Rock Formation and Earth’s Climate History
Formation of Rocks
Rocks are formed through various processes:
- Cooling of magma
- Disintegration, transport, and deposition
- Precipitation of inorganic salts in water
- Condensation of gases containing mineral particles
- Deposition of animal and vegetable remains
- Partial or complete recrystallization of minerals in a rock due to high temperatures and pressure
In short, igneous rocks are formed by the crystallization of magma. These rocks may be basic, acidic, or intermediate, depending on their composition, and plutonic, volcanic, intrusive, or extrusive, depending on whether consolidation has occurred within the Earth or on its surface. Igneous rocks, once consolidated, are fragmented or disintegrated into smaller fragments, particles, or clasts by weathering. These fragments form sediments, which are then transported and accumulated in a basin, where they can experience a hardening or compaction process called diagenesis or lithification, thereby forming sedimentary rocks.
Sedimentary rocks are formed by layers upon layers of different grain sizes. Through lithification, sands become sandstones, silts become shales, calcareous material becomes limestone, gravels become conglomerates, and so on. Sedimentary and igneous rocks can undergo the action of high temperatures and pressures, caused by different factors that lead to mineralogical and textural contrast. Such a change does not involve crossing through the liquid state, and this process is called metamorphism, resulting in metamorphic rocks.
Finally, both igneous, sedimentary, and metamorphic rocks, under the action of strong pressures and high temperatures with special conditions, can undergo melting or remelting to return to the primitive magmatic state, thus beginning a new cycle.
Formation of Sedimentary Rocks
In general, sedimentary rocks are found stratified and are formed in four ways:
- Deposition of debris from the disintegration of preexisting rocks
- Precipitation of inorganic salts in water
- Deposition of organic substances (plants and animals)
- Condensation of gases containing mineral particles
Compaction of Sedimentary Rocks
Sedimentary rocks are compacted by the following process: When a deposit is buried by the accumulation of new material, a local settlement occurs under the load, with the removal of excess water. Finally, a bond is established or strengthened, and the sediment acquires an appreciable degree of robustness. During compaction, water is expelled, and the individual particles are pressed closer together by the weight of overlying sediments.
Cementation is a process by which the mineral matter carried in solution by groundwater is deposited between grains, binding them together. Among the substances that cement sedimentary rocks are calcium carbonate (calcareous cement), silica (silicon cement), and minor amounts of iron oxides (ferruginous cement), clay, and gypsum. Chemical changes include reduction, especially of iron compounds by organic matter, and the destructive distillation of organic matter.
History
Throughout the 4.6 billion years of Earth’s existence, numerous events have occurred, from geological cataclysms and atmospheric chemical changes to global climate shifts and the appearance of life. At present, the average land temperature is 15°C and is in thermal equilibrium. However, the geological and paleontological record indicates that Earth’s climate has experienced variations over time. Climate can substantially affect the atmosphere and hydrosphere, which have undergone significant changes in composition and characteristics.
The atmosphere was formed by the degassing of the planet, resulting in a proto-atmosphere with plenty of water vapor and CO2, which would have been slightly reducing. Condensation of water vapor formed the oceans some 100 to 200 million years after the planet’s origin. By 3.8 billion years ago, conditions were suitable for life on Earth. The atmosphere’s composition has changed since its origin in various aspects. The CO2 concentration began declining, going from 20% to 0.03% today. This decrease in CO2 occurred during the process of fixing carbon to form carbonate rocks, such as limestone. This process led to a decrease in the greenhouse effect and a reduction in the average global surface temperature.
Moreover, the atmosphere changed from being anoxic to having an oxygen concentration of 21% today, becoming oxidizing. This process involved living organisms releasing oxygen through photosynthesis. About 600 million years ago, the oxygen concentration was similar to the current level. Paleoclimatology, the science of climate throughout Earth’s history, is based on the study of paleoclimatic indicators (e.g., tillite, stratification). Considering the historical record, it can be concluded that the climate has fluctuated between extremes. During colder periods, large masses of ice covered the continents, while during warmer periods, the thermal gradient between the equator and the poles decreased significantly.
Cold periods are called glacial periods, and warmer periods are called interglacial periods. With the exception of glaciations that occurred between 2.5 and 2.3 billion years ago, glacial periods are relatively recent, with the last glaciation occurring around 1 million years ago. The absence of early glaciations on Earth can be explained by the strong greenhouse effect that existed due to high CO2 concentrations at the time. Considering their distribution and duration, it can be concluded that the normal climate is a climate of gases, and the seven glaciations that have occurred are anomalies.
Within a glacial climate, there are oscillations between periods of maximum cold (glacials) and periods of milder climate (interglacials). These periods usually last slightly less than 100,000 years. Within these periods, there are shorter climatic oscillations, lasting around 1,000 years, called climatic optimums and climatic crises.
Main Causes of Climate Change
The main causes that can explain climate change are:
Changes in Astronomical Causes or External to the Planet
a) System rotation around the center of the galaxy: Ice ages may be related to the period of the Sun’s rotation around the center of the galaxy and its passage through the galaxy’s equatorial plane. This leads to a reduction of solar energy reaching Earth due to an increased quantity of diffuse matter. This movement lasts about 250 million years.
b) Variations in the energy emitted by the Sun: These can be progressive or periodic:
- Progressive: Caused by thermonuclear fusion processes, resulting in the Sun emitting less energy at its origin.
- Periodic: There are cycles of solar activity, like sunspots, that repeat every 11 years and could be the cause of climatic optimums and crises.
c) Variations in Earth’s orbit: These are known as Milankovitch cycles and affect Earth’s insolation and, consequently, temperatures.