The Caves of Central Oregon

with descriptions and figures of common and uncommon lava tube features
By Matt Skeels, Photos by Brent McGregor

Last updated 2022.06.14
The Monastery Cave showing Brent McGregor at a lava fall.

       Bend and its sister cities of Redmond, La Pine, and Sisters, act as a central hub for caving activities in Oregon. They reside in Deschutes County which contains the most caves in Oregon with a current count over 858! It is projected that the amount of caves in Deschutes County will continue upward toward 1,000 as new caves are discovered each year.
       While 858 is a staggering amount of caves, it is not a very diverse group since 702 of those are lava tubes. Out of the remaining 156 caves, the majority consist of caves that originated from lava, usually in the form of OVCs (open vertical conduits), spatter cone caves, inflationary caves, fissure vents, bubbles, and lava molds. Including these with the 702 total, that leaves only 60 caves that were not specifically created by processes that involved actively flowing lava. However, many of the remaining caves are located in volcanic bedrock of one kind or another.
       Out of the 60 non-lava caves, most are fissures, which are giant cracks that have split the volcanic rock apart. Fissures form along rift zones which occur along shield volcanoes or calderas. Rift zones are created by internal pressure from upwelling magma below. The pressure of the rock splits the crust apart leaving fissures, some of which can be up to 150 feet deep in the central Oregon area.
       Rockshelters, sometimes called just shelters, are small overhangs of rock and also make up a good portion of the remaining 60 non-lava caves, even though a lot occur in volcanic related rocks such as welded tuff or andesite. Rockshelters can form in various ways, usually through erosional processes that carve out bits of rock over time. The most common erosional processes are water, wind, and gravitational erosion. Most rockshelters form by erosional means and can also be labeled as erosional caves. Shelters are prevalent throughout our area, often occurring in the rim rock of ancient lava flows, or in the eroded faces of welded tuff such as at Smith Rock.
        Some of the remaining caves that are not fissures or shelters are snow caves, also called ablation caves. Snow caves are not to be confused with glacier caves, which as the name suggests, form in glaciers. Snow caves in Deschutes County mostly occur in locations where heat venting occurs near volcano summits leaving melted snow cavities. These snow caves are usually a temporary occurrence and form each winter.
       Finally, a few talus caves exist and defined as loose boulders that have fallen in a manner that leaves interior spaces between them. They typically occur at the base of rim rock or along faults that have dislodged rocks haphazardly.
      This is the makeup of caves in Deschutes County, but in the greater central Oregon area the diversity increases a bit. While more lava tubes and other kinds of lava caves are found in the adjacent counties, some unique and fascinating “piping” caves exist. Piping is a form of water erosion that occurs internally within unconsolidated rock material. Central Oregon holds the unchallenged distinction of having the largest piping cave in North America.

Let’s talk lava caves

       The central Oregon region is dotted with cinder cones and volcanoes. Lava flows of andesite, basalt, rhyolite, and dacite ranging from pahoehoe, aa, to block lava. Lava caves can form in all of these different kinds of lava flows, but the large majority of them will be found in basaltic lava flows with a pahoehoe consistency. Lava tubes are the most common type of lava caves and generally speaking, they form when an active lava flow surface begins to cool and harden but the interior remains insulated forming horizontal conduits that channel the extruding lava down the path of least resistance. Once the lava ceases to flow, any remaining lava in the conduit drains and leaves behind a lava tube. Sometimes it can take a year or longer before it's cool enough to enter.
       Deschutes County currently has 702 known lava tubes, though many more are expected to be found, and yet still more are speculated to exist under the landscape but lack an entrance. Lava tube entrances form during the initial eruptive event or immediately after when the lava flow cools and contracts. Weak structural points in the lava tube ceiling cause it to collapse, leaving a hole to the surface. It's also possible that lava tube entrances never formed a roof to begin with. These are usually found very near the vent of the lava flow where the temperatures were too great to allow cooling or inconsistent flow volumes never allowed a roof to form.
       Lava tube systems are a series of lava tubes that belong to a single lava flow. A good example of this is the Arnold Lava Tube System (see photo just below). A system is also typically distinguished as having a main lava tube conduit, sometimes called the master tube, that functioned as a conduit through which most of the lava was channeled. This is a key distinction between other lava flows that have lava tubes within them, but lack a master tube. Instead, these flows have multiple tubes that dispersed the lava in multiple directions. This usually results in lava tubes of a smaller size.Wind Cave of the Arnold Lava Tube System   
       This region has two well known lava tube systems, the Arnold and Horse Lava Tube Systems, both of which are roughly 80,000 years old and were formed from a singular event on the north flank of Newberry Volcano. There are other lava tube systems in the area, but they are not as developed as the Arnold and Horse systems. Sometimes they can have as few as four tubes in the system, and others can have dozens of caves.
       You will rarely find a cave in central Oregon that is larger than those of the Arnold and Horse systems. These extensive systems are home to some of the biggest and longest caves in Deschutes County. The majority of the caves in the Horse system are on private property and are left to the whims and wishes of the landowners. By contrast, all but two caves of the Arnold system are on public land and they see a tremendous amount of impact from your average and not so average visitor. These caves get a fair share of trash and intentional vandalism every year, so it is imperative that we keep these caves safe and clean so we may enjoy them for years to come.
       The lava tubes of central Oregon are found pretty much everywhere. Out in the high desert, up in the mountains, and in the city of Bend or elsewhere. They have been here for thousands of years, in some cases hundreds of thousands of years. The Native Americans utilized them right up until westward expansion brought in European settlers. When the settlers formed the town of Bend, they mined Arnold Ice Cave after Hugh O’Kane cornered the ice market. Even now, lava tubes have been incorporated into the structures of residential homes, often used as a natural and cheap way to cool the house during hot summer months.
       There are three main types of lava tubes in our area: unitary, multilevel and multilateral. Boyd Cave near Bend is a classic example of a unitary tube. It is a single tube that does not have multiple passageways. Multilevel tubes are lava tubes with more than one level. The tubes sit directly over one another and are traversable through connections in the floor or ceiling. Multilateral tubes are lava tubes with connected paralleling tubes, sometimes with as many as three or four paralleling tubes.
       Similar to lava tubes, surface tubes form on the surface of lava flows as their name suggests. They usually have a narrow and small cross section, sometimes restricting exploration to only the smallest of animals. In some cases, they are large enough for a human adult to explore fifteen, twenty feet or more. Surface tubes are formed when internal lavas from a still active lava tube extrude to the surface. In some instances, surface tubes can connect to larger conduits below. Surface tubes, like lava tubes, can take on different shapes by having multiple interconnecting tubes.Worm Sign Cave Spatter cone caves are lava caves that form in, or from, spatter cones. Spatter cones are like miniature volcanoes that are located at the main vent of a lava flow. They sit atop emerging lavas and gradually from miniature cones, sometimes leaving internal cavities upon cooling (see photo right - photo by Matt Skeels). Spatter cone caves can simply be a hollow spatter cone, but others can take on similarities to surface tubes, and in some cases connecting a surface tube to a spatter cone.
       Open Vertical Conduits, or OVCs, are vertical shafts that form at the main vent of a lava flow. They are similar to spatter cones but are often much larger in scale, originate deeper under the surface and are typically adjoined to the base of a cinder cone. In most cases, OVCs are usually singular shafts and are examples where lava breached the surface long ago. In rare circumstances OVCs can overlie lava tubes and connect to them. OVCs in central Oregon range from 4 feet to 150 feet for the deepest known. To safely explore an OVC, training with vertical gear and the single-rope technique should be used.
       Inflationary caves are another kind of lava cave and often resemble lava tubes. Inflationary means the outer malleable crust of the lava flow inflates, or expands, creating caverns within. In some inflationary caves, as the surface is expanding, the exterior wall linings lose their hold on the more solid interior and can begin to roll up as gravity takes over. This is called scrolling. Lava scrolls are common in inflationary caves and many can be observed in the Matz Caves. Some other inflationary caves are created when pressure ridges (also called a tumulus or tumuli) have their internal lavas vacate. These kinds of lava caves are very uncommon. They are usually entered on the top of the pressure ridge through its tension crack.
       Fissure vents or rift tubes are hard to discern from regular lava tubes because they share so much in common. While technically speaking fissure vents can be considered a lava tube, they have a unique exception that sets them apart. They are lava tubes that form in rifts or fissures, usually at a lava vent from where the lava is extruding. Formation happens when internal pressure from lava cracks the surface and forms a fissure. Subsequent lavas then flow into this rift forming narrow lava tubes with high ceilings. There are only two examples of rift tubes in our area.
       Lava bubbles can form in basalt, but our most common examples in central Oregon formed in rhyolitic obsidian flows. The formation of bubble caves happens when pressurized bubbles form in partially molten rhyolite. These bubbles can reach the surface leaving depressions or partial collapses when they "pop". The obsidian bubble caves of central Oregon are remnants of this activity and some of the only known examples in the world.
       Lava tree molds, sometimes erroneously referred to as lava tree casts, are empty cavities that once held the trunk of a tree. Typically formed in aa flows, a tree is partially or wholly engulfed by the advancing lava. As the tree burns, the lava cools just enough to form a crust around the tree trunk. The tree trunk completely burns away leaving a hollow interior. Both vertical and horizontal tree molds have been found, sometimes leaving their bark imprints in the mold lining. In Washington state, some horizontal tree molds connect two or more trees together. In California, one particular lava tube has vertical tree molds within the main passage and they appear as skinny lava poles within the lava tube.

Breakdown in Pictograph Cave.

Common and uncommon features of lava tubes

       Features of lava caves and lava tubes have been documented over the past decades. Some are rare, while others are commonplace. The lava tubes of the central Oregon region share many attributes. Probably the most common attribute is “breakdown.” Breakdown is a feature that forms while the lava tube is cooling and contracting. Walls, or parts of ceilings, collapse leaving boulders inside the cave (see photo right). Over time, parts of a lava tube can continue to collapse, though the majority of lava tubes have withstood thousands of years and survived hundreds of earthquakes without losing their integrity.
       Sand is also another common feature and can invade a subterranean cavity by creeping through cracks on the surface and washing down with rain water (see Figure 1). It may also drain into a cave from a natural collapse entrance from rain runoff. Over time, the sand and dirt collect in areas and can be acted upon by dripwater to form elaborate shapes called “sand castles.” The commercial Lava River Cave used to have some outstanding sand castles, but generations of visitors have treaded on them reducing them to a ghost of their former self. Sand castles are one of the rarest formations in central Oregon and when they do exist they are not extensively developed. Only a handful of lava tubes have them and only three outstanding examples remain (see Figure 2).
    Ice formations in Infinity Cave.   Original features from lava are very common. They have names like “original floor” or “original lining” which means the floor and walls (respectively) appear as they did when the lava stopped flowing many thousands of years ago (see Figure 3). Plenty more flow features exist like cauliflower aa, which is a kind of original floor that shows the transitional stage from pahoehoe to aa inside the lava tube. It forms spiny or knobby bumps in the floor. Another uncommon original feature of lava tubes are shelves. They represent previous lava flow levels within the tube. (see Figure 4)
       Perhaps some of the most photogenic features of lava tubes are the delicate stalactites and stalagmites. Both are formed by dripping lava. A stalactite forms on the ceiling where lava seeps from ceiling cracks or from behind a wall lining. If this lava drips onto a stationary floor, it will collect into slim or stout spires called stalagmites (see Figure 5). Contrary to some beliefs, these stalactites and stalagmites will not grow back if damaged. Like all lava features in a lava tube, once the lava has stopped flowing it is done forever. This is why it's important to maintain a hands-off approach to these and other features. One misstep and you will have destroyed a feature that lasted for millenia.
       Secondary mineral formations are uncommon in lava tubes, but a few kinds exist. Gypsum minerals can form along the ceilings, walls, and floors and look like a white thick crust. Gypsum usually forms where water has invaded a cave in small quantities and air exchange has been kept to a bare minimum. One cave in particular, Feather Bed Cave, was broken into by a pipeline crew. It previously had no entrance and its floor held a bed of gypsum powder one foot thick (see Figure 6).
       Coralloids are another kind of secondary mineral that is very common in the semi-arid lava tubes of central Oregon. They are primarily siliceous in nature and very small. They look vaguely similar to coral, hence the name, but often take on the color of the surrounding lava rock. The size of coralloid is usually about the size of a bead, but larger specimens can be as large as a dime (see Figure 7). They are extremely fragile and unlike other features they will grow back, albeit over hundreds to thousands of years.
       Another very common mineral formation is ice (see photo above). While not typically thought of as a mineral, it is nonetheless present in many lava tubes during winter months and late spring. While it does have the benefit of growing back rapidly every year, it is still treated delicately and not to be touched so others can admire its unique subterranean character. In some caves, ice remains year round and does not melt out during the summer. In these cases, it is very important to maintain a no-hands approach. Sometimes lava tubes can be referred to as “ice caves” which is misleading, because a lava tube is not primarily made out of ice. If a cave harbors ice year round it may be referred to as an ice cave.
       Lava falls, like the name implies, resemble a waterfall but consist of lava rock. Unlike a waterfall, lava falls are no longer flowing, but are frozen in time, capturing the remnants of lava as it cascaded down an abrupt drop in the floor (see Figure 8). The only freestanding lava falls known are in Manjanggul Cave, South Korea. 


Wolff's Teepee Cave

Figure 1 - The sandy floors of Wolff's Teepee Cave.

Matt Skeels with the sand castles of Eternity Cave

Figure 2 - Some of the last remaining sand castles in Oregon. Note that some are half as tall as Matt Skeels in this photo.

Lost and Found Cave

Figure 3 - Original floor and walls in Lost and Found Cave.

Kara Mickaelson in Infinity Cave.

Figure 4 - Benches in Infinity Cave. Usually benches overhang a little.

Sam Loomis by a large diameter stalagmite

Figure 5 - Sam Loomis by a large diameter stalagmite.

Gypsum bedding

Figure 6 - Thick gypsum deposits in a cave of eastern Oregon.

Large coralloids in Stookey Ranch Cave

Figure 7 - Large coralloids in Stookey Ranch Cave.

Brent McGregor at the top of a lava fall in Dynamited Cave

Figure 8 - Brent McGregor on top of a lava fall in Dynamited Cave.

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