Fourmile Hill Tree Molds Geologic Area
A mold of two trees in the basalt at Fourmile Hill.
Fourmile Hill Tree Molds Geologic Area is a ten–acre site on the north flank of Medicine Lake Volcano. It includes dozens of molds that formed when molten lava oozed and flowed through an ancient conifer forest. From violent volcanic eruptions to calmer, “Hawaiian–style” lava flows, the rocks of Northern California record a rich and varied volcanic history. Clues from the Fourmile Hill Tree Molds help scientists understand the complex geologic history of this region.
Geologic Background
The dark rock you see here is called basalt. It formed just over 12,000 years ago, when molten lava erupted from nearby volcanic vents called spatter cones. The lava spilled over the landscape like thick molasses, flowing around the trees near the vents, then smothering nearly everything in its path. While walking over the ancient lava flow, you will see a number of hollows ranging from 1–4 feet in diameter in the basalt; these are molds created as the cooling lava flowed around the tree trunks of a post–glacial forest.
Geologic Processes at Work
Scientists can examine many details of this area to help understand the physical processes which shaped the landscape we see today and to understand the ancient history of our earth and climate.
In order to create the tree molds, the lava was relatively fluid, indicating that the trees were near the eruptive vent. Otherwise, the trees probably would have been pushed over by cooler, more viscous lava at the toe of the lava flow. The trees are believed to be Western White Pines based on bark impressions. Can you find bark impressions inside the tree molds?
This is one of several lava flows that erupted at about the same time. Since the vents follow northeast–trending fissures, the eruptions may have been part the Basin and Range Extension, which encompassed a large portion of the western United States.
Other notable geologic features at this site include lavacicles (lava drips), pahoehoe (“ropey”) lava and 'a'a (“clinker”) lava.
Generalized map of the Fourmile Hill Tree Molds.
References
Donnelly–Nolan, J.M., in press. Geologic Map of Medicine Lake Volcano, northern California: USGS Scientific Investigations Map 2927, scale 1:50,000.
Johnston, D.A., and Donnelly–Nolan, J.M. eds., Guides to some volcanic terranes in Washington, Idaho, Oregon, and northern California: US Geological Survey Circular 838, p 171–176.