Geology Cafe

Tectonics and Volcanism: Forces That Build Up the Land

Tectonics is the study of the major structural or deformational features of the Earth's crust, or of a region, and their relations, origin, and historical structural evolution. Tectonic forces are associated with the uplift of mountain ranges and subsidence of basins, metamorphism, folding, faulting, and motion of continents due to plate tectonics. Tectonic forces are responsible for the gradual upward and downward movements of the crust, and the sudden earthquake-associated displacements along faults. Tectonic forces are what raise the land, defying gravity—the universal force that holds the Earth together. Tectonic forces compress, stretch, and rotate rocks in the earth's crust.

The movement of molten rock also raises the landscape, either by intruding and displacing existing rock, or through volcanic eruptions and accumulation of volcanic materials on the land's surface (lava flows, volcanoes, ash and cinder deposits, etc.). Volcanism includes geological processes that result in the expulsion of lava, pyroclastics, and gases at the Earth's surface. Heat flow in the Earth's mantle and crust are the driving force of plate tectonic motion and is responsible for tectonic deformation of the crust (folding and faulting) and igneous activity (intrusion and volcanism).

As tectonism and igneous activity raise the landscape, surficial processes of weathering and erosion work together to wear down the landscape. Typically in any region multiple geologic processes going on simultaneously, inconsistently, over time. The appearance of the landscape is a cumulative result of these processes.
California faults and geology
Click on thumbnail images for a larger view.
Faulting is responsible for many landscape features, including both uplifts and down-dropped basins. This pull-apart basin with ephemeral sag pond is along the San Andreas Fault in the southern Carrizo Plain National Monument, eastern San Luiz Obispo County.
Death Valley is a large pull-apart basin between the Black Mountains on the east, and the Panamint Range on the west. A great fault system runs along the eastern side of the basin at the foot of the Black Mountains, and is responsible for the development of the great amount of relief in the valley. Map of major faults in California (with bedrock geology after The Geologic Map of California [Jennings, 1977]). California has extensive fault systems, both ancient and active. The San Andreas Fault is perhaps California's best known fault, but many other major earthquake-producing faults occur throughout the coastal mountainous regions, along the Sierra Nevada Mountains, and throughout the Mojave and Great Basin regions. The mountain ranges and intervening basins formed as a result of tectonic forces in the earth's crust. and their associated fault systems
San Jacinto Peak rises along the west side of Coachella Valley near Palm Springs. Coachella Valley is a rift valley associated with the San Andreas Fault System in southern California. This view is looking west from Keyes View overlook in the Little San Bernardino Mountains in Joshua Tree National Park. The combination of fault-bounded uplifted mountain blocks and sediment filled basins produces a classic Basin and Range landscape in the eastern Mojave Desert region. This view is looking west from the top of Turquoise Mountain near Baker. This view encompasses portions of Silver and Soda dry lakes; the Soda Mountains cross above the center of the image; the distant peak is Cave Mountain.
Tectonic stresses cause fractures in brittle crustal rocks. If there is no apparent offset along a fracture it is called a joint; whereas fractures that display offset are called faults. Bedrock of all kinds can display jointing and faulting. Fractures in the earth become migration pathways for groundwater. These joints in weathered granite are along the Cima Road in the Ivanpah Mountains in the Mojave National Preserve. Tectonic forces can also fold rocks in addition to fracturing them. These sedimentary layers of marine chert and shale display complex folding. This outcrop is along the Summit Road in Mount Diablo State Park, Contra Costa County.
Tectonic and erosional processes proceed at a relatively faster pace in California relative to most areas within the mainland United States. This image shows relatively young stream floodplain deposits that have been folded upward, and then beveled off by stream erosion. Today, small landslides occurring in the poorly consolidated sediments in the stream cut bank. Winter floods wash away the loose material. This cutbank on Tres Pinos Creek along Panoche Road near the town in Paicines, San Benito County, California.
Igneous Processes
Igneous processes help to raise or build up the landscape.

Plutonism involves molten rock (magma) moving below the surface. Magma migrating toward the surface or filling a subterranean chamber can displace the surface upward. Uplift and erosion have exposed ancient plutonic throughout extensive regions of the California landscape, perhaps most notably in the Sierra Nevada and in Peninsular Ranges in the southern part of the state (see Fig. 1).

Volcanism
occurs when molten rock and gases erupt at the surface, producing layers of volcanic ash and debris, and lava flows that can spread across the surface or build up into volcanoes.

As erosion strips away materials from the landscape, more durable rocks are typically last to break down. Often the harder igneous rocks found in stocks, sills, dikes, laccoliths, and batholiths are preserved as elevated landscape features. In contrast, the softer material (ash, cinders, and lava flows) are more susceptible to weathering and erosion over time.
Volcanic Features
Cinder Cones National Historic Landmark in the Mojave National Preserve. Cinder Cone is part of a young volcanic area that began forming about 7.6 million years ago with the last eruptions only about 10,000 years ago.
Volcanic/igneous areas of California
Volcanoes and Volcanic Areas of California


Volcanic features dominate the landscape throughout a large portion of northeastern California. This view in Lava Beds National Monument shows a dark basalt lava flow in the foreground and of several cinder cones in the distance. Lava Beds National Monument is located on the north flank of the Medicine Lake Volcano, a great shield volcano and caldera of Pleistocene and Holocene age on the northern Modoc Plateau. The ridge in the distance in the lower picture is a great fault scarp on the volcanic field.
Devils Postpile Devils Postpile National Monument is famous for its cliffs of basalt columns and blocky talus slope formed from the collapsing columns. The rocks formed from an ancient lava flow that descended along an ancient stream valley. The unusual polygonal joints formed as the lava cooled and shrank.
Pinnacles National Monument Pinnacles National Monument encompasses rocks that formed from volcanic eruptions during the formation of the San Andreas Fault. The Pinnacles volcano spit in half on the fault, and the two sides are now separated by nearly 200 miles by movement along the fault over the past 23 million years.
The Granite Mountains in the Mojave National Reserve are also remnants of great igneous plutons that formed long ago. The softer rock that surrounded the plutons long ago has worn away by erosion. The massive cliffs of Yosemite Valley are made up of massive igneous plutons that formed and cooled long ago. The volcanoes that probably once existed above them have long since eroded away from the crest of the Sierra Nevada region.
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8/28/2012