Site Tour: North Crater Flow

NPS Illustration

North Crater Flow Trail

Background Information

The North Crater Flow Trail is one of most heavily visited trails in the park. The trail is fairly narrow making it difficult for a large group to gather in any one spot. It is important that you stay on the paved trail at all times. Walking on the lava in this area can easily break its fragile surface. Evidence of this damage can be seen along the trail as exposed red colored lava. Because of the wide range of volcanic features found on the North Crater Flow Trail, it is an excellent first stop.

Trail Head

Most geologists agree that volcanic activity seen on the surface today began at Craters of the Moon about 15,000 years ago. Since then, there have been eight periods of volcanic activity. Each period lasted from a few years to a few hundred years. The quiet time between periods of volcanic activity could be as short as several hundred years or as long as 3,000 years. The average time between periods of volcanic activity has been about 2,000 years. North Crater and the North Crater Lava Flow formed during the most recent eruptive period that ended approximately 2,000 years ago. Big Craters, the Spatter Cones, and the Blue Dragon Flow (Indian Tunnel, Boy Scout Cave, and the other lava tubes in the Caves Area) all formed during this same eruptive period. The following stops describe features that are encountered when following the trail in a clockwise direction.

Stop #1

Pahoehoe Lava

There are two general types of lava flows found at Craters of the Moon, pahoehoe (pah-hoy-hoy) and aa (ah-ah). Both of these words are Hawaiian since much of what we know about volcanic activity came from the study of active volcanoes in Hawaii. Pahoehoe is a smooth lava that often forms ropes and coils. Pahoehoe means "ropy" in Hawaiian. Pahoehoe flows are very hot, around 2,000 degrees Fahrenheit, and have been clocked flowing at speeds of more than 30 miles per hour. However, much of the pahoehoe at Craters of the Moon was very viscous or thick and was very close to being aa. Even in large channels this thick lava was probably only flowing at 3-6 miles per hour. Pahoehoe flows are often seen flowing like glowing orange rivers. As the surface of a pahoehoe flow cools it forms a sort of "skin." The hot, fast-moving lava beneath pulls, pushes, and twists this cooler top layer into the weird ropes and coils. These ropes and coils can give you a hint to the direction the lava flowed. Pahoehoe is the most common type of lava flow found at Craters of the Moon.

Stop #2A

Triple Twist Tree

If you were to look at a cross section of a tree or the top of a stump, you would see the tree trunk is made up of a series of concentric rings. Since a living tree adds one growth ring each year, you can determine the age of the tree by counting the number of growth rings. A core sample taken from the triple twist tree, showed it to be at least 1,350 years old.

A molten lava flow destroys all plant life in its path. After the flow cools, new plant life slowly begins to return. This returning plant life provides scientists with one way to date lava flows. By comparing the amount of vegetation on one flow with the amount of vegetation on an adjoining flow, geologists can determine which of the two flows is older. To get an actual date on a lava flow, geologists can date a tree growing on the flow. In the case of the triple twist tree we know that the tree began growing after the last eruption, so the lava flow must be at least 1,350 years old.

Geologists can also use a method known as "radio-carbon dating." Samples of charred vegetation such as sagebrush, limber pine, or even pine cones can be dated using this method. Another method scientists can use to obtain dates far older than living vegetation can provide is called "paleomagnetic measurements." As the lava cools, tiny magnetic crystals align themselves in the direction of the earth's current magnetic field. Each lava flow at Craters of the Moon reflects the direction of the magnetic field when the lava erupted and cooled. Scientists can correlate this information to the earth's changing pattern of magnetism and arrive at a date when the lava was deposited.

Stop Number 2B

Pressure Ridges and Squeeze Ups

Pressure ridges are a common feature of pahoehoe flows. They form as the flow's crust is pushed upwards by the pressure of the moving lava beneath. The arched crust generally cracks open, and may allow molten lava to "squeeze up" to the surface like toothpaste out of a tube. Pressure ridges are usually a few feet high and about 100 feet long, but they can be as high as 50 feet and over half a mile long.

Stop Number 3

A'a Lava

A'a flows have a rough, jagged surface and are extremely sharp. In Hawaiian, aa means "hard on the feet." Most lava flows emerge as pahoehoe and somewhere down the slope change into aa as the lava cools, loses much of its gas, and becomes thicker. As the flow cools, it begins to slow down, moving as a mass of broken rubble. As compared to the faster moving pahoehoe flows, aa travels very slowly. The a'a flow moves on top of a hidden molten core. As the clinkery outer surface slides down the front slope of the advancing flow, you can sometimes get a glimpse of the glowing interior. An advancing aa flow moves like a giant, slow moving tank or conveyer belt. Cooling lava rock that slides down the front of the flow is overridden by the advancing mass. Occasionally, a pahoehoe flow will burrow under an aa flow or emerge through a tunnel, leaving the appearance that an aa flow produced pahoehoe lava. Within the very viscous pahoehoe found here shearing caused by such things as flowing over a rough surface, down a steeper slope, or over a ledge can produce an aa like surface and then when the lava flows back onto a smooth surface or onto a lower gradient slope switch back to a smooth ropy appearance. Walking on an a'a flow has been compared to walking on millions of cups and saucers stacked in a loose pile.

Stop #4

Blue Dragon Lava

The blue color comes from a thin outer layer of lava that contains titanium magnetite crystals. These abundant crystals reflect the intense blue light. One of the youngest and largest lava flows in the park, the Blue Dragon Flow, gets its name from this blue lava. Robert Limbert, one of the park's earliest explorers, thought the name "Blue Dragon" was perfect. He compared the cracked and veined surface to the scales of a prehistoric reptile. Limbert also thought that squeeze ups looked like the legs and claws of a dragon.

Some of the lava is red where the blue lava has been broken and removed. This red coloration occurs because basaltic lava rock is high in iron. Metals such as iron rust when they are in contact with oxygen. Oxygen can be found in the air we breath, steam or water, and in the volcanic gases trapped within the cooling lava rock. The gases and water vapor trapped beneath the glassy congealed crust of the lava makes an ideal environment for oxidation to occur, turning the material beneath the crust a red color.

Stop #5

Rafted Blocks

Rafted blocks, or monoliths, are sections of a broken crater wall that have been carried along on top of an aa lava flow. Only aa flows are thick enough to "float" the blocks to their new locations. Although geologists can't be certain, they now believe that as many as four separate flows came from a vent at or near North Crater. The first, a particularly thick type of aa called block lava, flowed northward down the flank of North Crater into the valley between Sunset Cone and Grassy Cone located across the highway. The eruption was so violent that it destroyed the north flank of North Crater and possibly other cinder cones that might have stood in its path. As the eruption continued, two more aa lava flows rafted large blocks of the broken crater wall for several miles. Later, one more eruption produced a smaller pahoehoe flow. Viewed from the highway today, North Crater looks as if a giant "bite" was taken from it's northern side.

Stop #6

Block Lava

Block lava is a variety of aa lava characterized by large irregular blocks of very dense lava with smooth surfaces. Block flows are thicker than a'a or pahoehoe, and when they are molten, they creep along at very slow speeds.

End of Trail

Summary

The eruptions at North Crater produced block, a'a, and finally pahoehoe lava. Thick, slow-moving a'a lava is capable of "floating" large sections of crater wall for several miles. Hot, fluid pahoehoe flows like a river of molten rock, crusting over to form pressure ridges and a ropy or wrinkled surface. As we have seen, lava can come in many different colors, from basic black, to blue, to rusty red. Why do we say that Craters of Moon has a violent past, a calm present, and an uncertain future? Remember that the last eruption at Craters of the Moon ended approximately 2,000 years ago and that the average time between periods of volcanic activity is 2,000 years. Geologists predict that the forces needed to produce a volcanic eruption are slowly building. This means a new eruption could occur at any time; next year, 300 years, or 1,000 years from now. It seems safe to say that more volcanic eruptions will occur in the not too distant future.