Geology of Tacoma, Washington

Introduction

Tacoma, situated in Pierce County, Washington, is a city with a dynamic geological landscape. Its position along the Puget Sound and proximity to the Cascade Range gives it a unique geological profile that has influenced its development and natural environment. This article provides an in-depth look at Tacoma’s geology, including its bedrock, glacial history, and landforms.

Geological Setting

Tectonic Setting

Tacoma lies in the Pacific Northwest, a region characterized by complex tectonic interactions. The area is situated on the boundary between the Pacific Plate and the North American Plate. This tectonic setting is primarily influenced by the subduction of the Juan de Fuca Plate beneath the North American Plate along the Cascadia Subduction Zone.

Regional Geology

The geology of the Puget Sound region, including Tacoma, is shaped by a combination of tectonic activity, volcanic processes, and glacial movements. The region is part of the Cascadia Range, which extends from British Columbia to Northern California. The geological history of this area includes volcanic activity, sedimentation, and glaciation.

Bedrock Geology

Pre-Tertiary Rocks

The bedrock in Tacoma primarily consists of rocks from the Pre-Tertiary period, including metamorphic and intrusive igneous rocks. These rocks form the foundation upon which the more recent geological formations have been laid.

Metamorphic Rocks

Metamorphic rocks in the Tacoma area include schist and gneiss, which originated from pre-existing rocks that underwent metamorphism due to high pressure and temperature conditions. These rocks are often found in the foothills surrounding the city.

Intrusive Igneous Rocks

Intrusive igneous rocks, such as granite and diorite, were formed from magma that cooled and solidified beneath the Earth’s surface. These rocks contribute to the geological complexity of the region and can be found in various locations around Tacoma.

Tertiary Volcanics

The Tertiary period was marked by significant volcanic activity in the Pacific Northwest. During this time, volcanic rocks such as basalt, andesite, and rhyolite were deposited in the region.

Basalt Flows

Basalt flows from volcanic eruptions during the Tertiary period are prominent in the Tacoma area. These flows are characterized by their dark color and fine-grained texture. They form the base for many of Tacoma’s geological formations.

Rhyolite and Andesite

Rhyolite and andesite are volcanic rocks that were formed from more explosive volcanic activity. These rocks are less common but are present in the geological record of the region.

Quaternary Geology

Glacial History

The Quaternary period, particularly during the Pleistocene epoch, was marked by extensive glaciation in the Tacoma region. The most recent ice age left significant geological evidence in the area.

The Vashon Glaciation

The Vashon Glaciation was the last major glacial event to affect the Tacoma area. During this period, the region was covered by the Vashon Glacier, which advanced and retreated multiple times, shaping the landscape through erosion and deposition.

Glacial Retreat

As the Vashon Glacier retreated, it left behind various glacial landforms, including moraines, outwash plains, and glacial erratics. These features are still visible in the landscape today and provide insight into the region’s glacial history.

Glacial Deposits

Glacial deposits in Tacoma include a variety of sediments left by the retreating glaciers. These deposits play a crucial role in the region’s current topography and soil composition.

Till

Glacial till is an unsorted mixture of clay, silt, sand, and gravel deposited directly by the glacier. It is found throughout the Tacoma area and forms the basis for much of the local soil.

Outwash

Outwash consists of sand and gravel deposited by meltwater flowing from the glacier. Outwash plains are common in the region and contribute to the fertile soil found in parts of Tacoma.

Landforms and Topography

Puget Sound Lowlands

Tacoma is located within the Puget Sound Lowlands, a region characterized by its flat to gently rolling terrain. This lowland area was shaped by the glacial activity of the last ice age.

Glacial Erosion

The glaciers that covered the region eroded the landscape, creating broad, flat valleys and depressions that are now occupied by bodies of water such as Puget Sound.

Estuaries and Wetlands

The lowlands also include estuaries and wetlands, which were formed by the interaction of glacial meltwater with the sea. These areas are ecologically significant and support diverse plant and animal life.

Tacoma’s Hills and Valleys

Tacoma’s landscape is further characterized by its hills and valleys, which were formed by both glacial processes and tectonic activity.

Hill Formation

The hills in Tacoma, such as those in the surrounding foothills, were formed by a combination of volcanic activity and erosion. These hills offer scenic views and are a prominent feature of the local topography.

Valley Formation

The valleys in Tacoma were shaped by glacial meltwater and erosion. These valleys often contain rivers and streams that contribute to the region’s hydrology.

Geological Hazards

Earthquakes

Tacoma, like much of the Pacific Northwest, is susceptible to earthquakes due to its location near the Cascadia Subduction Zone. The region has experienced significant seismic activity throughout its history.

Major Earthquakes

Historically, the Tacoma area has been affected by several major earthquakes, including the 1965 Olympia earthquake. These events highlight the importance of understanding and preparing for seismic hazards.

Earthquake Preparedness

Local agencies and residents are encouraged to prepare for earthquakes by following guidelines for building safety, emergency response, and community preparedness.

Landslides

Landslides are another geological hazard in the Tacoma area, particularly in areas with steep slopes and loose soil.

Causes of Landslides

Landslides can be triggered by heavy rainfall, earthquakes, or human activities. Areas with glacial deposits are particularly prone to landslides due to the loose nature of the soil.

Mitigation Measures

Efforts to mitigate landslide risks include slope stabilization, proper drainage, and land use planning. These measures help reduce the impact of landslides on communities and infrastructure.

Human Impact and Utilization

Mining and Quarrying

Tacoma’s geology has led to the extraction of various natural resources, including sand, gravel, and rock. These resources have been used in construction and infrastructure projects throughout the region.

Historical Mining

Historically, mining operations in the Tacoma area focused on extracting valuable minerals and aggregates. These operations have played a role in the city’s development.

Modern Quarrying

Today, quarrying operations continue to extract construction materials. These activities are regulated to minimize environmental impact and ensure sustainable resource management.

Urban Development

Urban development in Tacoma has been influenced by its geological features. The city’s growth has required careful consideration of the local geology to address issues such as soil stability and flood risk.

Infrastructure Development

Building infrastructure in Tacoma requires an understanding of the local geology to ensure stability and safety. This includes considerations for foundation design, drainage, and erosion control.

Environmental Considerations

Urban development also involves addressing environmental impacts related to geology. Efforts are made to balance growth with environmental protection, particularly in areas with sensitive geological features.

Conclusion

Tacoma’s geology is a fascinating subject that encompasses a wide range of geological processes and features. From its tectonic setting and bedrock formations to its glacial history and landforms, the geology of Tacoma has significantly shaped the city’s landscape and development. Understanding this geological context provides valuable insights into the region’s natural history and helps guide future development and environmental management efforts.