Deserts , geology?

Question

question about Caliche (calcrete):

Caliche forms in desert climate through the dissolution and reprecipitation of _________ during and after rainsorms.

I am not sure what should be in blank. chemicals out of rock ???
please help,

Answer 1

Caliche is a general term for any secondary calcium carbonate (CaCO3) that forms in sediments or in voids and crevices within bedrock just below the surface in semiarid regions, as a result of soil-forming processes (pedogenic caliche) or ground-water evaporation (ground-water caliche); it is material left behind by the evaporation of ground water or soil moisture that is no longer present at that level, although ground water may be present at much lower depths beneath the caliche.

Caliche has several forms:

thin, white crusts or rinds on individual pebbles and fillings in pores and crevices in soil or bedrock;
discrete, hard, white nodules or lumps;
or thick, massive, rock-hard accumulations that cement gravel, sand, and fines of a sediment, producing a dense and impermeable layer that resembles fresh-water limestone. Such massive caliche layers (calcretes) are common in deserts at depths of a few centimeters to about 2 m. The layers are a few centimeters to several meters thick.
Occasionally, caliche acts as a barrier to percolation of soil moisture from precipitation, helping to retain seasonal moisture near the root zone in vegetated areas. Some alluvial fans eventually become so plugged with caliche that surface runoff can no longer percolate into the gravel, producing short-lived but disastrous flooding in their terminal regions.
Origin

In arid and semiarid regions, the CaCO3 comes from capillary rise and evaporation of CaCO3-charged ground water, from calcareous dust blown by wind and then driven into the soil by episodic rainfall, and from infiltration of soils, sediments, and rocks by runoff from areas containing sources of CaCO3 (primarily limestones). In vegetated areas, CaCO3 can precipitate out around the roots of plants. The relative contributions to caliche formation by these various processes, and the time relations represented by the different types of caliche in general, are not well defined.

Because water must be present in the soil or at the water table to evaporate and leave behind the CaCO3, formation of caliche requires a climate that is semiarid to subhumid, but caliche commonly persists as a relict feature in areas whose climate has changed to arid or extremely arid, as have parts of the Sahara and the southwestern U.S. It does not persist in areas that have become wetter, because there it is dissolved and leached from the soil. In the Sahara, the removal of overlying soil layers by wind has exposed the calichified zone of underlying alluvial sediment. The exposed caliche (calcrete) weathers to a dark-gray, very hard "kunkur" that resembles bedrock. In many areas, especially in broad alluvial valleys, it is only thinly veneered by windblown sand and provides a solid substrate beneath the sand plain. The presence of caliche less than a meter beneath loose sand in arid regions can be detected on many L band radar images acquired by spacecraft. Certain tones, textures, and colors on Landsat multispectral images can also be used to delineate large exposed areas of caliche deposits.

Significance

Because caliche is common in sediments of alluvial plains, these plains will support vehicular traffic, and movement across areas underlain by caliche can be rapid. The presence of massive, hard caliche (calcrete) beneath a few centimeters of loose surficial sand makes these surfaces easily trafficable, for the caliche will support trucks and other wheeled vehicles, whereas deep, soft sand will not (see Summary for Sand Plains/Sand Sheets). For trenching, such caliche is an impenetrable barrier to all but mechanized equipment. Trenches dug with a backhoe in a thick caliche zone have vertical sides that stand up with little support. Sediments below the caliche, however, are likely to be loose.

In semiarid areas, downward percolation of water from rainfall and runoff is inhibited by the presence of caliche layers, and grasses and shrubs in these areas may be sustained by soil moisture from precipitation better than vegetation in areas lacking the caliche at shallow depths. Caliche at depth, however, prevents ground water from rising to the surface. Where caliche is nodular or broken by erosion, gravels of rounded caliche are a common surface lag, whose presence is highly indicative of a caliche layer at some depth. In some areas, such as parts of northeastern Saudi Arabia and southwestern Iraq, thick layers of calcrete have been partially dissolved by rain and ground water, and have developed solution pits (sinkholes) that can be hazards to cross-country travel (Chapman [1]; Felber, et al. [2]).

Foreign Names and Synonyms

(Common names are in bold) Hardpan, calcrete, kankar or kunkur (India, Egypt), duricrust (some), croute calcaire (Tunisia), carbonate platform (Egypt).

Answer 2

calcium carbonate (CaCO3)

Many southern Arizona soils have layers of caliche either on or under the surface. Caliche is a layer of soil in which the soil particles have been cemented together by lime (calcium carbonate, CaCO3).

Caliche is usually found as a light-colored layer in the soil or as white or cream-colored concretions (lumps) mixed with the soil. Layers will vary in thickness from a few inches to several feet, and there may be more than one caliche layer in the soil.

http://ag.arizona.edu/pubs/garden/mg/soils/caliche.html

Answer 3

In all likelihood, it's Calcium Carbonate. For the record though, any "Carbonate" can leach up into the soil from below. Also, in Chile and Argentina, caliche can be composed of Sodium or Potassium Nitrate.

Answer 4

The answer is Calcium Carbonate (CaCO3).