The Subsurface Life in Mineral Environments (SLIME) Team Frequently-asked questions
Cave Ferromanganese Deposits (a.k.a corrosion residues)

Desert/Rock Varnish Ferromanganese Deposits

Lava Tube Microbial Mats

Sulfur Cave Microbial Ecology---Cueva de Villa Luz

Aeromicrobiology in Caves

Antibiotic production by cave microbes

Cave Pool Precipitates Arthropod Communities of
Carlsbad Cavern and Lechuguilla Caves

Human Impact in Caves

Q: How do caves form?
A: The simplest definition of a cave is a hole in the ground, but caves are far more complicated than just simple holes. A more complex definition is that caves are any natural space below the surface which extends beyond the twilight zone, and that is accessible to humans. Some would debate that spaces beneath the surface of the earth, without openings to the surface, should be considered caves as well. Caves can be classified by the type of rock they form in and how they form. The most common caves are formed in limestone and as lava tubes in basaltic rock. Other types of caves are less common and form in gypsum, granite, talus, quartzite, ice, and sandstone.

The process of cave formation is an extremely long process because the natural mechanisms are so slow. There are three main mechanisms that aid in the formation of caves: rainfall, sulfuric acid and flowing lava.


Water does not form caves through underwater streams like many people believe. The process of cave formation with water all begins with rain. This rain seeps into the surface of the earth and absorbs CO2, carbon dioxide given off by decaying vegetation in the soil. This absorption of CO2 begins a chemical reaction, which makes the water into a weak form of carbonic acid that can eat away at rock by dissolving calcite from the rock. As this acidic water reaches the cave it releases the gas, carbon dioxide, into the cave. Carbon dioxide then aids in the creation of speleothems, like stalactites and stalagmites.

You can see an animation of this process.

Flowing Lava

Did you know that one of the main reasons scientists believe that there may be life on other planets is because of lava tubes? From photographic evidence scientists have determined that Mars at one time experienced a large number of volcanic eruptions. From this fact scientists have developed the notion that there may also be lava tubes and caves on Mars. The lava tubes on Mars would form the same way they have here on earth. As lava flows out after a volcanic eruption the surface lava cools more quickly than the lava under the surface and solidifies. When the volcano stops erupting the rapidly flowing lava may drain from under the solid surface lava and leaves an empty tube of space behind. And just as they do on earth caves on Mars may protect life from the effects of planets surface environment and allow them to survive!

Sulfuric Acid

Sulfuric acid is the most widely produced chemical in America . It is used in the production of gasoline and in some drugs. Naturally occurring sulfuric acid is just as important, for instance it is a key element in cave formation. Sulfuric acid forms mainly from microbes that live in oil fields beneath the earth surface. These microbes consume the oil from these oil fields surface and release hydrogen sulfide gas, which rises through cracks in the cave, until it encounters the area of the cave that contains oxygen of the cave, also known as the oxygenated zone. The sulfide gas combined with oxygen forms sulfuric acid, which dissolves the limestone in the cave walls and begins to form a cave.

Q: How do cave decorations (speleothems) form?
A: Cave decorations are officially called speleothems. The most famous speleothems are stalactites (they hold "tight" to cave ceilings) and stalagmites. We have already discussed how water is important to the formation of forming caves. Water is also important in the formation of speleothems, but cave decorations form from the exact opposite process involved in cave formation. Carbonic acid develops back into water because the carbon dioxide that the water had absorbed is released. This water then flows into the cave it then begins to decorate the cave by releasing minerals. The water forms speleothems in five different ways: dripping, flowing, seeping, and forming pools. Dripping: Stalactites (insert pic of stalactite) Stalagmites (insert pic of stalagmite) Flowing: Flowstone (insert pic of flowstone) Seeping: Helicite (insert pic of Helicite) Pools: Calcite Rafts (insert pic of calcite rafts)
Q: What are the different types of caves?
A: Needs to be written
Q: Are microorganisms actually found within the rock walls?
A: Yes, microorganisms have been detected by microscopy in the punk rock zone as well as in the ferromanganese deposits (a.k.a. corrosion residues). The microbial shapes we see include cocci, rods, filaments, and stalked bacteria such as:
Q: How can I protect caves?
A: Just our (human) presence in caves can be a threat to the cave and the life that lives in it. When we walk through the cave we may accidently break a formation (speleothem); crush insects under foot without realizing it; drop food particles, skin cells, or hair; or compact the soil, depriviing microbes and tiny insects of needed oxygen. Thoughtless people may write their names on the walls or kill bats that live in the cave out of fear of these animals.

There are also much larger scale threats. In rural areas, many people see sinkholes and cave entrances as convenient dumping sites, throwing cars, refrigerators, garbage, or dead cows into the sinkhole. Toxic substances leaching from cars and appliances, and organic carbon from decaying animal flesh enter the cave system. One cave in Indiana was on the receiving end of a massive gasoline spill that killed most of the life in the cave. We have a very poor idea of how connected the surface of the Earth is with the subsurface environment. What we do aboveground can really impact caves and our water supplies that often flow through these caves.

Q: How much organic carbon is available to the microorganisms in a cave such as Lechuguilla Cave?
A: Lechuguilla Cave is a low-nutrient (i.e. organic carbon) cave. There are no running streams or flowing water in the cave and small amounts of organic carbon enter the cave through dripping water that may take years to infiltrate certain areas. We have measured organic carbon and found it to be low: 0.002--0.103% in bedrock and 0.006--0.014% in punk rock.
Q: Aren't the corrosion residues simply detrital products?
A: No, we have determined they the mineral composition changes significantly from that observed in the bedrock and punk rock to that found in the ferromanganese deposits (a.k.a. corrosion residues). The bedrock at both Lechuguilla and Spider Caves consists of dolomite and calcite, in varying proportions, with trace amounts of clastic minerals such as quartz; clays including kaolinite [Al2Si2O5(OH)4], illite [(K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]], montmorillonite [(Na,Ca)0.3(Al,Mg)2Si4O10(OH)2·n(H2O)], and dickite [Al2Si2O5(OH)4]; potassium feldspar; apatite and Fe- and Ti-oxides. These accessory minerals are detectable by XRD only after acid dissolution of the carbonate minerals, and quartz usually makes up the bulk of the resulting insoluble residue. Secondary ion mass spectrometry (SIMS) was used to look for trace levels of Fe, Mn, Sr, and rare earth elements. In unaltered bedrock samples, Fe and Mn concentrations are in the 30-50 ppm range. In comparison, the mineralogy of the ferromanganese deposits is usually dominated by the aluminum hydroxide nordstrandite [Al(OH)3], by oxide minerals or by clays, mainly illite. The predominant oxides are goethite [α-FeO(OH)], lithiophorite [(Al,Li)Mn4+O2(OH)2] and todorokite [(Mn2+,Ca,Mg) Mn4+3O7·H2O] with subordinate lepidocrocite [γ-FeO(OH)] and/or hematite [Fe2O3]. Quartz is usually a minor mineral in the ferromanganese deposits. Additionally, a suite of phosphate minerals (including apatite and REE-rich phosphates such as monazite, CePO4) have been semi-quantitatively analyzed by energy-dispersive spectroscopy (EDS).
Q: Where have you published the results?
A: See our Publications page.
Q: What is "moon milk" (moonmilk)?
A: Moonmilk is a milky white coating on cave walls or other speleothems. Sometimes it is a thick cottage-cheese like layer but more commonly it looks like a thin (<2 mm) white powder or crust, often just on the tips or ends of bumps (popcorn, pool fingers, pool crust, etc.). When wet it has a greasy feel, like butter or crisco but dried forms are crumbly and gritty. The mineral can be calcite, hydromagnesite or aragonite but you really can not tell from the appearance.

There is an active argument amongst scientists on what role, if any, microbes play in the formation of moonmilk. Some workers interpret is as entirely abiologic but the presence of abundant microbes found by others suggest biologic involvement.

Q: How do isotopic studies tell us whether a speleothem formed under the influence of microbes or by purely chemical processes?
A: Isotopes are forms of the same element that have different numbers of neutrons in the nucleus. Carbon (C) has two stable isotopes, 12C with six neutrons (6 protons + 6 neutrons = 12) and 13C with seven (6+7=13) and one radioactive isotope C (6+8=14). Since 13C is thus heavier than 12C, the ratio between the stable isotopes can be measured in a mass spectrometer and compared to a standard. Positive values (e.g. 13C = +1) indicate more 13C than the standard.

Water contains dissolved CO2 that through a series of reactions is included in calcium carbonate. The mineral, therefore, records the carbon isotopic value of the water. The water value, however, can be changed by microbes as most microbial processes favor either 13C or 12C. Sometimes this change is very large (either positive or negative) and easy to identify; more often is it subtle and we need to analyze associated abiologic calcite to compare differences rather than simply values.