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Geology...


(Reprinted from "Caves of Mount St. Helens - Guidebook")


Mount St. Helens National Volcanic Monument contains one of the largest concentrations of caves in the Northwest. The geology and caves found here are unique in their variety of sensitive resources, and young age. Unfortunately, unlike limestone caves, the appreciation of lava caves hinges upon a heightened awareness of volcanology through education. Lava tubes are generally not an "ohhh, ahhh" type of experience.

Mount St. Helen is one of the most active volcanoes in the Cascade Range. The most recent eruptive episode began on March 20, 1980, after 120 years of dormancy. On the morning of May 18, 1980, following two months of small steam and ash eruptions, the bulging north side of the mountain collapsed, creating an avalanche of rock, mud, and ice which traveled 15 miles down the North Fork, Toutle River Valley. The avalanche was followed by a huge steam explosion which leveled nearly 150 square mi1es of forest and killed nearly everything in its path. This explosion marked the beginning of a magmatic eruption that lasted for the next twelve hours, ejecting nearly three cubic kilometers of tephra, and sending an ash cloud 14 miles (20 km) into the atmosphere. Ash fell extensively throughout eastern Washington and western Montana and within a month the ash cloud had traveled around the world. Over the next few years, steadily diminishing eruptions built a succession of lava domes on the crater floor, blew them away, and built them again. As the explosions decreased in intensity and degassed lava oozed from the vent, a huge lava-dome nearly 900 feet high grew in the crater. The last lava was added to the dome in 1986; since then only steam emissions have occurred.

While this eruptive episode was dramatic, it produced no new lava tubes. The eruption that produced the caves occurred 1,950 years ago when fissures opened high on the volcano. Fluid basalt, similar to the famous eruptions of Kiluea Volcano in Hawaii, flowed mostly to the south, invading the valleys of the Toutle and Lewis Rivers. Even though lavas flowed down several sides of the volcano, they are best seen on slopes above the Lewis River. Due to the abundance of caves found here, geologist have named this rock unit the "Cave Basalt." The Cave Basalt Flow is almost a mile wide and about nine-miles long. Sixty four lava tubes had been identified by 1994.

Comparing the volume of the Cave Basalt with historic eruptions in the Hawaiian Islands, it becomes clear that the eruption duration spanned months to a year. The relatively quiet eruption was probably not spectacular after the initial eruptive phase. Lava most likely flowed quietly from fissures and was carried away from the vents through lava tubes. Occasionally lava would break out of a tube and would show its yellow heat before quickly cooling to a jet-black surface. On steep slopes lava would sometime be seen flowing over abrupt drops, accompanied by small plumes of fume and water vapor. It would have been possible to walk around on the active flow in complete comfort. The routes of lava tubes would have been marked only by small amounts of fume rising from cracks, or small of sulfur encrustation.

If we could have watched the flow advance, lava moving through the forest would transfix us because of the magnitude and ferocity of change. We see the lava extending itself in a series of thin overlapping lava "toes". These miniature flows, perhaps only a foot thick, break out of the flow margin and move outward until their surce is depleted. A dark crust quickly forms over their surface as they advance. Other toes stack between those already in place, piling up against logs trees, until the forest is completely invaded by lava. Lava covering vegetation quickly ignite it causing explosive bursts of flame along the flow margin. The air reeks of wood smoke. As trees and logs are surrounded, they burn off near the lava surface and fall. By this time the lava surface has cooled, so they lay in great piles.

Sometimes a second advance of lava would cover the already felled trees, leaving perfect molds that can still be seen today. Lava tree and log molds are very well preserved in the Cave Basalt. Some of the caves have log molds intersecting the passages. These molds are easily identified by the pattern of charred wood etched into the basalt.

Near the center of the flow, and particularly in older parts, discrete lava channels developed and carried lava to the advancing front. These lava channels would become roofed over with lava solidifying toward the center from the channel walls. As solidified crusts thickened and become able to support themselves, lava tubes were formed. Tubes not only conducted lava but also allowed lava to travel many miles with little heat loss. The lava tubes acted as an extension of the vent, allowing lava to travel great distances.

In Ape Cave, and other nearby caves that carried lava for long periods, lava cut downward into the pre-flow land surface, in a process called "thermal erosion". This erosion created passages with high narrow cross-sections. In some cases wall linings gradually closed the tube above the moving lava. creating horizontal partitions and stacked passage levels. In other places, the walls collapsed when the buoyant support of fluid lava was withdrawn, exposing soil baked red by heat from the lava. As the lava level dropped, leaving an opening above the flowing stream, hot gases began to remelt the wall and ceiling surfaces, forming a dark shiny glaze. In places, the glaze slumped to form a pleated pattern.

Lava oozing from upper walls and ceilings created stalactites, and globules dripping to the floor created stalagmites. Lava formations are not common in the caves and where they do occur they are usually small and fragile.

Lateral "flow marks" (minor ledges) along the walls mark stages of lava decline in the tube. When the lava level dropped and then stabilized for a short time, a flow mark was produced along the wall, much like a bathtub ring.

Lava tubes are frequently obstructed by blocks of solidified rock rafted along in the lava stream. These frequently wedged at constrictions forming jambs of lava blocks or as isolated pieces such as the "Lava Ball" in Ape Cave. Over time some blocks would remelt and vanish while others remained sintered in place by a coating of once molten lava. Temporary obstructions caused lava to back up, rupture the tube roof, and spread across the surface. These breakouts often added to the thickness and strength of the tube roof. In places a new crust formed over breakout areas; other times they remained open as skylights. Where the lava supply persisted, the solidified crust was raised by the force of molten rock injected beneath. This process thickened the flow while creating a heaved surface of pressure ridges and broken plates.

FOLLOWING THE ERUPTION


At the end of the eruption molten lava remaining in the tubes gradually drained away, leaving an open cave. Tubes take a long time to cool, as much as two years. During this cooling breakdown of the walls and ceiling begin to occur. Breakdown is caused by contraction-cracking of the cooling walls and ceilings. Cooling lava shrinks .5% -3%, leaving the cracks seen today in walls, ceilings, and floors. When this takes place, some areas are unsupported, promoting collapse. If collapse extends to the surface, entrances are formed. Not all entrances are collapse related however. Many caves are accessed through skylights. Understanding the process will help in assessing the hazard potential of an entrance.

On the flow surface vegetation gradually began to return. First arrivals are air and animal borne spores of moss and ferns. Bare rock surfaces develop thick coatings of moss and lichens; gradually pioneering plants root where enough soil has accumulated to allow growth. Over hundreds of years, trees invade the flow, creating the start of a new forest.

When lava surrounded trees and logs, tree molds (casts) formed. Tree casts are usually vertical holes, extending all the way through the lava flow, some as much as six-feet in diameter. Log casts are horizontal openings, where the lava covered felled trees. Some are large enough to crawl through, such as the "Log-crawl" along the 'Trail-of-Two-Forests' interpretive trail. Log molds are floored sometimes by a honeycombed impressions created where molten rock infiltrated burning charcoal as the tree was consumed.

Filling of caves begins soon after formation. At Mount St. Helens, sandy floors are developed where volcanic ash, pumice, and other debris have washed in through entrances or cracks. Most sand deposits come from a 450 year old 4 eruptive sequence called the Early Kalama Period. Following the 1980 eruptions, heavy rains and snow melt washed fresh ash from the upper slopes of Mount St. Helens onto the Cave Basalt Flow. This entered several caves, depositing sand and gravel on the floors. Hopeless Cave, a small lava tube up slope from the main entrance to Ape cave, was completely filled. As you walk Ape Cave Trail you cross one of these alluvial flows.

THE WIND DOES BLOW!


When more than one entrance exists, significant air movement may takes place. Cave wind is caused by differences in air temperature inside and outside the cave. During the winter, warm cave air rises like smoke in a chimney, and pours from upper entrances. This chimney effect reverses itself during the summer when cool cave air drains down- slope and rushes out through lower openings or cracks. Cave wind in Ape Cave has been measured at seven (7) miles per hour.

LAVA KARST!


At Mount St. Helens, a watershed of approximately 17 square miles drains entirely beneath the Cave Basalt Flow. All precipitation falling in the watershed disappear into the lava, either directly or as sinking streams. Two large springs, one at Dry Creek, and the other at Cougar Creek are resurgences. Water has been dye traced from upper Cougar Creek at Grass Lake three miles underground to Cougar Creek Spring, in a little less than three days. This rapid ground water transit can only be accounted for by open channels beneath the lava.

In Little Red River Cave, Lake Cave, and Ape Cave, water channels have developed along the contact between the overlying lava and buried paleo soils. Water collecting beneath the lava enters the caves at points where wall linings have collapsed exposing the buried soils. These erosion channels are as much as several hundred feet long. Over the last few years, similar erosional caves have been found along the margins of the Cave Basalt Flow. Some carry water all year and are still being surveyed. The "master" channels which carry most of the water flow to the large springs have not been entered. but are expected to be similar to those already found.

Two caves, Little Red River and Lake Cave, contain seasonal streams and lakes at their terminus. Lake Cave only has water during the winter months, but Little Red River Cave has perennial flow, and a semi-permanent lake. The unusual name of this last cave comes from iron oxide stains found in the stream bed.

CAVE SOFTLY!


Caves at Mount St. Helens contain a variety of sensitive resources. These include rare or endangered plants and animals, geologic features, formations, unusual hydrology, deposits of bones, and unique strains of microorganisms that have been undisturbed for thousands of years.

The ultimate goal while visiting a cave is to avoid having a negative impact on cave resources. Many cave formations are extremely fragile. To prevent damage, stay on well worn or marked trails. If Sand Castles  (drip eroded deposits of volcanic sand) or clay covered floors are encountered, avoid walking across them. Your footprints may destroy thousands of years of natural sculpting and development.




Lava stalagmites and bead-like globules of lava drip are found on the floor in some caves. These are fragile and will be crushed if stepped on. Avoid these areas or stay on established trails. At no time is it justifiable to break formations or cause damage for the sake of exploration. Responsible cavers always back away from entering areas where their presence could cause damage.

Many lava tubes have tree roots hanging from the ceiling where they enter through cracks. These are a primary source of food for cave invertebrates, rare cave adapted insects that must survive in a food-poor environment. Avoid contact with roots or disturbance to any other organic material found in caves. Even removal of human transported wood from caves should only be done after examination by a cave biologist to make certain that cave insects are not using it as a food source.

Aquatic invertebrates living in pools or cave streams are particularly vulnerable. Avoid unnecessary wading or walking in permanent water bodies. Never wash your hand or rinse mud off boots or clothing in cave pools or streams.




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