Stratigraphy is the science that studies, describes, and interprets the arrangement of layers or strata of rocks and other materials in the earth's crust. It is a branch of geology, a science that studies everything related to the structure of planet Earth.
Stratigraphic experts study the sequence of strata or layers that make up rocks, whether these layers are horizontal or vertical. We speak of strata or layers because the rocks that make up the earth's crust are arranged in that way, in layers one above the other..
These layers are mainly the product of sedimentation processes that consist of the slow progressive accumulation of sediments or material remains carried by water and wind. Sediments accumulate in the lower parts, mainly seas and lakes and there they are trapped by the weight of successive additional sediments.
In such a way that after millions of years they form compact layers that harden like sedimentary rock. Additionally, there are other types of rocks, such as igneous and metamorphic..
The first products of the lava from the volcanoes when cooling and the second formed by pressures at great depths and outcropped by the movements of the crust. All this is shaping the succession of layers that form the crust.
The layers or strata that stratigraphy studies are observed on the earth's surface when the layers are exposed in the so-called rocky outcrops. They can also be studied in mines, quarries, excavations or wells.
Stratigraphy studies the arrangement of layers of rocks and other materials in the earth's crust. It defines its shape, composition, physical and chemical properties, origin, age and evolution, that is, how they have changed over time..
This allows us to understand the geological processes that have occurred on Earth, such as major floods, earthquakes, movements of continental plates and others..
The stratigraphy identifies each layer, the types of rocks that compose them, their age and makes the stratigraphic cartography. This consists of maps where the location of the layers and their characteristics are indicated. The maps indicate to which period of the Earth's geological history each stratum or layer belongs.
A stratigraphic record is a set of data obtained by describing the rock layers that allows us to understand the evolution of the planet. On the other hand, when sediments are deposited, for example in a lake, remains of animals, plants and other organisms are also deposited when they die. These remains are compacted and form fossils, which are used to study the evolution of life.
Fossils can be bones or petrified shells or marks left by living things or their body parts. In turn, the presence of certain types of fossils helps to establish the age of the layer or stratum where they appear. Stratigraphy also studies these fossils, establishing what is called the paleontological content.
To carry out its work, stratigraphy has a series of principles.
This principle states that the natural laws that acted to form the ancient sedimentary layers are the same that even today shape the earth's crust. Therefore, by understanding how they work today, we can understand how these layers were formed in ancient times..
Here it is established that there is a logical time sequence in the processes that create and arrange geological layers. That is, the formation of the rock layer is prior to any alteration that we observe affecting it..
For example, a fault is a breaking and lifting of the sedimentary layers. According to this principle, it must be considered that what caused this break occurred after the rock was formed. Likewise, if there is a type of rock forming part of another larger rock, it means that the latter was formed after the rock that is included in it..
It proposes that the strata or layers are deposited horizontally one on top of the other, therefore those below are the oldest.
This principle is related to the previous one, noting that no matter how the strata look today, they originally formed horizontally. On the other hand, the different layers have lateral continuity throughout the sedimentary deposit and therefore the layer has the same age along the entire horizontal plane..
In addition, these layers will remain horizontal unless some force acts to deform or break them. Therefore, when seeing a fault or a cut in the sequence of layers, we must consider that some subsequent force acted..
For example, when an earthquake breaks the sequence of strata and lifts up a section, which is called a geologic fault..
Living beings evolve continuously and irreversibly, that is, species change and never return to what they were. Therefore, if we get the same types of fossils in two different layer sequences, we must consider that those layers are the same age. Thus, they were formed at the same time when there were those animals or plants that left their fossils there.
Likewise, comparing two similar layers we find different fossils, we must consider that these layers were formed at different times.
For example, if there is a dinosaur fossil in one layer and an elephant fossil in another, the first layer is the oldest. This is because dinosaurs lived before current elephants.
Stratigraphy, as a branch of geology, has various areas of specialization depending on the focus of attention of the statigrapher or person who exercises this profession.
This sub-discipline of stratigraphy tries to identify cyclical phenomena in the history of the Earth that leave a mark on the sedimentary layers. Understanding by cyclical occurrence that they are phenomena that happen from time to time.
In this case, they are phenomena that are repeated separated by long periods of time, thousands or millions of years. An example of the application of cyclo-stratigraphy is trying to establish the relationship between geological strata and terrestrial cyclical changes..
This is because the sediments that form the layers are affected by the current climate at the time of deposit. So by studying the stratigraphic record, that is, the geological layers, we can know what climate there was at the time they were formed..
This specialty is the product of the union of stratigraphy with geochemistry. Being this last discipline dedicated to the chemical analysis of rocks, sediments and fossil remains. For example, the study of the proportion of stable oxygen isotopes, extracted from seashells found in sediments.
This ratio varies depending on the temperature and other factors that existed when the shell was formed. Which allows us to know what environments existed when the sediments were formed and how they have changed over time.
It is based on the changes that the Earth's magnetic field has undergone throughout its existence. Rocks have the ability to slightly magnetize in a direction parallel to the magnetic field during their formation, which is reflected in their structure..
Therefore, you can know how the magnetic field was in the period in which that particular rock layer was formed..
So it is possible to define zones of layer sequences with the same orientation of the magnetic field. This makes it possible to map the sequences of layers in different places on the planet that were formed at the same time, calling these areas magnetozones..
The latter case is a branch of paleontology, the science that studies living things from the past, through the study of their fossils. This branch is the one in charge of identifying the fossils found in the geological layers.
Thanks to stratigraphy we can know the geological history of planet Earth, including when and how the different physical landscapes were formed. In the same way, we can know the evolution of life, by studying the fossils found in the different strata or layers..
It allows us to understand the different geological processes that affect the Earth and their effects on the territory and human constructions. For example, the presence and characteristics of certain strata give us information on which areas may suffer more earthquakes or landslides..
It also introduces us to the different materials in the earth's crust, their properties and distribution on the planet. These materials in turn can be subsequently extracted, since they are useful for various industrial applications.
For example, rocks that serve as raw material for construction or metals of various kinds. In the same way, to be able to build on an area, it is necessary to know the characteristics of the rock layers that exist under the place..
Similarly, stratigraphy allows us to know the environments and climates of the past, as well as their changes over time. Which in turn allows us to understand the current processes of climate change.
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