(By W. Kelly, 2010)
National Parks throughout the United States feature landscapes that are topographically rich and can tell a story about our home – the living Earth. The western United States in particular is the location of multiple National Parks (Figure 1) that are experiencing major changes right now, even as you read this sentence! Two dominant forces control the way the landscape looks in the western half of the United States: subduction and continental rifting. If you ever travel to the state of Nevada, you’ll be standing directly ontop of a continental rift zone called the Basin and Range Province.
Figure 1: Tectonic plates make up the outer crust of the Earth, and move across its surface through time. Plate boundaries are areas where plates intercept and are the locations of spectacular geological processes. The small red box indicates our location of interest. (United States Geological Survey)
The Earth is divided into many different tectonic plates, like puzzle pieces, that fit together and shift across the surface of the Earth through time (Figure 1). These tectonic plates float ontop of the underlying mantle, which is composed of more dense material, primarily molten iron and magnesium rich silicate rocks.
Tectonic plates meet at plate boundaries, where exciting geological processes occur. Three plate boundary types include: convergent, where tectonic plates collide; divergent, where tectonic plates pull apart; and transform, where tectonic plates slide past each other. The western half of the United States is dominated by continental rifting, where a new divergent plate boundary may form in the future. This area is referred to as the basin and range province (Figure 2). Continental rifting occurs in a zone when a continental plate, like North America, is being ripped apart into two plates, resulting in a rough terrain of down dropped blocks of earth.
Figure 2: The western half of the United States is dominated by extensional forces that form the Basin and Range Province, where continental rifting pulls the Earth’s surface apart, allowing magma to pour out onto Earth’s surface. (United States Geological Survey)
Further to the west, the Cascade Mountains form a chain of volcanic summits that reflect another primary process that shapes the western United States. Some convergent boundaries are referred to as subduction zones. These are areas where a thin and dense oceanic plate is forced down into Earth’s interior by an overlying thick and buoyant continental plate (Figure 3). Just that is occurring off the western coast of North America. An oceanic plate, called the Juan De Fuca plate, is diving beneath our feet, and leaving it’s footprint on Earth’s surface as it’s sweat melts continental rocks and forms the volcanic Cascade Mountains.
Figure 3: Subduction of the Juan de Fuca plate underneath North America, creating a volcanic arc, or chain of mountains that become volcanically active intermittantly as the process of subduction continues.
Many National parks and National Monuments in the western United States are located within active zones of continental rifting or subduction and because of this, feature landscapes largely controlled by related volcanic processes. Some of these park service sites include:• Lava Beds National Monument, CA • Capulin Volcano National Monument, NM • City of Rocks National Reserve, ID • Craters of the Moon National Monument and Preserve, ID • Newberry National Volcanic Monument, OR • Sunset Crater Volcano National Monument, AZ
Figure 4: A close-up map view of the western US, where the continental rifting and subduction leave their mark through National Parks and Monuments. Lava Beds National Monument is located in North-eastern California next to the Medicine Lake volcano.
Lava Beds National Monument is located in Northern California, and is nestled between the Basin and Range province to the east and the Cascade Mountains (a series of subduction-related volcanic mountains) to the west (Figure 4). The Monument sits on the flank of a large shield volcano and is unique because it has been impacted by geologically recent volcanic activity related to both the Cascadian subduction and basin and range rifting. A series of volcanic outlets surround Lava Beds National Monument, supplying the area with a variety of volcanic products such as lava flows, cinder cones, lava tubes, and lava domes. Lava flows in this area are Pleistocene to Holocene in age (figure 5), the most recent being 1100 years old, and are typically basaltic-andesite in composition.
Figure 5: The Geologic timescale, dividing the life of the Earth into a series of Epochs, Periods, Eras, and Eons. The lava flows that dominate Lava Beds National Monument are relatively young. USGS
Although not directly inside the park, Medicine Lake volcano has provided Lava Beds National Monument with primarily basaltic composition lava flows that have left their trace as lava tubes and lava flows. The Medicine Lake volcano formed about a million years ago and is considered one of the largest shield volcanoes in the entire Cascade Mountain range. A shield volcano has a low and broad shape that is controlled by the abundance of low viscosity lavas that have erupted from it's summit and flanks. These low viscosity lava are thin and runny, like maple syrup, and can therefore travel long distances from their source. A few lava flows from Medicine Lake have been more viscous, like the famous “Glass Mountain” obsidian flow that was perhaps similar to the consistency of molasses, having a high silica content and oozing slowly down the volcano caldera 885 years ago. Some of these flows produced ash fall that blanketed Lava Beds National Monument with pumice hundreds of years ago.
There have been many other sources of volcanism for Lava Beds National Monument that are related to the huge Medicine Lake volcano. For instance, lava flows also exist just outside of the Medicine Lake caldera and extend into the Monument boundaries. Over 200 vents exist within this area that trend roughly north-south, illustrating the direction of tensional forces that are actively pulling North America apart. In general, these vents produce lava spatter cones and flows including high viscosity lava domes.
One such source is Mammoth Crater. Mammoth Crater is in the far south of the National Monument and lies NNE of Medicine Lake volcano. Most of the lava flows observed within the monument are remnants from Mammoth Crater eruptions. It is also primarily these lava flows that have provided the monument with its 400 or so lava tube caves (figure 6). Eruptions that occurred 30,000 to 100,000 years ago left broad swathes of lava across the Monument. The lava tubes formed as the outer layer of lava cooled and solidified into rock, leaving the lava underneath to flow freely while maintaining relatively high temperatures (see the Lava Tubes Volcano World page!). These lava tubes acted as conduits through which lava was transported eastwards through the Monument until the eruption from Mammoth Crater ceased. Today, a network of interweaving lava tubes is left behind in Lava Beds National Monument that can be explored like caverns along the Monument’s “cave loop drive”.
Figure 6: An explorer traverses a lava tube named “Catacombs Cave” in Lava Beds National Monument. (National Park Service)
Four other prominent lava flows are visible in Lava Beds National Monument. The Schonchin flow covers the entire center of Lava Beds Monument. The Schonchin Butte (Figure 7) erupted this lava flow about 30,000 years ago out of a cinder cone that also spewed spatter and ash. Devils Homestead flow and the Ross flow are located in the western portion of the Monument. Devils Homestead flow (Figure 8) originated from Fleener Chimneys, another spatter cone that grew by the accumulation of projectile blobs of lava. Devils Homestead is considered an aa flow, basaltic in composition and with a blocky, uneven surface that collected and cooled between 2,000 and 8,000 years ago. The Ross flow is similar, but came from a different spatter cone vent called Black Crater. Lastly, the Callahan flow is the youngest in Lava Beds National Monument. It covered the southwestern corner of the Monument when this basaltic to andesitic lava flowed out of Cinder Butte only 1100 years ago. Lava Beds National Monument Geologic Resource Evaluation Scoping Summary Although none of these lava flows formed the spectacular lava tube caves, they have created interesting and unusual topography that make Lava Beds National Monument unique among other park sites.
Figure 7: The Devils Homestead flow located in western portions of the Lava Beds National Monument preents achallenging surface upon which to walk.
Figure 8: A large cinder cone, Schonchin Butte, stands out in the middle of Lava Beds National Monument.
Lava Beds National Monument Geologic Resource Evaluation Scoping Summary
Figure 1: NPS Park Sites.
Figure 4: Shaded Relief Map of the western United States. Take note of the long, parallel mountain ranges separated by down-dropped valleys. (Modified from Lillie 2005.)
Landscapes in the western United States are characterized by long, parallel mountain ranges separated by down-dropped valleys (Fig 4).
Eventually the asthenosphere upwells so close to the surface that magma that erupts onto the surface. Continental rifting processes are showcased in the steep fault escarpments and active volcanoes that characterize the western United States.
a) Plate divergence sometimes pulls a continent apart.
b) The upper part of the plate is cold and brittle. It breaks along normal faults (the blocks fall down), causing earthquakes and blocks of mountains ranges separated by rift valleys.
c) The hot, ductile upper mantle rises like a hot-air balloon, elevating the topography and generating magma that leads to volcanic activity along the fault boundaries. (From Lillie, 2005.)
Some of the National Parks and Monuments formed by continental rifting are:
• Capulin Volcano National Monument, NM
• City of Rocks National Reserve, ID
• Craters of the Moon National Monument and Preserve, ID
• Newberry National Volcanic Monument, OR
• Sunset Crater Volcano National Monument, AZ
A hotspot is thermally expanded buoyant mantle (bigger hot mantle that floats), which lifts an overlying plate. As hotspot material rises, the pressure drops so the hotspot begins to melt producing magma.
Hawaiian shield volcano Haleakala (right) is one of the shield volcanoes in Hawaii. (others include Mauna Loa (below) and Mauna Kea. (Photos by Robert J. Lillie.)
As heat input wanes, so does the volcanism. In a continental setting, dark basaltic magma is also produced in the early stages of hotspot volcanism, however as heat input wanes volcanism continues. As heat input wanes, the silica-poor basalt still rises toward the surface; as the magma rises, it melts its way trough thick, silica-rich continental crust forming shallow silica-rich (rhyolitic) magma chambers. The rhyolitic magma is thick and sticky, much like the magma produced at a subduction zone. This succession of erupted material results in fields of dark fluid basalt lava flows and cinder cones that are covered by rhyolite volcanoes.
Features at Yellowstone National Park record this sequence of (Fig. 7).
Fig 7: Hotspot magmatism at Yellowstone National Park. The lower chamber, formed at the base of the continental crust contains basalt magma that melts the mantle as the hotspot rises. The upper chamber below the surface contains rhyolite magma formed as the basalt magma melts its way through the crust and is enriched in silica. (From Lillie, 2005)
• Yellowstone National Park
• Craters of the Moon National Park and Preserve
• Hawaii Volcanoes National Park
• Haleakala National Park
Some of the most spectacular volcanoes on Earth are associated with subduction zones!
Right: The upper picture was taken at Crater Lake in 1941.
Left: Mt. Ranier in 1914.
Mt. Mazama, the volcano that erupted to form Crater used to look a lot like Mt. Ranier, however when it erupted, the top collapsed in on itself and filled with water over time to produce the lake we know today. The small cone at Crater Lake is a cinder cone called Wizard Island. (National Park Service)
A subduction zone forms when continental crust and oceanic crust collide. The continental crust is thicker and more buoyant than the oceanic crust so the oceanic crust subducts beneath the continental crust. As the plate sinks deeper, it can reach depths of 50 to 100 miles (80-160 kilometers) were it is so hot that the crust releases fluids trapped inside.
The fluid melts some of the silica-rich minerals in the overlying material producing dark, silica-poor basaltic magma. The basaltic melt migrates upwards and becomes more silica-rich it melts its way toward the surface. Sticky, silica-rich magma erupts at the surface forming steep-sided volcanoes.
Where plates converge, the thin, dense oceanic crust sinks beneath the thick,
buoyant continental crust. Volcanoes form where the subducting oceanic plate gets hot
enough to “sweat” fluids and initiate melting. (Modified from Lillie, 2005.)
Subduction zones produce volcanic arcs, curving chains of steep-sided volcanoes, for example the Aleutian Islands in Alaska. Volcanoes associated with subduction zones generally have steep sides and erupt explosively. Why are the subduction zone volcanoes so steep? The lavas erupted there are rich in silica. Silica acts as a thickening agent (like flour in gravy) so silica-rich magmas are thick and pasty. When the magma erupts, the lava is so thick that it can’t flow very far. Instead, it sticks to the sides of the volcano forming a tall, steep-sided cone. Sometimes these volcanoes explode (for example, Crater Lake, Mt. St. Helens, Lassen, and Mt. Ranier). Thick silica rich magma does not release the gasses very easily so they build up inside the magma chamber. As the gasses collect in the magma chamber, the pressure rises. It is possible for the pressure to rise so much that the chamber cannot contain the magma and it erupts explosively.
Some US Parks that have active volcanoes formed as the result of subduction related processes are:
Other national parks lie along ancient subduction zones. They used to contain volcanoes long ago, but over time the tall volcanoes were eroded away. Parks that show remnants of ancient volcanism are (these will be links to individual park pages):