Geological Time and Earth’s Processes: Dating, Weathering, Erosion, and Sedimentation

Posted on Dec 14, 2024 in Geology

Geological Time

Geological time is the period between the origin of the Earth and today. It is measured in millions of years. Geologists have always tried to sort all the events chronologically, using two methods: absolute dating and relative dating.

Absolute Dating

Absolute dating consists of assigning a specific date to an event. Various methods can be used for absolute dating, the most common being the radiometric method. This method relies on the existence of unstable atoms that undergo radioactive decay, where a parent element or isotope is converted into a child element. When half of the parent element has become the child element, this period is called the half-life or disintegration period. Knowing the rate of decay of an isotope, the amount of the parent element, and the amount of the child element, an exact date can be determined. Thus, the origin of the Earth could be dated to 4,500 million years ago.

Environmental Disadvantages:

  • Only occurs in magmatic rocks.
  • Only applies to rocks that are isolated and do not undergo erosion.
  • The amounts of radioactive elements are so small that they are difficult to measure.

Relative Dating

Relative dating is when events are arranged chronologically but without giving exact dates.

Methods used:

  1. Principle of Superposition of Strata: Within a series of layers, one layer is more modern than those under it and older than the one above it. Likewise, any event is younger than the rocks it affects and older than those it does not affect.
  2. Principle of Actualism: According to this principle, the processes currently operating on the Earth’s surface are essentially the same as those performed in earlier times.
  3. Principle of the Study of Fossils: Paleontology is the science that studies fossils. Fossils are always formed in sedimentary rocks. Only the hard parts of living beings fossilize. Sometimes the fossil is just a trace or a mold. Fossils are very useful for relative dating. Fossils provide two types of information:
    1. Organic paleontological information, indicating the environment in which the fossil lived and therefore the environment in which the rock containing the fossil was formed.
    2. Temporal information. Fossils can tell us the geological period in which the rock was formed because each fossil species lived during a specific period.

Earth’s Changes

The Earth has changed constantly throughout its history. Its surface, atmosphere, climate, living organisms, and so on have all changed. These changes have been explained by several theories:

  1. Catastrophism: According to this theory, changes occur when major disasters occur.
  2. Uniformitarianism or Gradualism: According to this theory, changes occur very slowly and over the long term.
  3. Neocatastrophism: Change is very slow, and abrupt changes are superimposed on it.

Major Divisions of Earth’s History

To facilitate its study, Earth’s history has been divided into several periods. These stages are:

  1. Aeons
  2. Eras
  3. Periods
  4. Epochs
  5. Ages

We are currently in the Phanerozoic eon, the Cenozoic era, and the Quaternary period. The following criteria are considered when establishing the divisions between stages:

  1. Extinction and appearance of new species.
  2. Existence of large tectonic episodes.
  3. Major changes in climate.
  4. Major changes in sea level.

Weathering

Weathering is the disintegration of rocks by mechanical, chemical, and biological effects. Rocks formed inside the Earth at high pressures and temperatures. When they come into contact with the atmosphere, their physical and chemical conditions change, leading to a series of changes that fragment the rocks into smaller pieces, which can then be eroded.

Chemical Weathering

Chemical weathering is the set of processes carried out by water or by atmospheric agents such as oxygen and carbon dioxide. Rocks disintegrate more easily with this type of weathering, and mineral grains lose their grip and dissolve or are altered by physical agents.

  • Dissolution: This consists of the incorporation of molecules of a solid into a solvent such as water. Many sedimentary rocks composed of salts left after the evaporation of water are dissolved through this system.
  • Hydration: The process by which water is chemically combined with a compound. When water molecules are introduced through the crystal lattices of the rock, pressure is exerted, causing a volume increase, which in some cases can reach 50%. When these materials dry, the opposite effect occurs, generating a contraction, and they break down.
  • Oxidation: Oxidation is caused by the action of oxygen, usually when it is released into the water. In oxidation, there is a simultaneous decrease as the oxidizing substance is reduced by seizing electrons, while the substance that loses electrons is oxidized. Rocky substrates with red, ocher, or brownish shades, so abundant, are produced by the oxidation of iron in the rocks.
  • Hydrolysis: The chemical breakdown of a substance by water, which in turn also decomposes. In this process, water is transformed into ions that can react with certain minerals, breaking their crystal lattices. This is the process that produces most of the clay materials we know.

Mechanical Weathering

In mechanical weathering, rocks are broken into smaller pieces by the effect of frost and sudden temperature changes.

Biological Weathering

Living beings weather rocks. For example, tree roots can act mechanically on rocks.

Erosion

Erosion is the wearing away of rocks by water, wind, ice, or the particles carried by these agents. Erosion is always associated with and simultaneous with the transport of the torn fragments.

Agents of Erosion:

  • Wind: A transport agent that acts in deserts and areas without vegetation.
  • Glaciers: Linked to certain high altitudes.
  • Marine Erosion: Coastal waves exert erosion in some places, resulting in characteristic morphology.
  • River Channels: Rainwater leads to small currents of water that flow down slopes and eventually form rivers.
  • Slope Erosion: Phenomena resulting in the displacement of weathering products under the action of gravity, often aided by rainwater.

Sedimentation

Erosion products can be carried to certain places where they are deposited. They are buried and undergo a series of transformations that can convert them into sedimentary rocks. Sedimentation occurs in the oceanic environment, in areas near coastlines. Sedimentary facies maintain the environmental features that allow sediment to be reconstructed. There are three types: continental, transitional, and oceanic.

Magmatic Intrusive: Plutonic, granite, syenite. Metamorphic Origin: Dynamic (clay slates, roofing), Crystalline (gneiss), Thermal Origin (marbles).