronology, if a fully anchored chronology is available.

Luminescence dating is the estimate of the amount of time elapsed since an earth material was either fully exposed to sunlight (in the case of sediment) or heated (in the case of lavas). However, if the sample wasn't fully saturated, it can yield too young an age. This methodology measures the photons emitted from an irradiated material when reheated or stimulated with light. The amount of light produced from the sample is proportional to the radiation dose accumulated when compared to a calibrated sample.

Types of luminescence analyses include optically stimulated, infrared, cathodoluminescence, and thermoluminescence. The anlayses are generally performed on silt and sand samples between 100,000 and 350,000 years in age.

The fission-track method of dating operates under the assumption that spontaneous fission of ²³⁸U and other isotopes generate fission tracks in a crystal lattice, and uses the observed track density and measured U concentration within a mineral separate, along with decay constants for spontaneous fission, to determine the age of the sample.

Generally, this analysis is performed on zircon and sphene, due to their excellent track retention properties, although it is also performed on other mineral separates, such as apatite, quartz and plagioclase, or on glass. This analysis is typically performed on relatively young samples, due to the effects of fading or annealling of tracks upon reheating. This method can also be used to determine thermal histories of rocks, because tracks are erased within different minerals at different temperatures.

The Rb-Sr (rubidium-strontium) radiometric method employs the decay of ⁸⁷Rb to stable ⁸⁷Sr to determine the age of a mineral separate or bulk rock sample by using abundances of both Rb and Sr and the ⁸⁷Sr/⁸⁶Sr ratio, assuming an initial ⁸⁷Sr/⁸⁶Sr ratio, and assuming re-heating did not occur after crystallization. Age and uncertainty are assigned by fitting isochrons to the analytical data.

This method is most suitable for determining time of crystallization in acidic and intermediate igneous rocks, although it can also be used to determine timing of metamorphism in metamorphic rocks, and it may be able to determine timing of source rock crystallization or metamorphism in detrital grains within sedimentary rocks.

Conversely, rare earth element samarium (Sm) and its daughter element, neodymium (Nd), are most useful in determining ages of ultramafic and mafic igneous rocks. The decay of ¹⁴⁷Sm to ¹⁴³Nd, using the corrected ratio of ¹⁴³Nd and ¹⁴⁴Nd for reference, can be used to generate the slope of an isochron when measured in a suite of related samples to determine age.

Fossils are traces of microscopic to megascopic remains of plants or animals found within sedimentary rocks, and their identification and association with a known period of time in the stratigraphic record led to the original development of the geologic time scale. Fossils may consist of partly to wholly remineralized portions of the plant or animal, or may merely be an impression or cast of the organism. The specialized branch of paleontology involves using detailed stratigraphic information and the representative fossil assemblages to determine the relative age of a formation. Empirical radiometric methods of determining age are used to further refine age estimates provided by the fossil record.

Geochemistry is the study of the chemicals found in rocks, minerals, and fluids. Elements are all around us and are often found as compounds in the environment. When the elements silicon (Si) and oxygen (O) combine, they create an oxide. Oxides are the building blocks which make up the planets and solar systems, and are known as "major elements". There are also "trace elements" which are found in low concentrations in rocks. By studying these major and trace elements, scientists can track the geologic processes that have created or changed the rocks and minerals that we see today.

There are a variety of ways that scientists analyze a rock or mineral for its chemistry. The two most common methods are X-ray diffraction and mass spectrometry.

X-ray diffraction

With X-ray diffraction, X-rays are directed towards and are absorbed by the sample. The sample then re-emits its own X-rays that are collected by a detector. The energy and wavelength of the collected X-rays are characteristic of the type and concentration of elements in the sample. This is a non-invasive technique that can be used on delicate objects as no sample is lost or changed in the process.

Mass spectrometry

Mass spectrometry involves crushing or dissolving a sample to analyze its chemistry. The sample is superheated into a plasma by lasers or ionization if solid or liquid. The plasma is then directed through a tube bent around a magnet. The magnet attracts each element towards it differently depending on the charge of the element. Where and how often an individual atom strikes a detector informs scientists both what and how much element is in the sample. This is an invasive technique that will destroy the sample.

Mysterious features in the landscape can be found in the southern Puget Lowland, called Mima mounds. Named for the type locality at the Mima Mounds Natural Area Preserve, Mima mounds are circular, chest-high dome-shaped mounds found in randomly spaced clusters, and are composed of dark gravelly soil.

There are many theories as to how the Mima mounds formed. Some postulate they formed from giant burrowing pocket gophers, others think they were created during earthquakes, and another contingent supports the idea that the clay in the soil shrinks and swells with varying water conditions. Weirdly, in the southern Puget Lowland, most of the Mima mound fields lie solely within recessional glacial outwash deposits. Whatever their cause, they're pretty fascinating features.

Map showing Mima mound areas within the southern Puget Lowland.

Retired Survey geologist Josh Logan examining a Mima mound in a excavation. Note the rich soil within the mound directly above recessional sand and gravel.

This lidar image of Tenalquot Prairie doesn't have the measles. It has the Mimas.

A geologic map classifies rocks and deposits into geologic map units, that are displayed as colored or patterned areas on the map based on unique rock type, age, or depositional setting.

In this example, each colored polygon represents a geologic map unit. The unit symbols labelling each polygon signify the units' age, depositional setting, rock type, and formal or informal name. For example, the unit symbol "Qgt_v" tells us that its age is Quaternary (less than about 2 million years old), it is composed of glacial (g) till (t), and is part of the Vashon Stade (v).

Contacts are lines that geologists use to mark the boundaries of geologic units, and line symbology will usually denote the confidence the author had in the location or identification of that contact. Solid lines denote higher confidence whereas dotted lines denote more uncertainty. Sometimes, the contact between two units is actually a fault.

During the Pleistocene, glaciers covered much of western and northern Washington. Geologists can sometimes find evidence that locates the southern limit of glaciation. These locations are depicted as lines on some geologic maps.

Cartographers use points on maps to denote localized features. On a geologic map, these locales can include: geochemistry or geochronology sample locations, important geologic units too small to show at map scale, wells or borings, or paleomagnetic sample locations.

Cartographers use lines on maps to denote localized linear features that add context and meaning. On a geologic map, these lines can represent: eskers, glacial ice limits, terrace or fault scarps, cross section lines, shorelines, streams, supplementary contours, geophysical survey lines, flow lines on lava flows, and landslide movement directions.

Dikes are vertical or near-vertical tabular igneous intrusions that cut through bedrock, forming linear trends across the Earth's surface. Dikes are typically represented on a geologic map by a line.

Sometimes, geologists need to show the aerial extent of something that's not a geologic unit. Examples include areas of mineralization or alteration, fault zones, outcrops, geomorphic features, and mineral resources.

These polygons are typically overlain on top of geologic units so that both can be viewed simultaneously.

Maps are made at varied scales. Map scale determines how much territory is shown by a map. For instance, at a scale of 1:100,000, one inch on the map equals 100,000 inches in the real world—about 1.6 miles.

In Washington State, common geologic map scales are 1:24,000, 1:100,000, and 1:250,000. All of Washington has been mapped at 1:100,000 and 1:250,000 scales, but most of the state remains unmapped at the more detailed 1:24,000 scale. Land owners, planners, decision-makers, and others commonly need information at 1:24,000, or even at a more detailed scale.

Use your mouse to hover over the photo to compare geologic mapping of the Skokomish-Union area in Mason County at various scales. The left map was mapped at 1:100,000 scale and the right map was created at 1:24,000 scale.

Extent of the Puget Lobe of the Cordilleran Ice Sheet during the latest ice advance.

Ice sheets are glaciers that can be thousands of feet thick and are exceptionally talented at transforming landscapes. It was during the Vashon Stade (the latest ice advance and retreat about 15,000&ndash–12,000 years ago) that the Puget Lowland got the extreme make-over that we see today.

As glaciers scour the landscape they flow over, sediment is collected and transported within, beneath, and above the ice. Glaciers are capable of moving the smallest of clay particles to very large boulders. Sediment is also transported in milky, silt-laden meltwater streams in front of the glacier's toe.

The glaciers left behind many features in the landscape that are fun to identify.

Fluting

Moraines

Eskers

Eskers are features that are often found near the terminal moraines. Eskers are formed when sub-glacial rivers transport pebble to cobble sized gravel, and the deposits are exposed once the glacier has retreated. The result is a sinuous ridge of gravel that runs roughly parallel to the direction of ice flow.

Use your mouse to hover over the photo.

Kettles

Glacial Till

Glacial till (diamictite) is unsorted glacial sediment. It is deposited directly at the base of glacial ice. Generally, it is composed of hard-packed clayey silt mixed with numerous irregularly shaped cobbles and boulders that were entrained in the ice as it plucked and abraded the surface.

Glacial Outwash

Glaciers produce a lot of meltwater, which subsequently transports vast amounts of sediment. Outwash is a general glacial term describing silt, sand, and gravel deposited in front of a glacier by the meltwater. The sediment is transported either within glacial streams or directly into lakes in front of the glacier's toe. Stream-transported outwash is typically found in broad outwash plains, which are braided stream complexes.

Outwash deposited in front of an advancing glacier will be subsequently overridden by the ice and will become compacted, whereas outwas deposited during glacial retreat does not get overridden and is generally less compacted.

Kames and Kame Terraces

Kames are the remnants of sediment that were deposited in depressions on the glacier. These sediments collapse once the ice has melted.

Kame terraces are geomorphic landforms left behind when glacial ice buttressed the deposition of sand and gravel along the sides of a glacier and subsequently melted. Kames look like long flat benches and are found along the sides of glaciated valley walls.

Alpine vs. Continental Glaciation

Geologists use the term 'continental' glaciation to refer to the widespread ice sheets that repeatedly advanced and retreated during the last ice age. In Washington, ice sheets up to 1 mile thick covered all of northern Washington, with a longer lobe reaching southward to Tenino in the Puget Lowland.

Alpine glaciation refers to the localized advance and retreat of ice due to cold climates in alpine settings. Numerous alpine glaciers are long-lived features, covering much of Washington's high elevation peaks. These glaciers expand their footprint during continental ice sheet advances.

More Information

Check our page on Washington's glacial geology for more information about how Washington was shaped by glaciers.

Dikes are vertical or near-vertical tabular igneous intrusions that cut through bedrock, forming linear trends across the Earth's surface. Dikes are typically represented on a geologic map by a line.

Oil and gas are naturally occurring resource deposits formed as a result of organic material that is buried and then subjected to heat and pressure. Oil seeps in Washington were first reported in 1881 along the sea cliffs on the west side of the Olympic Peninsula. Natural gas seeps associated with mud cones and mounds were also reported in this same general area. We access these resources by drilling wells. In 1965, the Jackson Prairie natural gas storage project was developed, which remains the Northwest's largest natural gas storage facility.

A total of 862 exploratory wells have been drilled in Washington to date, with 249 reporting shows for oil and gas.

For more information about oil and gas, go to our webpage.

The extraction of materials from a site such as sand and gravel, stone, clay, coal, industrial minerals, metallic substances, peat, or topsoil requires an active reclamation permit. Surface mines are regulated under the Surface Mine Reclamation Act enacted in 1971, which requires a permit for each mine with more than 3 acres of disturbed ground or a high wall that is both higher than 30 feet and steeper than 45 degrees. The Surface Mine Reclamation Act was amended in 1993 to ensure that every mine in the state is thoroughly reclaimed.

The Department of Natural Resources is responsible for seeing that reclamation is completed after surface mining and has exclusive authority to regulate mine reclamation and approve reclamation plans. The basic objective of mine reclamation is to re-establish the vegetative cover, soil stability, and water conditions at the site. Mines may be reclaimed for fish and wildlife, grazing, forestry, wetlands, residential, and commercial and industrial uses. Mine operation, which includes all mine-related activities except reclamation, is specifically regulated by local governments or state and federal agencies, exclusive of the Department of Natural Resources.

For more information about the Surface Mine Reclamation process, or to access fee schedules or forms, go to our webpage.

Much of our geothermal data is obtained from temperature-gradient wells or water wells where logging equipment was used to gather detailed temperature profiles. A temperature gradient is then calculated from this data in degrees C per kilometer.

Water well records where temperature was recorded can also be used to estimate a representative temperature-gradient using an assumed surface temperature in the area of the well. These data typically hold less scientific value compared to geothermal wells logged specifically for temperature data.

For more information about geothermal resources in Washington, visit our webpage.