Glad You Asked: Cinder Cones of Southwest Utah: What Exactly Are They and How Did They Get There?
by Jackson Smith
Utah boasts stunning natural beauty, characterized by an array of spectacular geological features. One of these striking features is the cinder cone volcanoes that stand out as towering landmarks. Utah claims more than 150 cinder cone volcanoes, found throughout the southwestern quadrant of the state.
Hancock Peak cinder cone on the Markagunt Plateau, east of Cedar Breaks National Monument, Garfield County. Photo courtesy of Robert Biek, 2012.
Cinder cone volcanoes, also known as scoria or pyroclastic cones, are relatively small and symmetrical volcanoes, and as their name suggests, conical in shape. They form through explosive eruptions caused by the release of gas-rich magma from a volcanic vent. Expanding gas drives magma vertically to the surface where it violently erupts into the air and solidifies as fragmented volcanic material called cinders or scoria, which then fall to accumulate around the vent. The cinders gradually build up to form a cone-shaped structure. If there is a persistent wind blowing during the eruption, the cones can take on a more asymmetrical shape as they form. One of the most well-known cinder cones in the world is the Parícutin cinder cone that erupted suddenly out of a corn field in Mexico in 1943 and within a decade, grew to 1,391 feet high.
Many Utahns are familiar with at least a few of the cinder cones found in southwest Utah. Their presence and prevalence stems from the dynamic and complex forces of plate tectonics and geologic events set in motion millions of years ago, even though some of the cinder cones are only thousands of years old.
The Earth’s crust is broken up into sections called tectonic plates, which steadily move slowly around the planet over millions of years, in many cases smashing into and overriding each other. Starting more than 150 million years ago, an ancient oceanic plate called the Farallon plate was forced under the western side of the continental North American plate in a process called subduction. Most of the time, subducted plates take a steep dive into the Earth’s interior. Instead, the more-buoyant-than-normal Farallon plate, with a little extra pushback from the North American plate, leveled out and advanced far eastward underneath the crust in Utah and much of western North America.
A) The Farallon oceanic plate subducts under the North American continental plate. B) The subducted part of the Farallon plate levels out and pushes its way to the east, causing the uplift of mountain ranges like the Uintas. C) Subduction slows and the distal end of the Farallon plate detaches and sinks, allowing the hot inflow of the asthenosphere to push upwards, melting the lithospheric mantle, and creating magma that causes extraordinary volcanic eruptions. D) After the Farallon plate is fully subducted, a transform boundary forms (red line) between the North American plate and the laterally moving Pacific plate. As the two plates grind past each other, the hot and soft North American plate stretches and extends toward the west and the crust thins allowing basaltic and less viscous magma from the asthenosphere to erupt and create cinder cone volcanoes.
Over time as it progressed farther east, the Farallon plate could not remain flat and its trajectory steepened downward again, and even rolled back toward itself. As subduction slowed, a section of the Farallon plate broke off and sank deeper down into the planet’s interior. The sinking of the detached section and the initial rollback motion facilitated the migration and flow of hot material upward from a deeper zone called the asthenosphere, which in turn helped to melt the lowest layer, called the lithospheric mantle, of the overriding North American plate. This magma rose through the crust of the North American plate towards the Earth’s surface and caused extremely explosive and enormous volcanic activity throughout Utah—but did not form small cinder cone volcanoes at first.
The hot material of the asthenosphere that did not erupt continued to push upwards against the crust of the North American plate for millions of years (even to this day) making the crust hotter and softer.
By about 17 million years ago the bulk of the Farallon plate was subducted and the North American plate encountered, and is still in contact with, the Pacific plate. Unlike the subducted Farallon plate, the Pacific plate laterally grinds past the North American plate at a transform boundary commonly known as the San Andreas fault. This oblique tectonic motion coupled with the previous long period of heating and softening from the hot asthenosphere is stretching, elevating, and thinning the continental crust across the Basin and Range Province from western Utah to eastern California. In turn, this stretching and thinning facilitated a transition from enormous explosive volcanoes, such as the Tushar Mountains (which are part of the Marysvale volcanic field) to the many small cinder cones found today across southwest Utah. The magma that fed these cinder cones was mainly composed of the upwelling asthenosphere and is therefore less viscous (basaltic composition, instead of andesitic composition)—generating far smaller and less violent eruptions.
A) The Farallon oceanic plate subducts under the North American continental plate.
B) The subducted part of the Farallon plate levels out and pushes its way to the east, causing the uplift of mountain ranges like the Uintas.
Over time as it progressed farther east, the Farallon plate could not remain flat and its trajectory steepened downward again, and even rolled back toward itself. As subduction slowed, a section of the Farallon plate broke off and sank deeper down into the planet’s interior. The sinking of the detached section and the initial rollback motion facilitated the migration and flow of hot material upward from a deeper zone called the asthenosphere, which in turn helped to melt the lowest layer, called the lithospheric mantle, of the overriding North American plate. This magma rose through the crust of the North American plate towards the Earth’s surface and caused extremely explosive and enormous volcanic activity throughout Utah—but did not form small cinder cone volcanoes at first.
The hot material of the asthenosphere that did not erupt continued to push upwards against the crust of the North American plate for millions of years (even to this day) making the crust hotter and softer.
C) Subduction slows and the distal end of the Farallon plate detaches and sinks, allowing the hot inflow of the asthenosphere to push upwards, melting the lithospheric mantle, and creating magma that causes extraordinary volcanic eruptions.
D) After the Farallon plate is fully subducted, a transform boundary forms (red line) between the North American plate and the laterally moving Pacific plate. As the two plates grind past each other, the hot and soft North American plate stretches and extends toward the west and the crust thins allowing basaltic and less viscous magma from the asthenosphere to erupt and create cinder cone volcanoes.
By about 17 million years ago the bulk of the Farallon plate was subducted and the North American plate encountered, and is still in contact with, the Pacific plate. Unlike the subducted Farallon plate, the Pacific plate laterally grinds past the North American plate at a transform boundary commonly known as the San Andreas fault. This oblique tectonic motion coupled with the previous long period of heating and softening from the hot asthenosphere is stretching, elevating, and thinning the continental crust across the Basin and Range Province from western Utah to eastern California. In turn, this stretching and thinning facilitated a transition from enormous explosive volcanoes, such as the Tushar Mountains (which are part of the Marysvale volcanic field) to the many small cinder cones found today across southwest Utah. The magma that fed these cinder cones was mainly composed of the upwelling asthenosphere and is therefore less viscous (basaltic composition, instead of andesitic composition)—generating far smaller and less violent eruptions.
Location map of cinder cone volcanoes in southwest Utah.
Some cinder cones are relatively large, like the “E” Hill cinder cone (Utah’s tallest) that rises around 1,000 feet above the city of Enterprise, but they are still the smallest volcano type. Accordingly, there are many basaltic lava flows in Utah where the source vent or cone has entirely eroded away, such as Utah’s oldest basalt, the 19-million-year-old Mosida Basalt, west of Utah Lake. The oldest basalt flow tied to an existing cinder cone comes from the dual Dickinson Hill cinder cones southeast of Panguitch, dated to be about 5.3 million years old. The Markagunt Plateau boasts over thirty cinder cones, most less than 1 million years old. The youngest basalt in Utah comes from the Ice Springs cinder cone in the Black Rock Desert of Millard County, which likely erupted about 10,000 years ago (previously reported ages of 600 to 720 years have recently come into question).
Utah’s cinder cones stand as a testament to the geological processes that have unfolded for more than 150 million years, capping an ongoing narrative of incredible volcanic and tectonic forces.
