The result of the planet's internal activity modifies the earth's surface. These are the so-called "internal geological agents". The new crust soon receives the "attack" of other agents, the external ones, that erode it.
Erosion produces residues of rocky material of various sizes that, over time, are fragmented into smaller pieces. Gravity and transport by the action of water or wind deposits and accumulates them in the lowest areas of the land relief. This results in the appearance of sediments that are deposited in layers, which we call "strata." Later, the lower layers, which support more weight, are transformed into new rocks, the sedimentary rocks.
Formation of strata and sedimentary rocks
The surface of the planet is broken, due to erosion, into more or less large pieces, from the blocks of rock to the very thin slime, passing through all sizes of gravels and sand. The force of gravity and the drag of water tend to deposit these fragments in the low areas where, at times, huge amounts accumulate.
These materials are forming successive layers that we call "strata". The type of stratum depends on the weather and the erosion that occurs in each era. This makes his study interesting to know the conditions of past eras. The branch of geology that studies sediments and strata is called "stratigraphy."
The superficial strata of the sedimentary areas usually have a soft consistency, often being ideal for agriculture. But over time, as new layers accumulate, the lower ones have to bear more weight and their particles, under greater pressure, are compacted. This, together with the increase in temperature, cause chemical changes that eventually turn the sediment into hard rock.
The age of the fossils
Sedimentary processes can occur anywhere on the earth's surface where there is erosion, but not all deposited material becomes sedimentary rock. since erosion itself can drag sediments before they harden. Basically, sedimentary processes are of three types:
Sailors, deposits are formed on the continental shelf and in the abyssal areas.
Continental, materials accumulate at the foot of mountain ranges, in glaciers, along river basins and in deserts.
Of Transition, which is the sedimentation that takes place at points of contact between the sea and the continents, such as swampy areas and deltas.
Organic remains trapped in sediments can be fossilized if they become metamorphic rocks. The set of the various strata that contain the same types of fossils is called "stratigraphic unit."
The thickness of a sediment allows to deduce the time it took to form, if the sedimentation rate is known. Each type of sediment indicates some characteristics of the time in which it was formed, such as rains, glaciations, desertification, ... All this allows geologists to know the age of the sediment and, therefore, also the age of the fossils it contains, resulting A good dating method.
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