The majority of all fossils are found in sedimentary rocks versus igneous or metamorphic rocks—by far. In rare instances fossils are also preserved in some volcanic rocks and deposits. Altogether, there is a much richer connection between volcanoes, volcanic rocks, and fossils than might be expected.
Volcanic regions tend to be teeming with life because volcanic soils are fertile and their mountainous terrains offer a wide range of habitats for plants and animals. Although volcanic eruptions are hazardous and may lead to the death of plants and animals, they sometimes produce deposits that preserve evidence of past life. Many volcanic deposits accumulate rapidly, a condition which generally favors the preservation of animal or plant remains. However, the high temperatures of lava flows often incinerate everything they encounter.
Depending on how explosive an eruption is and the composition of the magma, volcanic eruptions may produce either lava flows or fragmented volcanic material known as tephra. Fossils in lava flows are quite rare and generally restricted to molds of trees that were enveloped by advancing lava.
Fossils are more likely to be present within tephra deposits. During vigorous eruptions, tephra may be propelled tens of thousands of feet into the atmosphere. The smallest particles (volcanic ash) may form ash clouds that are dispersed great distances by winds prior to falling to the ground. These ash-fall deposits are important to the science of paleontology in two ways:
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Ash deposits are somewhat akin to sedimentary rocks in the ways that they accumulate, so can preserve the remains of plants and animals in their deposits. Many ash deposits accumulate as ash particles fall the Earth’s surface. Others can be reworked, or transported by streams or winds from previous airfall ash deposits.
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Ash deposits represent an instant of geologic time and contain minerals from which geologists can obtain their numeric ages, and by extension the date the fossils within them. Therefore, ash layers provide important information on the ages of fossil-bearing rocks.
Lahars (volcanic mudflows) are another type of volcanic deposit that can preserve fossils, especially petrified wood and other fossil plants. Lahars may form either during an eruption, or due to melting snow or heavy precipitation.
NPS photo by G. Jensen.
Due to their high temperatures, lava flows typically rapidly incinerate organic material. Therefore, fossils are rarely present in lava flows. The main exception to this rule are lava tree mold in basaltic lava flows.
Lava flows erupted on volcanoes such as Kīlauea in Hawai’i Volcanoes National Park frequently engulfs forests. Upon surrounding the trunk of a tree, the lava surface is chilled and can sometimes make a clear mold of the tree’s bark. After forming a mold, the tree then burns away leaving a hollow cylinder where the original tree stood.
Tree molds may be vertical when a standing tree was surrounded by lava, or horizontal when the advancing lava flow knocked it over.
Lava tree molds have been documented in five national parks:
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Craters of the Moon National Monument & Preserve, Idaho
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El Malpais National Monument, New Mexico
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Hawaiʻi Volcanoes National Park, Hawaiʻi
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Lava Beds National Monument, California
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Puʻuhonua O Hōnaunau National Historical Park, Hawaiʻi
Fossil tree molds are somewhat fragile features found near the surface of lava flows. They are rarely found in lava flows older than Holocene in age. The lava flows that contain tree molds in national parks are all very young, being no older than a few thousand years at most.
The volcanic mudflows known as lahars are a particularly common feature of composite volcanoes such as Mount Rainier. They are mixtures of water, volcanic ash, tephra, rock fragments, and chunks of ice that flow like wet concrete. Lahars travel down river valleys and may bury trees and forests under great thicknesses of volcanic mud.
Petrified wood is the most common type of fossil found in lahar deposits. Trees may get buried while standing, leaving in situ stumps, like those in Florissant Fossil Beds National Monument and Yellowstone National Park. The volcanic ash that is the main component of most lahars provide a ready source of silica that can contribute to the preservation of wood through permineralization and replacement.
Florissant Fossil Beds National Monument
Lahars are the key component of the geologic history of Florissant Fossil Beds National Monument that led to its exceptionally well-preserved fossils of insects and flowers, as well as the monument’s petrified tree stumps. The delicate plant and insect fossils that the monument is known for were preserved by Lake Florissant, which was formed by lahars that dammed the river. Other lahars buried a forest of giant redwoods and other trees, leading to the fossilization of massive tree stumps.
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Florissant Fossil Beds—On Top of a Forest, on the Bottom of a Lake
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Florissant Fossil Beds—Shadows of the Past - Florissant Fossil Beds National Monument
John Day Fossil Beds National Monument
The Clarno Nut Beds in John Day Fossil Beds National Monument contain one of the most diverse fossil floras in the world, with more than 175 species of fruit and seeds having been described, mostly of flowering plants (angiosperms). The exceptional richness of this fossil deposit makes it a Lagerstätte. This deposit was formed when a lahar from nearby volcanoes engulfed a hot, wet, semitropical forest. The abundant silica in the lahar deposits helped permineralize fruits and nuts—parts of plants that are rarely fossilized.
NPS Photo by P. Gamman.
The Clarno Nut Beds in John Day Fossil Beds National Monument contain one of the most diverse fossil floras in the world, with more than 175 species of fruit and seeds having been described, mostly of flowering plants (angiosperms). The exceptional richness of this fossil deposit makes it a Lagerstätte. This deposit was formed when a lahar from nearby volcanoes engulfed a hot, wet, semitropical forest. The abundant silica in the lahar deposits helped permineralize fruits and nuts—parts of plants that are rarely fossilized.
Yellowstone National Park
NPS Photo by William W Dunmire.
Yellowstone National Park contains perhaps the largest and most extensive occurrence of petrified wood in the world, all within in a series of lahar deposits.
Lahars from the Absaroka volcanoes near the park’s northeastern and northern boundaries buried huge sections of ancient forests. Some of the petrified stumps and logs were transported upright by the lahars, others were buried in place. Some of fossilized stumps are massive and are up to eight feet in diameter or more than 20 feet tall. The petrified wood is from sequoias, firs, and numerous deciduous trees.
Volcanic ash deposits may contain important fossils. It also contains minerals that enable scientists to measure its age, and thereby determine when animals and plants in the geologic past lived and died.
Volcanic ash is fine-grained material, less than 2 mm (0.08 in) across, ejected from a volcano during explosive eruptions. It consists of shattered bits of volcanic glass, crystals, and rock fragments. Vigorous eruptions may propel ash columns high into the atmosphere where it disperses with the wind. Particularly large-volume eruptions can produce thick ash deposits at great distances from the volcano from which they originated. Ash deposits like any other type of sediment can be reworked and redistributed by rivers and streams.
Since volcanic ash is mostly made of volcanic glass, it provides a ready source of silica which is important in fossilization processes such as permineralization and replacement. Silica from volcanic ash also can result in blooms of microscopic diatoms, which sometimes play an important role in fossilization in lacustrine (lake) environments such as at Florissant Fossil Beds National Monument.
Source of Silica
NPS photo by Stuart Holmes.
Volcanic glass is the primary component of volcanic ash. The silica in volcanic glass can easily be transported by groundwaters that flow through buried sediments because it is not trapped within a mineral’s crystal structure, like that in quartz and feldspar. During fossilization, this silica can either replace organic minerals or infill small pores in bone or wood in animal and plant remains that may be in the buried sediment.
The abundance of volcanic ash within the Chinle Formation is one of the factors that led to the abundance of petrified logs in Petrified Forest National Park. The logs that were deposited in the Triassic stream channels may not have been fossilized without the ready source of silica for groundwater during diagenesis (e.g., the process of turning sediment into sedimentary rock).
Diatom Blooms
NPS photo.
The presence of silica from volcanic ash is indirectly responsible for the many exquisitely preserved fossil insects and flowers at Florissant Fossil Beds National Monument. Lake Florissant formed after lahars dammed a river. Continued volcanic activity periodically washed volcanic ash into the lake where the presence of silica resulted in blooms of diatoms, a type of microscopic algae with cell walls made of silica. Eventually, the diatoms died and fell to the bottom of the lake. The paper shales that preserve the delicate fossils in the Florissant Formation are made of couplets of clay and diatoms.
Fossils in Ashfall Deposits
James St. John photo.
Heavy ashfall, even from volcanic eruptions far away, led to the death of prehistoric animals due to the inhalation of ash. The animal carcasses can then become entombed in ash. Ashfall Fossil Beds State Historical Park, a National Natural Landmark in eastern Nebraska, is a classic fossil locality containing fossils in ashfall deposits. A massive volcanic eruption 900 miles west in Idaho nearly 12 million years ago ejected voluminous ash into the atmosphere where winds blew it east. At Ashfall Fossil Beds, the layer of ash from this eruption is approximately 8 to 10 feet thick, encasing articulated skeletons of ancient horses, camels, and large rhinos that perished while breathing the abrasive ash.
Volcanic Ash as Geologic Timekeepers
Volcanic ash beds in sedimentary sequences contain minerals which enable the determination of their numeric age. Minerals such as zircon and feldspars contain tiny amounts of radioactive elements that allow geologists to use them as geologic clocks. If a volcanic ash contains fossils, or is above or below fossiliferous sedimentary layers, then it can be used to infer the age of fossils.
Volcanic ash beds, particularly those found within sedimentary sequences, are critically important geologic timekeepers because geologists do not have ways to measure the numeric ages of most sedimentary rocks.
Even without numeric ages, volcanic ash layers serve as critical marker beds withing sedimentary strata, helping paleontologists correlate between different fossil-bearing outcrops.
Hagerman Fossil Beds National Monument
Hagerman Fossil Beds National Monument contains steeps slopes with about 600 feet of strata that contain Pliocene fossils that span more than a million years of geologic history. The strata reflect a complex geologic history of faulting and scouring by rivers and lakes during deposition. These factors have made simple correlation of ash beds more complicated than in some areas.
The Hagerman Paleontology, Environments, and Tephrochronology (PET) Project has used the identification and correlation of at least 13 different volcanic ash beds to obtain the precise information needed to reconstruct ancient environments across the time interval from 4.2 to 3.07 million years ago. Laura Walkup of US Geological Survey did detailed mapping and correlation of these ash beds using their age and their unique chemical composition. The result of this study is that paleontologists at Hagerman Fossil Beds can identify 14 distinct faunal communities and ecosystems across that slice of geologic time.
Petrified Forest National Park
At Petrified Forest National Park, geologists dated zircons from volcanic ash in the Chinle Formation to provide age control for the fossils found in the park. Zircon contains a tiny bit of radioactive uranium so it can be used to obtain numeric ages. The Chinle Formation was deposited between about 227 and 205 million years ago, which was an important time for tetrapod, including reptile and dinosaur, evolution.
Petrified Forest National Park—Geologic Formations
Caption (image, right): Stratigraphic column of the Chinle Formation at Petrified Forest National Park showing numeric ages obtained from minerals in volcanic ash.
Scotts Bluff National Monument
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Two volcanic ash beds in Scotts Bluff National Monument occur within the Whitney Member of the Brule Formation.
Volcanic ash beds in Scotts Bluff National Monument served as important marker beds in the Oligocene sequence and also provide numeric ages that bracket the age of fossils. The Brule Formation contains rich fossil record including oreodonts, a type of herbivorous mammal, and dry-land-dwelling tortoise known as Stylemus.
Last updated: October 11, 2024