Geology: Lava Caves vs. Solution Caves

What is a cave?

 A cave is a naturally occurring underground chamber or passageway, usually a large interconnected system of passageways and chambers. The United States Geological Survey (USGS) calls caves, "a natural opening in the ground extending beyond the zone of light and large enough to permit the entry of man." There are several different types of caves, including erosional caves, glacier caves, fracture caves, sea caves, and more.  This page compares a specific lava cave with a specific solution cave, both of which are located in Arizona.  The first cave we will explore is the lava cave.

Grand view of the Lava River Cave in Flagstaff, Arizona

Lava Caves

What is a Lava Cave?

 A lava cave is a cave formed by the cooling of lava (liquid rock).  It is a primary cave, meaning it formed at the same time as the surrounding rock. The diagram below, from the US Department of Agriculture, demonstrates its formation visually:

Lava River Cave, Flagstaff, Arizona

This placard is on display at the cave entrance.

 General Characteristics of the Cave Today:

Temperature: Year round, the cave is between 32˚F and 40˚F.  Called a natural "ice box," the inside of the cave remains nearly frozen, even when it is 90˚F at the mouth.  This is because cool air falls into the cave during the winter and the darkly colored, dense, basaltic rocks in the cave act as a insulator to keep it inside during the summer. Interestingly, the warmest part of the cave is not just inside the mouth, but rather at the very end- nearly a mile away.

   

Biology: The cave is home to small animals such as porcupines, bats, and squirrels, according to the Coconino National Forest. However, visitors are unlikely to find the animals, as human activity scares them away.  You can still tell that the critters are there, though, because their droppings are all around the cave walls. The cave is also home to lichen, bacteria, and fungi.

Fungus

How did it get here?  This photo was taken about a half mile from the cave entrance.  A person (or perhaps an animal) may have dropped food or detritus (something biotic), and now a fungi has made its home here.

 Cave Geology and Features: Even though this is a lava cave, and not a solution cave, water still seeps in from the sides and deposits minerals. Although lava caves may have water features, these are not what lava caves are known for.  The defining features of these caves happened during their formation. For example, when the lava was still flowing in the cave, the heating and cooling of the surrounding rock caused pieces to fall. Some of these rock pieces were not fully melted back into lava, and instead remained preserved in the floor as "splashdowns." 

Example of a splashdown

Cooling cracks of various sizes are also notable features that are found on the cave floor. When the lava was very hot, it flowed relatively smoothly. As the lava cooled, it contracted, no longer able to take up as much space as before. Cracks formed in the newly formed basalt to relieve this stress. 

Example of a large crack in the cave floor.

Another crack.

Lava caves can also have some ceiling collapse. Much like the splashdowns, when it was hot the rocks surrounding the "lava river" expanded, and as they cooled, cracks formed. No longer able to stay attached to the rocks above, large chunks of the ceiling fell down into the hardened lava.

Ceiling collapse (aka breakdown)

Here are some more photos of the Lava River Cave:

Water droplets near the entrance

As a photographer, I wanted to highlight different features of the cave.  Given the amount of tourism this cave receives, I now consider graffiti to be a defining feature of this particular location.

One explanation for these horizontal marks is that they are "flow horizons".  Flow horizons form as the lava level goes up and down.  This feature could have been formed over the course of ten minutes or ten years.

Notice the paper products left on the ground from some visitors.  Don't they ruin this shot?  PLEASE, if you plan on visiting this, or any other cave, follow the Leave No Trace guidelines!

This is a "lava tongue."  As the lava flowed, it cooled.  More lava came in behind it and congealed.  Imagine having thick pudding, and then pushing a spoon through it.  Quickly move the spoon, then slow down and bring it back a little bit, then quickly move it again.  A similar shape would arise.

The background of this photo shows breakdown, which may have occurred when the ceiling was heated during formation or from recent events such as ice getting in cracks, expanding, and weakening the rock.

Possible ceiling collapse, as well as background graffitti

In the upper left corner, you can see water seeping in and depositing minerals.  The vertical lines along the upper wall may be "melt lines", caused by melting lava running down the wall.

Solution Caves

What is a Solution Cave?

A solution cave is a cave formed by the slow moving of ground water through carbonate and sulfate (such as Limestone) rocks. The water dissolves the rocks to from large underground caverns with many passageways and tunnels. The solution Cave we explored was made out of limestone and located near Payson, Arizona.

How the Cave was Formed:

Karst Topography, a feature of Limestone caves like the one we explored.

 

Scout Cave, Gila County, Arizona

General Characteristics of the Cave Today:

 Temperature: The cave remains a roughly constant temperature year-round.  Warm pants and a long shirt are recommended.  It can get a little chilly when you stand still, so be sure to bring a light jacket.

Biology: While we were in the cave we did not find any evidence of creatures living there, other than some crickets. One local from Payson, Arizona had a report of a lone bat in the cave, but while we were there we did not find the bat or any traces of bats in general. Lithotrophs, bacteria that would commonly eat rocks and walls of the cave, could be present here, however we did not find traces of them either. They could still be present in Scout Cave, but it would have to be in small quantities.

Cave Geology and Special Features:  The cave is formed from water flowing through limestone rock and is still active and growing today. While we did not find evidence of new water coming from the ceiling, some of the sides of the cave were slick with water. 

Notice the shimmer of water

Stalactites and stalagmites are dominating features of the cave and are formed by the slow moving of calcite rich water through the surrounding limestone. This water seeps into the ceiling of the cave and evaporates to slowly form hanging cones of calcium carbonate called stalactites. The water that doesn't go into making the stalactites, falls to the floor to evaporate there and make cones of calcium carbonate on the floor. 

Notice the abundance of stalactites on the otherwise smooth ceiling

When these cones come together, they make columns, another feature of the cave. Draperies are found in the cave and according to the National Park Service, draperies "form from the same chemical process as stalactites, but in this case, the water does not get a chance to drip from the ceiling. Instead, draperies form when calcite-rich water runs down an overhanging wall", and are held there by surface tension. When impurities, often iron oxide, are present in the water, they form into darker strips. This feature, when present in thin draperies, is known as cave bacon.

Notice the column to the left.  The features that have light shining through them are an example of cave bacon.

More photographs from Scout Cave:

Semi-transparent vertical flowstone (drapery) in a flowstone wall.  Each little cup (bottom third of the image) is ripple, almost like a rimstone layer.  Some soda straws can be seen in the background.

Question:  Why is that a white light, shone through a white rock, produces a red light?

One possible explanation is that the lattice structure of the rock filters out short wavelengths (bluer colors) and lets through longer wavelengths (more red).  Click here to learn more about light and the electromagnetic spectrum.

Circled in red are the remnants of rimstones/pools that no longer contain water.
In blue, you can see a difference in color- the top is a light beige and the bottom is almost brown.  These lines are like the rings left in a dirty bath tub.  The cave may have been flooded during the last glaciation (ice age), which occurred approximately 12,000 to 20,000 years ago.  The bump, circled in green, may have been caused by a vertical drip of water that deposited minerals and created an underground stalagmite. The pink shows a flat ceiling, which may have been caused by a less permeable material or an ancient breakdown.

View of Scout Cave from the back looking towards the entrance.  You can see a 6'2" football player in the middle of the image, and if you look carefully, a girl lit up by flashlights near the back.

Stalactites that grew until they reached another surface, forming columns.

The top of this image is another example of flowstone.  Notice the texture on the floor.  This area used to be submerged in water.  The little structures are corralloides, or grape clusters and formed below the water line.

Thanks for visiting!  If you have any questions, please feel free to contact us at asugeokids@gmail.com.