The process of understanding geology cannot be separated from an understanding of concepts and laws found in geology. This is the basis for studying geology to make it easier to understand it. The concepts that need to be known before studying geology are more in-depth, namely about the concept of arrangement, rules, and relationships between rocks in space and time where the space covered is where the rocks are formed and time is the length of time the rocks are formed based on the existing geological time scale.
Geological laws and concepts that become a reference in geology include the doctrine of Uniformitarianism initiated by James Hutton, the Superposition Law by Steno, the concept of Conformity and Unconformity, the regression-regression concept, the cross-cutting relationship, and others.
Stratigraphy is a study of the history, composition and relative age, and distribution of soil layers and interpretation of rock layers to explain the history of planet Earth. This study is a branch of geology which is often referred to as quee of geology. This study also appeared in the United Kingdom in the 19th century initiated by William Smith. The science is based on observations on several layers of soil that appear in the same order (superposition). At that time, it was concluded that the lowest soil layer was the oldest layer with a few exceptions.
Stratigraphy is the science of strata or stratum in which there is a layer of rock, distinguished from other strata located above or below it. William Smith earned the nickname “Father of Stratigraphy” was the first person to be aware of the fossils contained in sediments. More details about stratigraphy discuss the classification of strata based on the fossils contained in them. So, the definition of stratigraphy is a study of rock layers and the relationship of the rock layers with other rock layers with the aim of gaining knowledge and information about the history of the Earth.
In the stratigraphy, there are also basic principles namely the superposition principle (Steno, 1669), the original law (original horizontality), the principle of cutting (cross-cutting), the principle of lateral continuity (continuity). Besides that, in stratigraphy, there are concepts of conformity and unconformity.
Conformity is a relationship between layers of rock with other layers above or below it that is continuous. There is no interval of deposition time or sedimentation. In general, if the field can be indicated by the position of the layer (strike/dip).
Inconsistency is the relationship between one layer of rock with another rock layer (upper or lower limit) that is not continuous. This is caused by the presence of a cavity or space during deposition. In geology, 3 types of inconsistencies are known:
- An angular unconformity is one type of inconsistency whose form shows a layer that has been folded and erosion occurs, then above that layer, there is another layer of sediment.
- Disconformity is one type of inconsistency that has a relationship between layers of sedimentary rock separated by erosion fields. This phenomenon occurs because sedimentation stops for some time and causes the top layer to erode causing a rough layer.
- Paraconformity is also called pseudo alignment, which is the relationship between two layers of sediment which has an inconsistency that is parallel to the other sediment layers. In such cases, it is very difficult to see the limits of inconsistency because there are no erosion boundaries. The method used to see the peculiarities between these layers is to look at fossils in each layer, namely Paleontological analysis, namely by using or calculating the age range of fossils because each sediment has a different age and fossils that are buried in it must be different types too.
- Non-conformity is a discrepancy that occurs where there is a phenomenon of layers of igneous rock or metamorphic rocks which underneath there is a layer of sediment.
The following are things that cause unconformity:
- Splits in A Layered Structure (Discordance in Bedding):
The bedding itself is a layered structure. This structure is characteristic of sedimentary rocks which show a layered arrangement of sedimentary rocks with a thickness of each layer of less than 1 cm. The description of the form of inconsistency in the sedimentary layer is characterized by a layered structure (beds) at the bottom which is more folded than the folds on it which are horizontal.
- Surface Erosion
The structure of the rock layer at the top is above the erosion surface of the older rock layers.
- Basal Conglomerate
The lower rock layer which includes the upper part contains gravel from the older rock layers.
- Variation in the Deformation Section
Older rocks show folds and fractures or metamorphism.
- Veins and Intrusions
The vein is a layer, sediment from minerals or rocks containing irregularly shaped seeds and is different from rock formations around it. The picture of inconsistency this time is dike or in geological terms is a sheet of rock formed in fractures in the body of the rock that already exists there is a distance that is not too far away and there are veins that appear in old rocks.
- The difference in Rock Character
Intrusive frozen rocks or plutonic rocks that intersect with layers of sedimentary rock do not show any impact on each layer and not long after the erosion of the upper rock layers occurs.
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