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).
These landscapes are a result of continental rifting, or places where the continental crust is extending and thinning. As the crust thins, the hot, buoyant upper mantle (the asthenosphere) rises.
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):