Welded Huckleberry Ridge Tuff at Golden Gate above Mammoth Hot Springs Yellowstone's world-famous natural history is marked by such colossal volcanic events that their reflections in today's landscape are difficult to grasp and impossible to take in at just a glance, even for those familiar with the signs of past volcanism. Grand Canyon of the Yellowstone slices through thick accumulation of rhyolite The stunning features of Yellowstone National Park result from great explosive eruptions and profound collapse of the ground, enormously thick lava flows, uplift and extensive faulting, and the erosive power of flowing water and ice. For more than a century, geologists have discovered and analyzed evidence of the dramatic events that have shaped the land here. When combined with growing knowledge about how volcanoes work and the never-ending motion of Earth's surface, the evidence tells a remarkable story of the Yellowstone landscape. Top of six-sided column of basalt at Sheepeater Cliff Columns of basalt lava flow at Sheepeater Cliff, outside youngest caldera Toe of basalt lava flow, Kilauea Volcano, Hawai`i The volcanism most directly identified with the Yellowstone region has, during about the past 2 million years, built an immense volcanic plateau that straddles a high mountain divide—the Yellowstone Plateau volcanic field. This volcanic region has evolved through 3 cycles of voluminous outpourings of rhyolite lava and volcanic ash, each of them climaxing with one of Earth's greatest pyroclastic-flow eruptions and the resulting collapse of a central area to form a large caldera. Other eruptions have poured out basalt lava flows around the margins of the volcanic field. Yellowstone's volcanism is only the most recent in a 17 million-year history of volcanic activity that has occurred progressively from southwestern Idaho to Yellowstone National Park. At least six other large volcanic centers along this path generated caldera-forming eruptions; the calderas are no longer visible because they are buried beneath younger basaltic lava flows and sediments that blanket the Snake River Plain. Click on image for larger size and explanation; image is from Smith and Siegel (2000).

Three Volcanic Cycles of Yellowstone

Overview

Three extraordinarily large explosive eruptions in the past 2.1 million years each created a giant caldera within or west of Yellowstone National Park with the spread of enormous volumes of hot, fragmented volcanic rocks as pyroclastic flows over vast areas within times as short as a few days or weeks. The accumulated hot ash, pumice, and other rock fragments welded together from their heat and the weight of overlying material to form extensive sheets of hard lava-like rock. In some sections, these welded ash-flow tuffs are more than 400 m thick! These ash-flow sheets—from oldest to youngest, the Huckleberry Ridge, Mesa Falls, and Lava Creek Tuffs—account for more than half the material erupted from Yellowstone. The enormous outpouring of magma, 280 to 2,450 km³ during each explosive event, led to the collapse of magma-chamber roofs, causing the ground above to subside by many hundreds of meters to form the calderas.

Before and after these caldera-forming events, eruptions in the Yellowstone area produced rhyolitic and basaltic rocks—large rhyolite lava flows and some smaller pyroclastic flows in and near where the calderas collapsed and basalt lava flows around the margins of the calderas.

Welded tuffs form capping layer on top of Mount Everts Huckleberry Ridge and Lava Creek ash-flow tuffs At the top of Mount Everts east of Mammoth Hot Springs, a brownish-colored cliff held an important clue about the number of caldera-forming eruptions at Yellowstone. Click on image for the clue.

A general sequence of events was repeated in the evolution of each of Yellowstone's three volcanic cycles:

1. A broad area, larger than that which will become the caldera is slowly uplifted. This uplift reflects the development and rise of large volumes of rhyolite to form a magma chamber at shallow depths in the Earth's crust. Stretching of the crust above the inflating magma chamber leads to concentric and radial fracturing and faulting at the surface, typically accompanied by the extrusion of lava flows from these fractures.
   


2. At a critical stage in the evolution of the magma chamber, enormous volumes of the overpressurized rhyolite magma erupt explosively through the ring-fracture zone created above the magma chamber during inflation and uplift, producing extensive ash-flow sheets. As the eruptions partly empty the chamber of its magma, the roof of the magma chamber collapses along the same ring fractures to produce a large caldera.
   


3. Postcollapse volcanism includes the extrusion of rhyolite lavas and smaller explosive eruptions of pyroclastic flows within or adjacent to the the caldera. In the present-day Yellowstone caldera, lakes formed where streams draining into or along the margin of the caldera were dammed by these thick intracaldera rhyolite flows, including Shoshone, Lewis, Heart, and Yellowstone Lakes. Shortly following collapse, the caldera floor may be uplifted by hundreds of meters in a process known as resurgent doming; this uplift reflects renewed pressure as magma rises again into the magma chamber. Hydrothermal activity (such as hot springs and geysers) occurs during all three stages but, in the third stage, it becomes the dominant or only visible sign at the surface of magmatic activity below.

Thick rhyolite lava flows of the Madison Plateau	Scientists infer that rhyolite lava flows (left photo) as well as the caldera-forming ash-flow tuffs were fed from shallow magma chambers filled by the melting of rocks of the lower continental crust below Yellowstone. The heat needed to facilitate the melting process was supplied by the repeated injections of basalt magma from the mantle into the shallow crust
Click on images for a larger-sized image and description.
Undine Falls spills over basalt lava flow	Basalt lava flows (left photo), though subordinate in volume to rhyolites, have erupted throughout the 2.3 million-year volcanic history of the Yellowstone area. The magma feeding these eruptions originated in the upper part of Earth's mantle and resided only briefly in the crust before erupting at the surface.

Comparison of the three ash-flow tuffs of the Yellowstone Group and resulting calderas

Caldera-forming ash-flow tuff	Age (millions of years)	Volume erupted (km3)	Area covered (km2)	Caldera dimensions (km)	Caldera name
Lava Creek Tuff	- 0.640	1,000	7,500	85 x 45	Yellowstone caldera
Mesa Falls Tuff	-1.3	280	2,700	16 km in diameter	Henry's Fork caldera
Huckleberry Ridge Tuff	-2.1	2,450	15,500	75-95 x 40-601	Big Bend Ridge, Snake River, and Red Mountains caldera segments

¹Inferred first-cycle caldera boundary is irregular; caldera consists of three overlapping collapse areas.

Rim of the Yellowstone caldera (the youngest caldera)

Photograph courtesy of Bob Smith, University of Utah	Aerial view of the striking NW rim of the Yellowstone caldera and intracaldera rhyolite lava flows at Madison Junction in Yellowstone National Park. View is looking north. The steep-facing caldera wall, 500 m tall, formed when the area in the foreground collapsed during eruption of the Lava Creek Tuff 640,000 years ago. The thick West Yellowstone rhyolite lava flow erupted about 110,000 years ago, and the Nez Perce Creek flow erupted 160,000 years ago.
Faults associated with the NE margin of Yellowstone caldera Outermost fault associated with the NE margin of the Yellowstone caldera	These views of faults outside the caldera rim illustrate that faults related to regional tectonic stresses can be associated with caldera collapse along a zone of concentric ring faults. Click on images for a description and larger-sized images. Photographs courtesy of Bob Smith, University of Utah

Overall Stratigraphy: major volcanic units of the Yellowstone Plateau volcanic field

The recognition of the three volcanic cycles provides a natural basis for dividing the volcanic rock layers in the Yellowstone area. The major ash-flow tuffs that erupted at the climax of each cycle are the primary rock units of Yellowstone in areas beyond the caldera complex. The table below represents the work of geologists who identified, described, and mapped the different volcanic rock deposits in the Yellowstone area and determined the order in which they erupted during each of the three cycles of activity. The units are shown in stratigraphic sequence (the youngest at the top of the table, oldest at the bottom). Table is from Christiansen (2001).

Stratigraphic units of the Yellowstone Plateau volcanic field

Volcanic Cycle	Precaldera Rhyolite	Caldera-forming ash-flow tuff	Postcaldera rhyolite	Contemporaneous plateau-marginal basalts1
Third			Plateau Rhyolite2	Basalts of Snake River Group Osprey Basalt Madison River Basalt Basalt of Geode Creek Swan Lake Flat Basalt Basalt of Mariposa Lake
		Lava Creek Tuff (0.64 Ma)
	Mount Jackson Rhyolite Lewis Canyon Rhyolite			Undine Falls Basalt Basalt of Warm River Basalt of Shotgun Valley
Second			Island Park Rhyolite	Basalt of the Narrows
		Mesa Falls Tuff (1.3 Ma)
	Big Bend Ridge Rhyolite3
First			Big Bend Ridge Rhyolite3
		Huckleberry Ridge Tuff (2.2-2.1 Ma)
	Rhyolite of Snake River Butte			Junction Butte Basalt

¹ Plateau-marginal basaltic volcanism has been intermittently active throughout the history of the Yellowstone area. The basaltic volcanism is not inherently part of the rhyolitic cycles, but for descriptive purposes they are discussed together.
² The Plateau Rhyolite comprises more than 20 individual flows ranging in age from 70,000 to 160,000 years. Flows of the contemporaneous basalts (Madison River basalt, etc.) range in age from > 110,000 to < 640,000 years ago.
³ Part of the Big Bend Ridge Rhyolite comprises postcaldera flows of the first cycle and part comprises precaldera flows of the second cycle.

 

References

Christiansen, R.L., 2001, The Quaternary and Pliocene Yellowstone Plateau Volcanic Field of Wyoming, Idaho, and Montana: U.S. Geological Survey Professional Paper 729-G, 145 p., 3 plates.

Smith, R. B., and Siegel, L., 2000, Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks: New York, Oxford University Press, p. 242.

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