An entrance to a lava tube at El Malpais National Monument, New Mexico.
NPS Photo by Dale Pate.
Introduction
While limestone caves with stalactites and stalagmites might be what most people think of when they think of caves, there are several other types of caves. Second most common are probably lava tubes – caves formed due to volcanic eruptions.
Many volcanoes form parallel to the coasts near subduction zones, such as in the Cascade Mountains of Washington, Oregon and Northern California, including Mt Rainier National Park (Washington). Volcanoes also form above “hotspots” in the Earth such as at Yellowstone National Park (Wyoming, Montana, and Idaho) and at Hawaii Volcanoes National Park (Hawaii). Hotspots are locations where heat and molten rock naturally rise from deep within the Earth for thousands and even millions of years. There are different types of volcanoes and volcanic eruptions, but if the right eruption occurs, lava tubes can form.
Glowing lava flowing beneath a field of hardened lava, 1969–1974 eruption of Mauna Ulu. Hawaii Volcanoes National Park.
USGS photo.
Lava Tube Formation
Lava is thousands of degrees hot and can flow like a river. The top of this white-hot river will cool with exposure to the much colder temperatures in the air above. As it cools the lava solidifies into black stone while down below the molten river keeps flowing. The rock above keeps cooling. It thickens and widens and will finally form a roof across the molten rock below creating the ceiling of the cave. Eventually as the eruption ends or the lava flow moves to someplace else, the lava tube drains of molten rock leaving a cave tunnel behind.
When fluid, molten lava flows out of the ground, it works its way downhill. Soon the surface of this lava stream cools and hardens into a crust. Although the outer crust is hard, the lava inside is still molten, and continues to flow downhill. Once the molten lava has passed through, it leaves an empty tunnel called a lava cave, or more commonly, a lava tube.
NPS image by Joel Despain.
A Lava Tube in Mojave National Preserve (California). The lava tube was formed by an erupting cinder cone volcano about 27,000 years ago.
NPS photo.
Most often lava tubes are close to surface and not too deep underground. This makes collapses and entrances more common than in other types of caves. With more entrances there can be a closer connection between these caves and the surface. In dry parts of our nation, such as at Lava Beds National Monument in Northern California the caves are important sources of water and shelter for many animals and plants. Surface animals enter the caves seeking water in pools and from ice near the entrances. The Monument also hosts rare plants that are usually found further to the north or in cooler climates living inside of Monument cave entrances.
While most lava caves are shallow, some are buried by newer lava flows. Occasionally new lava may fill an older existing tube destroying it. This has been documented several times in Hawai'i Volcanoes National Park where mapped and documented caves no longer exist and have been filled with younger molten stone.
Many lava caves are single tubes that can extend for miles. But occasionally lava tubes can be quite complex with passage splits and junctions, multiple levels and even sudden drop offs, all of which reflects how the lava flowed and formed. A great example of this can be found in Catacombs Cave at Lava Beds National Monument in the Cascade Mountains of northern California. In this cave passages split in multiple directions and short pits connect levels.
Fire and Ice
Ice formations in a lava tube in El Malpais National Monument, New Mexico.
NPS photo by Nicholas Guarino.
Lava caves are born of fire, but high on mountains where they often form or in cold regions they can contain ice. Caves are very well insulated with constant temperatures reflecting the average temperature of the region where they form. Summer high temperatures and winter lows have almost no effect underground. Lava tubes in cold places eventually become cold once the volcanic eruption ends. Often this means cold enough for ice. In some instances the shape of cave passages may make a trap for cold air that sinks into the tube in the winter and remains cold through the summer. These caves lack stalactites and stalagmites, but they can have large, spectacular ice formations. Their wet surfaces and transparent nature make for a delightful light show in the ice caves. Ice caves are well represented in our National Parks with such caves found at Craters of the Moon National Monument (Idaho) and at Sunset Crater National Monument (Arizona).
Unfortunately, with our climate slowly warming, the ice caves are warming too. And warmer caves in some cases has meant too warm for ice. This has been very well documented at Lava Beds National Monument (California) by park staff and volunteer researchers with the Cave Research Foundation. The ice is slowly disappearing from many caves. Lovely translucent icicles are vanishing, and floors of cave ice are melting away to reveal the dark rock below. The loss of ice is permanently removing some beautiful features from these National Park caves. It could have a big impact on park wildlife as water supplies melt and dry up.
Underground Features
Like limestone caves, lava caves have lots of interesting features inside. Some have stalactite-like “lavacicles” hanging from the ceiling where molten rock had dripped. The drips can also hit the floor making stalagmite-like features that rise like stacks of tiny pancakes. Lava curbs and gutters form when molten lava sticks to cave walls. Sometimes blocks of rock get stuck in the lava flow and may end up in any location in the cave—stuck to the ceiling or wall or protruding from the floor. Lava can also be colorful. Usually when lava cools it becomes the black rock basalt. But impurities in the rock can color it orange or red, or rarely colors such as purple.
Lava stalactites form when lava is still flowing through the tube. As molten portions of the ceiling drip downward some portions cool and harden, forming stalactites. Craters Of The Moon National Monument & Preserve, Idaho.
NPS photo.
Lava caves are just as biologically diverse as limestone caves with many small, unusual animals inside. In Hawaiian lava tubes whole ecosystems with many different species have developed on the roots of trees that penetrate into the caves. In other locations, large bat roosts lie in the caves. They are also frequently used by bears, ring-tailed cats and other medium-sized mammals.
Roots hanging from the ceiling inside Nāhuku (Thurston Lava Tube), Hawaii Volcanoes National Park, Hawaii.
NPS photo by Dave Boyle.
Find Your Park—Lava Tubes
The following is a partial list of National Park Service units that include lava tubes:
