Explore Geology
geology fieldnotes title

Craters of the Moon

National Monument


cover of park brochure

park geology subheading
Photo of Craters of the Moon National Monument
Craters of the Moon National Monument, Idaho

"The strangest 75 square miles on the North American continent," one early traveler dubbed the Craters of the Moon landscape. Others deemed it "a weird lunar landscape," "an outdoor museum of volcanism," and "a desolate and awful waste." But this old landscape, showing our globe's awesome forces, eventually became an object of awe.

Geologists predict the landscape will sometime erupt again. Surface patterns and formations abound here which are typical of basaltic lava associated with volcanism the world over. "Where is the volcano?" you might ask. There is not just one, for here the Earth opened a great wound and lava spewed out. These fissure vents, volcanic cones, and lava flows of the Great Rift Zone began erupting about 15,000 years ago and ceased only 2,000 years ago.

Silent Evidence of a Powerful Past
Basalt lava flows are grouped by appearance. Most common here are Aa (pronounced ah-ah) and Pahoe-hoe (pronounced pa-hoy-hoy). These Hawaiian terms, one explorer said, mean "unfriendly" and "friendly," respectively! Aa cuts hands and boots, and means "hard on the feet". Pahoehoe is relatively smooth, and means "ropy." Pahoehoe lava was more fluid on emerging and it hardens in pleats like hot fudge poured from a pan. Aa lava was more viscous on emerging. Aa's highly irregular surface consists of rubble encrusted with stubbly spines, making it impassable to foot travelers. Pahoehoe contains more dissolved gas than aa and is more frequently associated with impressive lava fountains. A third lava flow form, blocky lava, is less common at Craters of the Moon. This type forms angular blocks that may be almost three feet wide.

photo of different lava types
top left: aa lava
right: pahoehoe lava
bottom left: lava bomb

There are three classes of bombs;
  • spindle,
  • ribbon,
  • and breadcrust.

Bombs ranging in length from ½-inch to three feet form as airborne blobs of molten lava cool and harden as they fall to Earth.

How Did Lava Tubes Form?
When fluid, molten lava flowed out of the ground, it behaved like a stream of water working its way downhill. But soon the "stream" surface cooled and hardened. This crust then insulated the molten lava inside, enabling it to keep flowing. The molten lava inside the crust eventually flowed out leaving the crust as the walls of a lava tube or "cave."

You can explore some of these caves at Craters of the Moon. Some contain stalactites that were created by the dripping of molten lava before cooling. Others contain ice year round. Some are inhabited by blind insects. In summer, swallows, ravens, and great horned owls nest near cave openings.

Exploring Craters of the Moon by the Loop Road
What appears monotonous at first is really a landscape full of detail and surprises. This brief guide to selected features helps you to see the park according to your own pace, schedule, and interests. The larger story unfolds as you tour the park by the loop road, after stopping at the visitor center, a good orientation point for beginning your explorations.

Visitor Center. Displays and a short video describe the park's lava phenomena, life, history, and the Earth processes creating them. Check on schedules of conducted walks and evening programs, and examine the sales publications about the park. Ask questions about both the park and your explorations.

The 7-mile-long loop road takes you deeper into the park's unique scenic attractions. Most of the drive is one-way. Several spur roads and trailheads enable you to explore Craters of the Moon even further. The trails invite foot travel. You can make the drive, including several short walks in your itinerary, in about 2 hours.

North Crater Flow. At this first stop, a short trail crosses the flow to a group of monoliths or crater wall fragments transported by lava flows. This flow is one of the youngest and here the Triple Twist Tree suggests, because of its 1,350 growth rings, that these eruptions ceased only 2,000 years ago. You see fine examples of both ropy pahoehoe- and aa lava flows on North Crater Flow. Just up the road is the North Crater Trail. Take this longer, steep trail to peer into a volcano vent.

Devils Orchard. After the road skirts Paisley Cone, on the east side stands Devils Orchard. This group of lava fragments stands like islands in a sea of cinders. A short spur road leads to a self-guiding trail through these weird features. As you walk this ½-mile trail, you will see how people have had an impact on this lava landscape and what is being done to protect it today. This barrier-free trail is designed to provide access to all people.

Inferno Cone Viewpoint. A volcanic landscape of cinder cones spreads before you to the distant mountain ranges beyond. Cool, moist north slopes of the cones have noticeably more vegetation than the drier south slopes. From the summit of Inferno Cone - a short, steep walk - you can easily recognize the chain of cinder cones along the Great Rift.

Big Cinder Butte towers above the lava plain in the distance. This is one of the largest purely basaltic cinder cones in the world.

Big Craters and Spatter Cones Area. Spatter cones formed along the Great Rift fissure where clots of pasty lava stuck together when they fell. The material and forces of these eruptions originated at depths of nearly 37 miles within the Earth. To protect these fragile volcanic features, you are required to stay on trails in this area.

Trails to Tree Molds and Wilderness. A spur road just beyond Inferno Cone takes you to trails to the Tree Molds Area, Trench Mortar Flats, and the Craters of the Moon Wilderness. Tree molds formed where molten lava flows encased trees and then hardened. The cylindrical molds that remained after the wood rotted away range from a few inches to just under three feet in diameter. Note: All backcountry camping requires a permit available at the Visitor Center.

Cave Area. At this last stop on the loop road take a ½-mile walk to the lava tubes and see Dewdrop, Boy Scout, Beauty, and Surprise Caves and the Indian Tunnel. You need to carry artificial light in all caves but Indian Tunnel.

park maps subheading

The General park map handed out at the visitor center is available on the park's map webpage.

For information about topographic maps, geologic maps, and geologic data sets, please see the geologic maps page.

photo album subheading

More photos and tips for taking photos at Craters of the Moon can be found here.

For information on other photo collections featuring National Park geology, please see the Image Sources page.

books, videos, cds subheading

Currently, we do not have a listing for a park-specific geoscience book. The park's geology may be described in regional or state geology texts.

Please visit the Geology Books and Media webpage for additional sources such as text books, theme books, CD ROMs, and technical reports.

Parks and Plates: The Geology of Our National Parks, Monuments & Seashores.
Lillie, Robert J., 2005.
W.W. Norton and Company.
ISBN 0-393-92407-6
9" x 10.75", paperback, 550 pages, full color throughout

The spectacular geology in our national parks provides the answers to many questions about the Earth. The answers can be appreciated through plate tectonics, an exciting way to understand the ongoing natural processes that sculpt our landscape. Parks and Plates is a visual and scientific voyage of discovery!

Ordering from your National Park Cooperative Associations' bookstores helps to support programs in the parks. Please visit the bookstore locator for park books and much more.

geologic research subheading

Information about the park's research program is available on the park's research webpage.

For information about permits that are required for conducting geologic research activities in National Parks, see the Permits Information page.

The NPS maintains a searchable data base of research needs that have been identified by parks.

A bibliography of geologic references is being prepared for each park through the Geologic Resources Evaluation Program (GRE). Please see the GRE website for more information and contacts.

selected links subheading

NPS Geology and Soils Partners

NRCS logoAssociation of American State Geologists
NRCS logoGeological Society of America
NRCS logoNatural Resource Conservation Service - Soils
USGS logo U.S. Geological Survey

teacher feature subheading

Craters of the Moon National Monument and Preserve is an out of this world outdoor classroom. Contact park staff at (208)527-3257 to make reservations to visit Idaho's national park with your students. You can also "Visit the Moon Without Leaving Idaho" with this on-line exercise in comparative planetology. Teacher's Workshops are offered by Monument staff each spring and summer.

More about the park's education and intrepretive programs is available on the park's education webpage.

For resources and information on teaching geology using National Park examples, see the Students & Teachers pages.
updated on 01/04/2005  I   http://nature.nps.gov/geology/parks/crmo/index.cfm   I  Email: Webmaster
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