Teepee (also spelled tepee or tipi) structures are

sedimentary structures Sedimentary structures include all kinds of features in sediments and sedimentary rocks, formed at the time of deposition. Sediments and sedimentary rocks are characterized by bedding, which occurs when layers of sediment, with different partic ...

interpreted to represent formation in peritidal environments. Teepees are largely the result of evaporation of water and subsequent

precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hai ...

of minerals within sediment, resulting in expansion and buckling to form a teepee-like shape. Their name originates from geologists working in the

Guadalupe Mountains The Guadalupe Mountains ( es, Sierra de Guadalupe) are a mountain range located in West Texas and southeastern New Mexico. The range includes the highest summit in Texas, Guadalupe Peak, , and the "signature peak" of West Texas, El Capitan, bo ...

, who noted their appearance in cross-section resembles that of a Native American teepee. Teepee structures have been observed in rocks more than 2.7 billion years old, and can be seen forming in modern environments on Earth, such as the Dead Sea.

Formation

Several mechanisms are involved in the formation of teepee structures, with displacive forces resulting from mineral precipitation likely the most important.

Displacive force from mineral precipitation

The precipitation of minerals (e.g.,

calcite Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scra ...

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and dr ...

halite Halite (), commonly known as rock salt, is a type of salt, the mineral (natural) form of sodium chloride ( Na Cl). Halite forms isometric crystals. The mineral is typically colorless or white, but may also be light blue, dark blue, purple, ...

) within sediments results in expansive forces. If supersaturated pore waters are maintained during mineral precipitation, the forces can be sufficient to deform sedimentary beds. This conditions would be particularly prevalent in areas with low rates of

and high rates of evaporation, such as sabkhas.

Hydration of salts

The hydration of some minerals is associated with a significant increase in volume. The hydration of

anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with t ...

to form

, for example, results in an increase of volume by 63%. In arid environments where anhydrite is widespread, a storm or

spring tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ...

could case rapid hydration and swelling, resulting in increased stresses and

buckling In structural engineering, buckling is the sudden change in shape ( deformation) of a structural component under load, such as the bowing of a column under compression or the wrinkling of a plate under shear. If a structure is subjected to a ...

within the sediment to form teepees. Teepee structure in the Belcher Islands

Thermal expansion

Different rocks expanded by different amounts when heated. A 10 m wide sheet of limestone will become approximately 4 mm wider when heated by 50°C, whereas a rock with significant amounts of

sulphate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride s ...

minerals may expanded by 12 - 24 mm in width. While this is likely insufficient for the generation of teepees by itself, it may work in conjunction with the other mechanisms.

Earthquake

Some have suggested that teepee structures may be the result of seismic activity deforming sedimentary layers.

References

{{reflist Sedimentary structures