A caldera is a massive, cauldron-like depression formed when a volcano collapses into itself. This feature is created after a colossal eruption empties the magma chamber beneath the surface. The immense weight of the overlying rock causes the ground to subside, leaving behind a vast, bowl-shaped basin. The McDermitt Caldera in the western United States is one such structure, representing a significant geological event and a feature of modern relevance.
Geographical Placement
The McDermitt Caldera straddles the state line between Nevada and Oregon in a remote, arid section of the northwestern Great Basin. Its boundaries encompass parts of Harney and Malheur counties in Oregon and Humboldt County in Nevada. The caldera is an oval-shaped depression, measuring approximately 28 miles (45 kilometers) from north to south and about 22 miles (35 kilometers) from east to west.
This large structure is situated within the broader Basin and Range Province, characterized by numerous north-south trending mountain ranges and valleys. The caldera is an endorheic basin, meaning water collected within it historically did not flow out to the sea. Key geographical markers, such as the Montana Mountains, which contain the caldera’s highest point, lie near its center, west of the town of McDermitt.
Geological Origin
The McDermitt Caldera formed during the Miocene Epoch, approximately 16.4 million years ago, as the result of a supervolcanic eruption. This event is considered one of the earliest large-scale eruptions associated with the Yellowstone Hotspot track, which stretches northeast across the western United States. The eruption was immense, evacuating an estimated 1,000 cubic kilometers of magma from a chamber below the surface.
The material ejected is known as the McDermitt Tuff, a rock layer strongly zoned in its chemical composition. Once the magma chamber was emptied, the overlying roofrock collapsed along a ring-fault system, creating the deep depression of the caldera. Following the main collapse, magma intrusion beneath the surface caused a post-collapse uplift, and subsequent volcanic activity continued for hundreds of thousands of years.
The newly formed basin collected water, creating a large, closed lake system. This environment became a repository for thick layers of volcanic ash and sediments eroded from the surrounding caldera walls. The combination of the chemically rich volcanic material and the isolated basin set the stage for the caldera’s unique mineral concentration.
Unique Mineral Composition
The McDermitt Caldera’s unique mineral composition is a direct consequence of its geological formation and post-collapse history. The sedimentary layers and volcanic ash that accumulated in the lake basin hold one of the world’s largest known clay-hosted lithium deposits. Lithium-rich magma, which fed the original eruption, served as the source for the metal.
As the hot volcanic ash, or tuff, settled into the caldera lake, it began to chemically break down and weather into clay minerals. This process, combined with the closed-basin hydrology, allowed lithium to be leached from the volcanic material and concentrated within the lake sediments over time. The lack of outflow ensured the increasing saturation of dissolved minerals in the basin.
The initial lithium-bearing clay that formed was primarily smectite. Later, hydrothermal fluids—hot, mineral-rich water heated by the cooling magma body—circulated through the sediments along ancient fault lines. This fluid chemically altered the original smectite clays into a different, more stable clay mineral known as illite.
This transformation process created an exceptionally high concentration of the metal in the illite claystone, such as that found in the Thacker Pass area within the caldera. The illite mineral itself has been analyzed to contain between 1.3% and 2.4% lithium by weight, nearly double the concentration of the more common smectite. The vast reserves of concentrated lithium clay are what distinguish the McDermitt Caldera as a feature of unparalleled geological and economic interest.