Field of Breadcrust Bombs

Of the 60 lava flows visible on the surface of the Craters of the Moon Lava Field today, 20 have been dated. The oldest is about 15,000 years old and the youngest about 2,000.

Types of lava

Some lava flows were very dense and have a surface of angular blocks referred to as block lava. Others have a rough, jagged, or clinkery surface called áa lava. Still others have a smooth, ropy, or billowy surface called pahoehoe lava. Three special kinds of pahoehoe may be observed in the Craters of the Moon Lava Field: (1) slabby pahoehoe is made up of jumbled plates or slabs of broken pahoehoe crust; (2) shelly pahoehoe, which forms from gas-charged lava, contains small open tubes, blisters, and thin crusts; and (3) spiny pahoehoe, which is very thick and pasty, contains elongated gas bubbles on the surface that form spines. The slabby and spiny varieties of pahoehoe are transition phases to áa.

Bombs

Four kinds of volcanic bombs are found at Craters of the Moon, all of which began as a volume of molten rock that is ejected into the air. If the lava gets twisted during its flight, it is called a spindle bomb and typically measures from a few inches to several feet in length. If it is very tiny and twisted, it is called a ribbon bomb. When the volume of lava forms a crust that is cracked by expanding gases as it flies through the air, it is called a breadcrust bomb, which exhibits a surface texture that resembles bread rising in the oven. If the lava mass does not completely solidify during flight, so that it flattens and spreads on landing, it is called a cow-pie bomb. Some cow-pie bombs are over 10 feet long.

Lava Tube Caves

Lava tubes are hollow spaces beneath the surface of solidified lava flows. They are formed by the withdrawal of molten lava after the formation of the surface crusts. Indian Tunnel, in the northern area of the park, has a 40-foot high ceiling and is 800 feet long. Bear Trap Cave, which lies between the Craters of the Moon and the Kings Bowl Lava Fields, is about 15 miles long, but is not continuously passable. Most of the Craters of the Moon lava flows are composed of pahoehoe and were fed through tubes and tube systems, although there are some sheet flows.

Other lava features

At Craters of the Moon, structures representing both inflation and deflation of the lava surface can be seen along with hot and cold collapses of the roofs of lava tubes. Inside lava tubes, one can see lava stalactites, remelt features, and lava curbs. In other places lava flows formed ponds, built levees, and produced lava cascades. Some lava flows produced small mounds (tumuli) or elongated ridges (pressure ridges) on their crusts. In some places, squeeze-ups formed when pressure was sufficient to force molten lava up through tension fractures in the top of pressure ridges or cracks in the solidified crust of lava ponds. Pressure plateaus were produced by the sill-like injection of new lava beneath the crust of an earlier flow that had not completely solidified.

When magma emerges at the surface along a segment of a rift, it often begins by producing a curtain of fire and a line of low eruptions. As portions of the segment become clogged, the fountains jet higher. If magma emerges at the surface highly charged with gas it sprays high in the air; the fire fountains that produced many of the Craters of the Moon cinder cones were probably over 1,000 feet high. Big Cinder Butte, the tallest cinder cone at Craters of the Moon, is over 700 feet high. The highly gas-charged molten rock cools and solidifies during flight and rains down to form cinder cones. If you look closely at cinders you will see that they are laced with gas holes and resemble a sponge.

Some vents along the rift ejected very fluid particles (spatter) that accumulated to form steep-sided spatter cones. Along eruptive fissures where a whole segment erupted, spatter accumulated to produce low ridges called spatter ramparts. Hornitos, also known as rootless vents, are similar in appearance to spatter cones. Hornitos form from spatter ejected from holes in the crust of a lava tube instead of directly from a feeding fissure. Craters of the Moon also has collapse features known as sinks or pit craters. During some eruptions, pieces of crater walls were carried off like icebergs by lava flows. These wall chunks are known as rafted blocks. The monoliths on the North Crater Flow Trail are excellent examples of these volcanic formations.