TITLE     INTRODUCTION      DETRITAL ROCKS       

BIOCHEMICAL ROCKS      ENVIRONMENTAL CLUES      CREDITS

CHEMICAL SEDIMENTARY ROCKS

evaporites        chemical limestone         tufa

As the name implies, chemical sedimentary rocks form due to chemical reactions.   These reactions occur within water that is saturated with positively and negatively charged atoms.   Such atoms are naturally attracted to each other, and will form ionic and covalent bonds as they crystallize into solid mineral crystals.   This process is called precipitation.   Precipitation can occur within an ocean, a lake, a cave, or where a hot spring flows out onto Earth's surface.   Minerals formed due to precipitation can be quite useful; for example, halite is the salt that people sprinkle on their hamburgers and french fries, and gypsum is the main ingredient in plaster of Paris and sheetrock.   As minerals precipitate from water, they tend to grow attached to previously formed crystals.   The resulting rock is composed of intergrown mineral crystals, usually all of the same composition.   The text and images below illustrate three of the most common types of chemical sedimentary rocks.

 

Types of Chemical Sedimentary Rocks

evaporites      Evaporite chemical sedimentary rocks form as water evaporates from a lake or ocean.   During the process of evaporation, water molecules change from the liquid phase to the gas phase, but atoms such as calcium, sodium, and chlorine are left behind.   As a result, the remaining water becomes enriched in these atoms which begin to precipitate from the water.   Evaporite minerals and rocks tend to form in arid climates where the rate of evaporation greatly exceeds rainfall.   Therefore, evaporites are common where lakes form and then evaporate in a desert (such lakes are referred to as playa lakes), or in shallow and warm arms of the ocean adjacent to desert areas.   Evaporites are light in color, and so can be readily recognized, even from a great distance.

Image 1 shows a dry playa in northern Morocco.   Image 2 is a playa lake in California that is in the process of evaporating.   Enlarge the image to see the rings of evaporites surrounding the remaining water.   Image 3 is a close up view of a playa lake surface, an extremely inhospitable environment for most life forms.   Image 4 show the remains of a bird that probably became poisoned by the salty water in a Mojave Desert playa lake.   If buried by more sediment, this carcass would make an excellent fossil.   Image 5 is a thick deposit of rock gypsum within a sequence of sedimentary rocks exposed in the Rocky Mountains of Colorado.   Development of such a thick layer of evaporite rock would have required continuous evaporation of an arm of the ocean over a long period of time.   Geologists interpret this layer of rock to represent a shallow inland sea that existed several hundred million years ago in what is now the mountainous state of Colorado.   Image 6 is a close up view of rock salt.   Enlarge this image to see the individual halite mineral crystals which are composed of atoms of the elements sodium and chlorine (NaCl).   (Thanks to Dr. Stan Finney of CSU Long Beach for the use of images three and five.)

   sed_evapplayaMorocco_1.jpg (22256 bytes)  sedlakeevaporites1.jpg (33142 bytes)  sed_evapplayacloseup_1.jpg (64095 bytes)  sed_evaptoxicsaltBIRD_1.jpg (60477 bytes)  sed_evapGypsumColo_1.jpg (56092 bytes)  sedrocksaltandhalite1.jpg (24369 bytes)

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chemical limestone      Most limestone forms due to biochemical processes, but the limestone called travertine forms in two different ways due to chemical reactions between charged particles called ions.

Wherever water saturated in calcium and carbonate ions evaporates or carbon dioxide escapes from the carbonate ion, layers of calcium carbonate will build up forming terraces and other odd shapes.   This typically occurs where hot springs flow out at the surface, but can also happen where a stream transporting lots of ions splashes or spills onto its banks.

         opalterrace.jpg (16682 bytes)    Travertine terraces of Yellowstone National Park (photo from the NPS web site).

 

           travertineJemez1.JPG (46965 bytes)     Travertine ledges and drapes along the Jemez River, New Mexico.

Travertine also forms underground as groundwater enriched with calcium and carbonate ions drips from the ceilings of caves and caverns.   This slow, repeated dripping allows the calcium and carbonate to bond together forming tiny crystals of the mineral calcite.   As more calcite crystals form they develop elongated structures called stalactites that hang downward from cave/cavern ceilings.   Water that drips to the floor of a cave/cavern can form stalagmites that gradually grow upward from the floor.   Sometimes stalagmites and stalactites merge together forming columns within a cave/cavern.   The images below are from Carlsbad Caverns National Park, and were taken by Park personnel.

         Carlsbadsmallstlacatites1.jpg (17253 bytes)                           Carlsbadstlagmites1.jpg (14229 bytes)                              Carlsbadsodastraws1.jpg (16303 bytes)  

                                       Tiny stalactites.                            Large stalagmites.                   Stalactites and stalagmites, and two columns.

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tufa      Tufa forms where a natural spring flows into a lake.   Precipitation of calcium, carbonate, and any other ions will occur instantaneously around the spring vent.   This leads the development of tufa towers or bulbous cauliflower-shaped structures that are relatively porous when inspected closely (image 4, below).   Image 1 shows the classic examples of tufa outcrops at Mono Lake, where tufa towers have been exposed due to artificial lowering of lake level.   Images 2, 3, and 4 are of the Trona Pinnacles in the Mojave Desert which developed in the past when a much wetter environment formed large lakes in this region.   The weird landscape around the Trona Pinnacles has been used in science fiction shows such as Star Trek and Planet of the Apes.   Relatively small, bulbous-shaped tufa deposits form where spring water seeps slowly into a lake, such as the tufa in image 5 which now sits high and dry on a ridge in the Mojave Desert near Barstow.        

sedMonoLaketufa1.jpg (34251 bytes)      sedTrona1Pinnacles1.jpg (45621 bytes)      sed_tufaTrona_1.jpg (184535 bytes)      sedTrona1closeup1.jpg (56205 bytes)     sed_tufaRBasin_1.jpg (44597 bytes)

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