tipos de rocas

rocas comunes :
Common soils of the world 
Soil formation depends on parent material, climate (temperature and moisture) and biotic factors (mainly acidity, soil moisture content and drainage). Since the minerals and elements that make up the parent material are well mixed over the planet, the main groups of soil follow the main habitat types of the world.
Here are the main soil orders, in alphabetical sequence.

• Alfisol: soils with clay-enriched subsoils. Mineral soil, relatively low in organic matter, high base saturation. Clay content increases with depth to an illuvial horizon of silicate clays (argillic horizon), which restricts soil depth and root penetration, resulting in poor cropyields. Soils are sensitive to crusting. Moisture is adequate to mature a crop in most years. Mostly derived from granites, gneisses and schists but occasionally from sandstone, mica, quartzite and shale.
• Andisol: volcanic soils of tuff and ash.
• Aridisol: soils of deserts and semi-arid regions, and related saline or alkaline soils. Mineral soil, relatively low in organic matter. Inadequate moisture to mature a crop without irrigation in most years. some pedogenic horizons.
• Entisol: soils without layering, except after ploughing. Mineral soil with weak or no pedogenic horizons. No deep, wide cracks in most years.
• Histosol: bog-type soils. Organic in more than half of the upper 80 cm.
• Inceptisol: very young soils with weakly developed soil layers, and not much leaching or mineral alteration. Mineral soil with some pedogenic horizons and some weatherable materials. Enough moisture available to mature a crop in most years. No horizon of illuvial clays. Relatively low in either organic matter or base saturation, or in both.
• Mollisol: grassland soils, mostly rich in calcium; also forest soils developed on calcium-rich parent materials. They are characterised by a thick surface layer, rich in organic material. Mineral soil with thick dark surface horizon, relatively rich in organic matter. High base saturation throughout. No deep, wide cracks in most years.
• Oxisol: soils that are more weathered than the ultisols, including most laterites. Mineral soil without weatherable materials. Inactive clays. No illuvial horizon of silicate clays.
• Spodosol: soils with a light, ashy gray A horizon, and a B horizon containing inorganic matter and clay leached from the A horizon. Mineral soil with an illuvial horizon of amorphous alumina and organic matter, with or without amorphous iron.
• Ultisol: soils similar to alfisols, but with weathering more advanced. They include some lateritic (highly leached) soils. Mineral soil with and illuvial horizon of silicate clays. Low in base saturation. Moisture adequate to mature a crop in most years.
• Vertisol: soils with upper layers mixed or inverted because they contain expandable clays (that swell when wet and crack when dry). Clayey soils forming deep, wide cracks at some time in most years.

The following table shows where the main soil orders in the world are found. Numbers are millions of hectares. (Various sources, from B A Stewart et al: Sustaining the world base of an expanding world agriculture, in Lal & Pierce: Soil management for sustainability, 1991). Of the total land area of 13,000 Mha, 8,800 Mha are productive land, forest or farmed. 

soil type	world Mha	USA	tropics	subtropic Africa	subtropic Latin Am	subtropic Asia
alfisols aridisols entisols histosols inceptisols mollisols oxisols spodosols ultisols vertisols mountains, lakes, etc.	1730 2480 1090 120 1170 1130 1120 560 730 230 2810	125 107 74 4.8 170 323 0.1 47 120 9.1 54	800 900 400 - 400 50 1100 - 550 100 600	466 440 255 - 38 - 188 - 24 51 -	107 33 17 - - 78 - - 8 - 70	121 47 - - 28 - - - 20 80 23

The world's ecosystems depend primarily on their plants, which in turn depend mainly on climate. Soil formation depends on the same climatic factors as these, and on the acids produced by the plants. It is not surprising then, that the main soil orders of the world follow the world's main ecosystems. The diagram above shows the vegetation type (and ecosystem) as a function of both temperature and rainfall. The boundaries between the various ecosystems curve downward, going from left to right (hot to cold). This is because more moisture is needed in warm climates, in order to offset their high evaporation losses. So, an amount of 1m rainfall in the tropics is barely enough for dry savanna grassland, but in the polar regions will lead to the formation of bogs. Important is not the amount of rainfall but whether it is more or less than natural evaporation. Below follow the soil characteristics, going from cold to warm climate.

• Subglacial desert: vegetation is almost absent, weathering is very slow and the soils formed are very shallow (lithosols). High mountain soils are similar but erosion is high.
• Tundra soil: Tundras. Dark-brown, peaty layer over grey horizons, mottled with rust. Vegetation is sparse, consisting of mosses, lichens, herbs and shrubs that do not produce very acid humus. Rainfall is low (20-100 mm/yr). The bases that are leached from the surface, accumulate in the lower part of the profile, where further drainage is impaired by permafrost. Acidity is insufficient to podsolise, but it reacts with iron to produce brown soils. There is often a humus-rich layer on the surface of the permafrost horizon.
• Podzol: Leaf litter over a humus-rich layer over a whitish-grey to greyish-brown leached layer. B horizon clayey and brown, acidic. Taiga coniferous forest and mountain coniferous forest. With a short summer and conditions not too frosty for coniferous trees, highly acidic humus is produced, causing podsolisation. In waterlogged places, organic matter accumulates and the reducing-conditions make gleisolic grey soils. Humus podsols are more common in Atlantic regions because of their higher rainfall. The podsolisation process leaches aluminum and iron bases into the B horizon.
• Brown forest soil (brunisol): Broad-leaved forest and meadows. With a longer summer, deciduous trees prevail, which produce milder, less acidic humus. These soils are brown and less leached. Where rainfall is low and parent materials are calcareous or basaltic, the formed soils are neutral (brunisol= braun erde= brown earth). In places with high rainfall, soils become acid (brun acide).
• Chernozemic regions, lightly leached dry soil: (chernozem= black earth) Black to grey-brown crumbly soil to a depth of 90-120cm grading through lighter colour to a layer of carbonate accumulation. Climate subhumid, temperate to cool. Temperate grasslands. A grassland vegetation with chernozemic soils occurs when spring is not dry, winter is relatively cold, and leaching rainfall is low. Calcareous, gypsic or salic horizons are frequent. Solonchaks, solonetz, and planosols are common where drainage is poor. Prairie soils of America are inbetween chernozemic and brunisolic soils.
• Cinnamonic regions, red and yellow subtropical soil: Subtropical scrub and grassland. In climates with dry seasons where soil is dried thoroughly to considerable depth, fine crystallites of irreversibly dehydrated iron sesquioxides (Fe2O3) are formed, giving reddish colour and cinnamonic soils. Organic matter decays rapidly and does not accumulate on the surface. Humus originates chiefly in roots and is well distributed along the profile.
• Desert soil: Light grey or brown in north, reddish in south. Low in organic matter. A carbonate layer generally within 30cm of the surface. Climate arid, cool to hot. Vegetation is very scarce in the desert (desert shrubs), and soil erosion is very severe from wind and occasional rains. Weathering is slow and shallow, and leaching is almost absent. Soils are very poor in organic matter and do not have a humic horizon. Because sodium elimination is difficult,  autochthonous soils are usually planosols and solonetz with very thin horizons. A calcareous horizon usually underlies the B horizon. Soils are often covered with desert pavement. Materials eroded from higher land, accumulate in depressions, forming dunes and alluvial soils.
• Kaolinitic regions: Tropical rainforests. The high leaching rainfall of tropical regions, results in the formation of two-layer clays (kaolinite). Soils are usually acid in the lower horizons, whereas the upper horizon may be eutrophic (rich in nutrients). A dry season activates decay of organic matter, favours fires and interferes with the production of acid humus, resulting in surface-dystrophic soils (low in nutrients). A long dry season may encourage ferrugination. Young soils from volcanic ash start as andosol, then becoming terra roxa (intergrade kaolinitic) and then kaolinitic. Terra roxa (red earth) is often made up of young soils formed easily from basalt, in erosion-prone mountainous tropical regions (coffee regions).
• Bog soils: Brown, dark-brown or black peaty material over soils of mineral matter mottled grey and rust. Vegetation: swamp, forest, sedges, grasses. Moist, cool to tropical climates. Gleisation.
• Alpine meadow: Mountain alpine soils. Dark-brown, organic-rich layer grading down at 30-60cm to grey and rusty soil, streaked and mottled. Cool climate. Vegetation: grasses, sedges, herbs. Some gleisation and calcification.
• Rendzina: Limestone soils. Dark-grey or black, organic-rich surface layers over soft light grey or white calcareous material derived from chalk, soft limestone or marl. Vegetation mostly grassland. The lime in these soils is derived from the parent material. Climate: all regions.
• Laterite: Rainforests. Thin organic layer over reddish, strongly leached soil, generally clayey and enriched in hydrous alumina or iron oxide or both; low in silica; generally deep soil. Tropical wet climates. Vegetation mostly forest.
• Saline and alkaline: Soils in which salts, including alkali have accumulated, generally in poorly drained, arid or semi-arid areas. Salts deposited as a result of evaporation. Vegetation: salt-tolerant species or none.

 

Carbon pools and fluxes in terrestrial ecosystems Although this diagram classifies soils and ecosystems slightly differently, it gives a good and exact overview of the productivity (right-hand column) and stored carbon in the soil (left) and above the soil (middle). The histograms are in amount of carbon per square metre and the red numbers are totals for each class, in Pg (billions of tonnes, Gt). The bold red numbers are world totals for each columns. Notice that the heighth of each rectangle represents the relative area covered by each type of ecosystem. It can be seen that warm grassland covers most of the continents and highland woods least.

Example: warm grasslands cover the largest area (vertical size), have a very low investment in canopy but moderate carbon investment underground, and they are moderately productive. 
Example: humid broad-leaved forests have high productivity and a very high investment in carbon above ground versus moderate investment underground. 
Example: tundra have very low investments above ground but very high amounts of carbon underground. They are the least productive ecosystems of all.
It has been attempted to align the ecosystems by latitude, although this was not strictly possible. Going from tundra to warm grassland (top down), one sees productivity increasing, then decreasing again. Likewise, the stored carbon above ground in roots and foliage follows the same trend, but the amount of carbon underground in roots and soil organisms decreases. Notice how grasslands have very little organic matter above ground, but ten times more underground. Arid soils have more biomass above than underground.
 

The map on right gives another picture of the world's climate. It shows the problem areas where rainfall either grossly exceeds evaporation (dark green) or the other way around (yellow). In these areas, sustainable farming is extremely difficult, even when enough drainage is applied in the wet zones or irrigation in the dry zones. In the wet zones the nutrients and fertiliser are leached away easily whereas in the dry zones salts accumulate in the soil. In the light green areas, farming can be done sustainably but much of this area is still covered in snow, ice and bare rock (not shown).