Cabin Meadow Pillow Lava Geologic Area
Cabin Meadow Pillow Basalt outcrop against forest background.
The Cabin Meadow Pillow Lava site is about 10 acres in size, 10 air miles NE of the community of Callahan. The main outcrop, located between Cabin Meadow and Rock Fence Creeks, measures about 25 meters in stratigraphic thickness. It consists of ancient rock that are about 415 million years old, from the early–Devonian period. The rocks in this unit are spilitic basalts, and contain visible minerals of albite plagioclase (the light minerals) and either augite or hypersthene pyroxenes (the dark green and black minerals). They are formed as pillow lavas, with individual pillows up to 1 meter by 2 meters in size.
Geologic Background
This site is by far the best example of pillow lavas known to exist on the Klamath Forest. They are named “pillow structures” because the features look like large pillows in the rock. Pillow structures are formed when lava flows from volcanic vents into water and cools. Pillows are a valuable aid to geologists in deciphering the origin of rocks by providing clues to the amount of tilting and deformation they have experienced, and the environment in which the rocks formed.
Geologic Processes at Work
Pillow lavas typically form along submarine volcanic vents, where fresh basaltic lava tumbles over the seafloor.
When molten lava oozes slowly into water, the outer–most layer quenches instantly, forming a rind of hot glass over a bulb of molten rock. Heat from the molten rock inside causes the bulb to expand, cracking through the glassy rind. The molten lava can then ooze out through the cracks, forming new bulbs. This process continues until the eruption ceases and lava stops flowing from the volcanic vents.
Bubbles of gas from within the hot liquid rock try to escape. The gas bubbles are often trapped within the pillow lava because they cannot break through the solid crust, forming what are called vesicles. The largest vesicles occur near the rind.
In cross section, lava pillows show a chilled margin sequence, consisting of a glassy outer rind, followed by a layer of spherulites (glass spheres; typically very small), covering a mass of very fine–grained volcanic rock. Pillow structures typically form from basaltic lavas which flow like molasses.
Nearby Underwater Volcanoes
Spreading Ridges - The Axial Volcano erupts along a ridge between the Pacific and Juan de Fuca tectonic plates. This is an area where the ocean floor is spreading apart, allowing magma to rise from beneath the oceanic crust.
Hot Spots - These are areas where magma rises from beneath the oceanic crust too, but have nothing to do with tectonic plates. Hawai'i is one such location.
Generalized Map of the Cabin Meadow Pillow Basalt Special Interest Area.
References
Lindsley–Griffin, N., et al. 2008, Paleogeographic significance of Ediacaran Cyclomedusoids within the Antelope Mountain Quartzite, Yreka terrane, eastern Klamath Mountains, California. (in press).
Lindsley–Griffin, N., et al., 2006, Ediacaran cyclomedusoids and the paleogeographic setting of the Neoproterozoic–early Paleozoic Yreka and Trinity terranes, eastern Klamath Mountains, California. p. 411–431
Mankinen, E.A., et al., 2002, Concordant paleolatitudes for Neoproterozoic ophiolitic rocks of the Trinity Complex, Klamath Mountains, California. p. 11–1/11–18.