Sedimentary Rock Formation and Plate Tectonics
Formation Mechanisms of Sedimentary Rocks
The formation of sedimentary rocks involves a set of physical, chemical, and biological changes that take place on the Earth’s surface and the outer part of the crust in contact with the hydrosphere, the atmosphere, and the biosphere.
The following processes occur in the formation of sedimentary rocks:
Weathering: The first step in the formation of sedimentary rocks. Rocks found on the Earth’s surface undergo alteration. This modification can be mechanical, due to ice melting and the growth of salt crystals, or chemical.
Erosion: When particles are torn out of the set where they are. This activity occurs under the effects of gravitational attraction, which tends to bring down objects, and also the impact of the action of water, air, ice, or other particles in motion.
Particles torn continue their journey through one of the agents available (water, air, or ice) to reach the site of deposition or sedimentation basin. The traffic from the area of erosion to the sedimentation basin is moving.
The set of mineral or organic material derived from preexisting rocks or the activity of living beings, following the transport process, is deposited to form sediments.
As the sediments are buried, they suffer a reduction of voids or pores and a loss of water, a process known as compaction. If salts dissolved in water fall into the blank spaces, the particles are joined together, producing cementation.
This set of changes affecting the sediment in a sedimentary rock is called diagenesis.
Characteristics of Sedimentary Rocks
The typical arrangement of sedimentary rocks is in layers or overlapping layers called stratification. Each layer is a sedimentation unit. Different episodes form sedimentary layers that overlap each other successively.
The conditions of formation of some sedimentary rocks also cause some internal rankings, top or bottom, visible at the level of capacity, called sedimentary structures. Sedimentary structures reflect a set of processes that can develop both during and after deposition.
The origin of these rocks on the surface in contact with the biosphere is likely to contain fossil remains of the activity of living organisms. These help to reconstruct the conditions of rock formation.
The study of sedimentary rocks is based on the principle of actualism, as set forth by James Hutton in 1788, which can be summarized with the following sentence: “The present is the key to the past.”
Classification of Sedimentary Rocks
If you stick to the mechanisms of formation explained, sedimentary rocks can be classified as detrital when they are the result of the mechanical process of sedimentation of particles, and non-detrital when they are not, and therefore involved in the formation of precipitation, chemical and/or participation of living beings.
Detrital Sedimentary Rocks
Depending on the size of the particles:
- The Rudites: Larger than 2mm.
- Clast: Larger than 2mm, round (pebbles), with sharp edges or cantelluts (cairells).
- Matrix: Smaller than 2mm, filling the gaps between the pebbles.
- Cement: Chemical origin that occupies the voids between pebbles and matrix.
- Clast: Larger than 2mm, round (pebbles), with sharp edges or cantelluts (cairells).
If there is no cement, there is no consistency, and pebbles can be separated easily. In this case, we speak of sediments and, particularly, if it is a set of pebbles, then called gravel. When there is cement, it is distinguished between conglomerates, which are composed of pebbles, and breccia, when it comes to cairells.
- Sand and Sandstone (Arenite)
Sands are incoherent deposits formed by particles of a size between 1/16mm and 2mm. Sandstones are rocks that correspond to cemented sand. The components that are distinguished are:
- Grains: Between 1/16mm and 2mm.
- Matrix: Lutitic fraction that fills the empty spaces.
- Cement: Chemical origin that unites the grains.
- The Mudstones:
Grouped into sediments and sedimentary rocks in which the grain size is less than 1/16mm.
- Silts: The size is between 1/16 and 1/256mm.
- Clay: Less than 1/256mm.
When the earlier sediments are compacted, they are called siltstone and claystone, respectively.
Non-Detrital Sedimentary Rocks
Sedimentary rocks that do not originate from detrital materials are transported in solution and accumulate as sediments or chemical precipitation.
Carbonate Rocks: Limestones and Dolomites
- Limestone: Very abundant on the Earth’s surface. All have in common a calcareous composition, a sedimentary origin of the formation process, and react with 10% hydrochloric acid.
- Limestone: Fine texture formed by the precipitation of “mud”, very small crystals (2 to 4 µm) of calcium carbonate, from the disintegration of shells and parts of organic origin.
- Fossiliferous Limestone, Lumaquela: When there is participation of living organisms from accumulations of skeletons.
- Oolitic Limestone: Radial or concentric spheres structure of 2mm. Originated by precipitation of carbonate around a tiny quartz grain or fossil fragment.
- Travertine: Carbonate precipitation around stems, leaves, or plants in continental environments: fluvial or lacustrine.
- The Dolomites:
They look similar to some limestones, with which they often arise together. They can be distinguished by the texture, which is generally more granular, or by a very weak or no reaction with 10% cold hydrochloric acid, which does occur in limestone.
Evaporitic Rocks:
Formed by the precipitation of salts dissolved in water exposed to strong evaporation.
Ordered from least to most soluble.
Organogenic Rocks (Accumulation of Organic Matter):
This includes rocks formed by the transformation of organic compound remains of organisms. These are coal and oil (petroleum), known as fossil fuels.
- Coals (mainland plant) come from the transformation of plant remains accumulated in wetlands. Their formation involves a process of decomposition of plant tissues (cellulose and lignin) in the absence of oxygen (anaerobic conditions) and the action of microorganisms (bacteria). This process, called carbonization, with the passage of time produces a progressive enrichment in carbon atoms. In addition, coals contain hydrogen, oxygen, and to a lesser extent, nitrogen and sulfur.
- Hydrocarbons and Oil (vegetable marine plankton) are rocks formed by a mixture of hydrocarbons, substances made up of hydrogen and carbon in a semi-solid, liquid, and gaseous state. Crude oil is a dark, oily liquid that gives off a strong odor and is lighter than water.
It is considered to be formed from the remains of organic matter accumulated on the seabed or lake. Planktonic organic matter undergoes fermentation in the absence of oxygen, creating an insoluble substance, sapropel, which is considered crude oil.
Once the oil originates, it migrates through the rocks until a penetrable or geological structure (trap) prevents it from accumulating in a porous rock (reservoir rock). Often, oil fields also contain natural gas and water, prepared according to their densities and always occupying the empty spaces (pores) of the rocks.
Plate Tectonics
Continental Drift
Alexander von Humboldt noted the similarity between the rocks of Brazil and Congo and the existence of similar fossils in both areas. To justify these facts, he had to imagine the existence of bridges of rocks above the ocean.
Alfred Wegener, in 1912, published the theory of continental drift. According to this theory, the continents are not fixed, but from a single continent (Pangea), they were fragmented and gradually shifted to arrive at the current appearance.
Wegener provided a series of tests that supported the validity of his theory:
- Geographic Evidence: Topographic coupling between the coasts on both sides of the Atlantic, as proved later, is still tight at the edges of the continental shelf.
- Geological Evidence: Correspondence between geological formations of the same age on both sides of the Atlantic. Wegener noted especially deposits of tillite, glacial sedimentary rocks that were on the margins of different continents.
- Paleontological Evidence: The existence of similar flora and fauna on both sides of the ocean could only be explained by the existence of transoceanic bridges or the fact that the continents were together in the past.
- Paleoclimatic Evidence: Regions and present very different continents had similar climates.
The theory of continental drift was not accepted by most geologists of the time. Wegener was not able to explain the energy that made the movement of continents possible.
Also accepted was the existence of a vertical movement of the continental crust on the ocean to explain the formation and maintenance of the mountains, as explained by the theory of Isostasy, by Clarence E. Dutton. According to this theory, there exists a dynamic balance between a very dense outer zone, supported on a denser mantle, so that any increase in mass is compensated by a collapse and all loss, with an elevation. This theory was largely accepted by the scientific community at the time.
According to Wegener, if one supports the idea that the continents rose and descended slowly on a viscous mass, one could also admit the existence of horizontal movements.
A more definitive explanation came later thanks to the discovery of oceanic ridges and paleomagnetism of rocks in the middle of the twentieth century. This evidenced the success of Wegener and allowed the scientific community to formulate the theory of plate tectonics, on which current concepts of geology are based.
Plate Tectonics
Research for military purposes to know the underwater topography and magnetic anomalies to uncover evidence that gave submarines lacked the scientific community to define the theory of plate tectonics. The lifting of deep seabed maps revealed the existence of mountain ranges with a depression in the inner central part of the ocean called oceanic ridges.
Studies of paleomagnetism on land-based targeting ferruginous minerals from rocks revealed that periodically there is a migration of the North Pole to the South Pole and vice versa. This allowed us to date the rocks of the seabed with a certain accuracy and revealed that younger rocks are next to the ridges and that older rocks are found as we move towards the continents symmetrically on each side of the ridge. This is because the central part of the new magma rises from inside the Earth, cools down, and creates a new oceanic crust.
Involving the formation of new crust, the surface of the Earth increases, which destroys it somewhere. These areas are known by the name of subduction zones, and it occurs when one plate sinks into the interior of the Earth. The plate that subducts corresponds to the so-called Benioff plane, which explains a phenomenon that seismologists observed in different parts of the Earth where earthquakes occurred on a sloping surface. The interpretation of the theory of plate tectonics is that this plane corresponds to the oceanic crust that is getting back towards the interior of the Earth. From these tests, plate tectonics was defined.
The lithosphere consists of a set of tectonic plates that move over the molten or partially molten material of the asthenosphere. In areas where the plates move, a new oceanic lithosphere is created, and in subduction zones, it is destroyed. The fusion of the plates can cause magmas that emerge on the surface as volcanoes, and friction between plates causes earthquakes.
There are two types of plates:
- Continental Plates: Correspond, roughly, to the hatched areas of the continents and are composed of the continental crust and upper mantle.
- Oceanic Plates: Occupy areas covered by sea and are composed of the oceanic crust and upper mantle.
Types of Plate Edges
- Diverging Edges: Are the edges of the plate that tend to move away. Also called constructive, expansive, or accretionary.
- Convergent Edges: The movement between the plates is to approach or collide. If one plate sinks beneath another, subduction occurs.
- Continental Plate – Continental Plate.
- Continental Plate – Oceanic Plate.
- Oceanic Plate – Oceanic Plate.
- Transform Edges: Oceanic ridges are cut by faults that displace the axis of the ridge, forming numerous fragments. These faults are called transform faults. The horizontal movement of the plates has no vertical component. In this type of contact, there is virtually no distortion, and lithospheric material is neither created nor destroyed.