Tubos Lávicos
www.goodearthgraphics.com/virtual_tube/virtube.html
All the photos on this page, except for the top one, were taken by scientists working for the U.S. Geological Survey based at the Hawaiian Volcano Observatory, and are used by permission of the USGS.
Nace un Tubo Lávico
Formation of lava tubes requires a special kind of volcanic eruption: those where fluid lava flows down the sides. The more explosive type of eruption known as pyroclastic, where rocks, cinders, and ash are ejected, don’t form tubes. Within fluid lava flows, there are two main mechanisms for forming tubes. In one type, the upper surface of the lava flow begins to cool, and the lava beneath continues to flow in tubular conduits beneath the surface. In another common scenario, channels carrying lava become roofed over as material accretes to the side, eventually forming a solid roof.
Due to the insulating effects of the hardened lava above, molten lava is able to travel considerable distance underground with very little cooling. In Hawaii, lava tubes have carried fluid lavas 50 or more miles from their source and thereby played a large role in shaping the islands.
Whether on the surface or underground, flowing lava behaves like most rivers would, flowing downhill, around obstructions. Lava flowing underground sometimes returns to the surface through existing skylights, or ruptures the surface when the flow volume becomes too high, creating a breakout entrance.
Skylights indicate an active tube flowing under a hardened crust
a closer view of an active skylight
Channelized lava with a large piece of hardened crust wedged in the center, which is one way in which a channel can roof over to form a tube.
another mechanism for a channel to roof over is by accretion of material along the edges that can coalesce to form a roof.
Flowing lava leaves its mark in many ways. The simplest of these are flow lines, etched into the walls of the cave or floor of the cave.. The lower photo shows a phenomenon sometimes seen in places where two major conduits converge, leaving a v‐shaped point aimed in the direction of the flow at the head of the passage wall that divides them. These are called “ship’s prows” for their resemblance to such a feature. The one below has an unusual hanging “mini‐prow” suspended above and pointing the same direction as the prow beneath it: downflow.
When bodies of partially hardened lava are pulled apart, by gravity or by flowing lava, stretch marks may result as shown in the first photo. Here, the stretched lava is on a portion of wall that has
been pulled downflow by the lava stream. Stretched lava may also
occur when a crusted floor slumps down on top of a contracting lava lake. Sometimes strands of lava, like stretched taffy, adhere to the two surfaces pulled apart, as shown in the lower photo.
While many lava tube passages tend to be sinuous passages, the cross‐
sections of lava tube passages vary from tall and narrow, to classic tube shape, to keyhole‐shaped. Passages tend to be tubular or oval‐shaped when they initially form, as in the top photo. As lava continues to flow through a passage over time, it tends to melt and erode down the floor and make the passage taller, as in the middle pair of photos.Passages can also be stacked, with new passages forming over older ones as underground channels roof over, as shown in the bottom photo. In some tubes as many as five distinct, stacked levels are known.
Lava flow characterized by ropy, billowy, bulbous textures. It is a Hawaiian word that literally means "smooth." It is the type of flow most likely to produce lava tubes. Lava tubes are usually floored with pahoehoe, but that pahoehoe often grades into a floor.
more frothy,
cauliflower a'a
The first photo is a classic example of ropy pahoehoe on a lava tube floor with a bit of glaze. The lower photo shows an unusual
variant found in some tubes that this author calls dendritic pahoehoe, but is also be a form of lava blade. It may result from lava flowing over Pele's hair.
dendritic pahoehoe
Lava flow composed of rough, jagged chunks, sometimes welded together but often loose, where the fragments are sometimes called clinker. Often a lava flow begins as pahoehoe lava and grades into aa as pieces of it start to cool and are carried along in the flow.
It is rare for lava tubes to form in a`a, but sometimes aa flows over older flows and enters pre‐existing tubes through skylights. Usually this ends up plugging up the passage inside, thereby segmenting a once‐continuous tube system.
In the photo below, cavers are preparing to enter a tube in the pahohoe lava. The darker material in the upper left is an a`a flow that came later, covering up portions of the pahoehoe flow. Fortunately for the cavers, it stopped just short of covering up the cave's entrance.
There are two common types of entrances to lava tubes. Either they are formed by collapse of the cave’s ceiling, or they represent the edge of a roofed‐over trench. If the collapsed entrance is relatively small in relation to the cave, roughly the diameter of the underlying passage, we tend to call it a skylight. The fourth photo shows a classic skylight as viewed from inside the tube. But perhaps the most common type of entrance is a collapse along a passage segment, which gives access to cave passage at either end. In Hawaii either of these collapse‐type entrances is known as a puka. More rarely, entrances may be found in cliff faces, such as those occurring in the ongoing eruptions of Kilauea in Hawaii, where tubes carry the lava all the way to the ocean and sometimes pour out of entrances in the cliffs.
Though initially formed by collapse, entrances may be modified by later lava flows, such as shown in the middle photo below. Here, a lava flow through the tube has overflowed one of the skylight entrances, depositing lava balls on the surface in the process, and has left a smooth surface where the lava rose up, before draining back into the entrance. The upper limit of the overflow is clearly visible here.
Depending on the location of a lava tube and the age of the lava, the entrance zone may form a unique ecosystem that harbors plant life not seen on the nearby surface. Particularly in arid regions, the tube may offer a supply of moist air that can harbor such things as ferns and mosses. In the bottom photo, ferns are found beneath a surface practically devoid of vegetation. Even in lush Hawaiian rain forest, atube entrance may provide a habitat for plants not seen nearby on the surface.
An opening to the surface formed where a portion of a lava tube's ceiling has collapsed. If this occurs after the tube has drained, a breakdown pile will be evident beneath the skylight. But if the tube was still active, or had subsequent flows of lava through it, the breakdown will be carried off or buried.
Surface skylights can be clearly seen during the current eruption sequence from Kiluea Volcano on the Big Island of Hawaii. Viewed from the air, they appear as a string of glowing red pearls against the blackness of the pahoehoe surface. Scientists at the Hawaiian Volcano Observatory use aerial photographs to map out developing lava tube caves as they carry hot lava to the sea. The first photo taken by an Observatory scientists, shows one of these windows into an active lava tube.
The photo below was taken from beneath a skylight in Maui's Ke'alaku Caverns. The cave is thousands of years old, so the surface is lush with vegetation.
In appearance, runners look like driblets of lava running down the wall of the cave. They are actually a form of tubular lava stalactite formed along a surface instead of free‐hanging. They may also form along the edge of a stalactite. Like tubular lava stalactites, they are formed of cooling segregated lava that is extruded from semi‐
crystalline wall matrices, propelled by boiling and escaping gases.
Many lava tube passages have a smooth, thin, metallic‐looking coating over the darker, coarser basalt underneath. Glaze frequently covers all the surfaces in a passage. The formation of glaze is a matter of dispute. Some feel that it represents remelting of the basalt from hot flows and gases in the tube. The Allreds (1998) suggest that glaze is "the continuous to discontinuous coating of magnetite crystals growing on either segregated or parent lava. Glaze grows after the segregated lava seeps or bubbles out". They found that the silvery luster of glaze is due to tiny facets of magnetite on the surface. Glaze can become red when oxidized, as near a skylight, or greenish when high in pyroxene.
Glaze is also used somewhat more generically to describe any lining with a smooth surface.
Pieces of lava tube ceiling or wall that have broken off and fallen to the floor. Much of this breakage occurs during the cooling phase, as shrinkage of the tube's lining occurs and contraction cracks form. Some ceilings in lava tubes contain very unstable rock that has not yet fallen to the floor, and caution must be taken during exploration not to dislodge it.
Breakdown of the ceiling may also make an opening to the surface, or skylight.
In the photo below we see breakdown that came from the collapse of the pahoehoe floor of an upper level. Clearly, the breakdown occured after the last sreams of molten lava had coursed through the lower level, or they would have been covered and possibly become welded breakdown.
Helictites are an eccentric or "vermiform" form of stalactite that twists and turns rather than growing straight up and down. They typically begin as tubular lava stalactites, but crystallization of the emerging lava as it cools pushes the lava in different directions. Often groups of these helictites will bend in the same direction down the passage, suggesting an influence of wind moving through the tube.
Helictites rarely have drip stalagmites lying underneath. Most likely, the crystallization at the ends of the tubes that redirect the flow also reduce it to the point that little material is ejected to fall on the floor beneath.
The top photo shows a classic group of helictites with some calcite or gypsum crust on them. In the second photo, some unusually long (over a foot) helictites formed on the end of some tubular lava statlactites. In the bottom photo, one can see just how densely these formations may cover the ceiling of a tube. Needless to say, these formations are extremely delicate and caution must be taken when exploring near them.
Smaller conduits carrying lava into a larger passage often seal off completely as the lava cools, if they haven't drained completely. In some cases a seal may form when breakdown blocks a passage and the flow is insufficient to overflow or carry off the breakdown.
In the photo below, it appears that at least four different flows were carried through an infeeding conduit before it sealed completely.