Lava tree molds: fossils of trees engulfed by lava flows

Trees caught in a lava flow can leave their shape behind in rock. These impressions, called tree molds, are trace fossils that form when trees engulfed by molten lava are not consumed at once by the heat.

A tree mold in volcanic rock. USGS photo.

How a tree leaves a mold

When lava touches a tree, the moisture in it is released as steam, which speeds up cooling and gives the tree time to leave an impression in the lava. Surface details such as bark patterns are sometimes preserved, even after the tree has decayed or been destroyed by the lava's heat.

Lava burning through trees at Hawaiʻi Volcanoes National Park. Image from the National Park Service's page.

There are two basic kinds:

Lava trees usually form when airborne lava spatter builds up on a standing tree during an eruption. The spatter partly buries the tree and leaves a thin conical shell of lava with the impression of a trunk on its inside surface. They can also form when a flow engulfs a standing tree and makes a shell, then drains away.

These upright tree molds formed when fast-moving lava flowed around trees. The cool, moist trunk makes the lava crust around it, and when the flow is short-lived it drains away and leaves lava trees standing. Hawaii Volcanoes National Park. NPS photo.

Where the National Park Service has them

A 2012 inventory of tree molds in National Park Service areas found them in five parks:

• Craters of the Moon National Monument & Preserve, Idaho
• El Malpais National Monument, New Mexico
• Hawaiʻi Volcanoes National Park, Hawaiʻi
• Lava Beds National Monument, California
• Puʻuhonua O Hōnaunau National Historical Park, Hawaiʻi

The five parks where the inventory found tree molds.

What scientists learn

Lava can also turn plants to charcoal, made by incomplete burning of organic material where oxygen is limited. Radiocarbon ages can be determined from the charcoal. USGS photo.

Tree molds kept in parks and other protected areas are chances for public education and scientific study of rare paleontological resources. Charcoal from burned trees can often be dated, which helps map the order of eruptions around a volcano. Molds can show what vegetation grew before an eruption, which helps define the habitat and climate then. The structure of lava in and around a mold can even show the direction, speed and thickness of a flow.

Sources

• National Park Service, Lava Tree Mold Fossils; rewritten in hubnx's own words. The page adapts the abstract of a 2012 conference presentation by V. L. Santucci, L. C. Walkup, T. Casadevall, J. C. Wood and T. Conners on an inventory of National Park Service fossil tree molds preserved within lava flows.