CRATERS OF THE MOON NATIONAL MONUMENT, IDAHO
FIELD TRIP STOPS – HOLOCENE BASALT FLOWS AND ASSOCIATED STRUCTURES
LOCATION: In south-central Idaho, along US-20 between Arco and Carey (midway between Boise and Yellowstone N.P.).
GEOLOGIC FEATURES: Basalt; lava flows; Pahoehoe flows; Aa Flows; Cinder Cones; Spatter Cones; Lava tunnels; Volcanic bombs; Tree Molds; Fissures; Pressure Ridges; Hot Spot.
DESCRIPTION: Craters of the Moon National Monument lies within the Snake River Plain of Idaho. It is composed of Holocene Basalt Lava Fields that are between 15,000 and 2,000 years old. Eruptions occurred along Fissures developed along a Hot Spot. Movement of the North American Plate in a southwesterly direction over the stationary Hot Spot has caused the cessation of volcanic activity at Craters of the Moon and promoted the current activity at Yellowstone N.P.
(1) What is meant by the Viscosity of a lava? How does it relate to the flow rate of a lava?
(2) Rocks of Craters of the Moon are composed of Basalt. How does Basalt differ from the lava of more explosive volcanoes such as Mt. St. Helens? Explain.
(3) How can the viscosity of a lava relate to its ability to form fields of large area such as found at Craters of the Moon?
(4) Describe how smooth ropey pahoehoe crust can form.
(5) What is a Vesicle in a lava rock? Aa flows have vesicles on their outer surface. How can these vesicles form?
(6) What is a Fissure? Are fissures likely to be formed during periods of compression or tension?
(7) CHALLENGE: If the North American continent moves to the southwest over the hot spot presently under Yellowstone, how would you expect the ages of volcanic activity to vary to the northest and southwest of Craters of the Moon?
Figure 1 - Craters of the Moon National Monument lava fields cover more thanr 600 square miles in area and contain about 60 separate lava flows made up of Basalt.
Figure 2 - The Monument has about 25 cinder cones up to about 800 ft. high. It also has Spatter Cones.
Figure 3 - Cinder Cones
Figure 4 - "Big Cinder Butte" is the tallest cone in the monument at more that 700 ft in relief.
Figure 5 - Aa Lava field and Cinder Cones. Aa Lava typically has its hardened surface boken into rough angular fragments having vesicles.
Figure 6 - One of the many fissures in the Monument. Eight fissure systems occupy the Monument, Most cinder cones are associated with a 28 mile segment of the 53 mile long Great Rift Fissure System.
Figure 7 - "Lava Cascades" at Craters of the Moon formed of pahoehoe lava.
Figure 8 - "Pressure Ridge" is a raised mound of hardened lava formed when the outer edge and surface of a pahoehoe lava flow hardened and the moving liquid lava underneath pushed up on the crust creating a ridge.
Figure 9 - Pahoehow flow with its typical smooth ropey surface.
Figure 10 - Spatter Cones are found at Craters of the Moon. These cones are fomed of welded lava fragments formed when blobs of molten lava are blown into the air from a lava fountain and lack the time needed to completely cool before hitting the ground.
Figure 11 - Entering a Lava Tunnel
Figure 12 - Entrance to Lava Tunnel (Tube). Lava Tunnels are formed when the outer portion of a lava flow hardens with the insulated liquid lava below the crust continuing to flow, sometimes leaving a tunnel as it evacuates the tube.
Figure 13 - Inside a lava tunnel looking out to its opening.
Figure 14 - "Tree Mold." When lava flows around a tree, the lava cools and hardens. The tree will burn away and leave an inprint of its bark in the basalt.
Figure 15 - Impression of Tree Bark caused by a tree falling into the still liquid lava. When the lava hardens from the contact with the cooler tree, an impression of the tree bark is preserved.