Tectonic Plates, Folds, Faults, and Earth’s Age
Transform Faults and Plate Tectonics
Transform faults are boundaries between tectonic plates where there is a lateral displacement of one plate with respect to the other. These boundaries are neither constructive nor destructive.
Mountain Ranges Formation
- Collision between two oceanic plates: One plate subducts under the other, originating an oceanic trench.
- Collision between oceanic and continental plates: Because the oceanic plate is denser, it subducts under the continental plate in an oceanic trench.
- Collision between two continental plates (mixed plates): The oceanic lithosphere disappears until the continents collide, forming an intracontinental mountain range. This range is formed with the sediments that had accumulated on the oceanic lithosphere between the two continents.
Types of Folds
Folds According to Layer Arrangement
- Anticline: The oldest layers are located in the core, and the most modern ones are on the outside. They are A-shaped.
- Syncline: The most modern materials are located in the core. They are V-shaped.
- Monoclinal or Knee Fold: Folds that have only one side; an inflection in the layers.
- Symmetrical Fold: The axial plane is vertical, and the two limbs are symmetrical.
- Asymmetrical Fold: The two limbs have different inclinations.
Folds According to Their Axial Plane
- Open Fold: The axial plane is vertical.
- Overfold: The axial plane forms an angle of less than 45ยบ with the vertical.
- Recumbent Fold: The axial plane is almost horizontal.
Faults: Definition and Types
Faults are fragile deformations that occur when rocks cannot absorb the stresses to which they are subjected. Stresses can be compressive, distensive, or shear.
Geometric Elements of Faults
- Blocks or Fault Walls: The two portions of rock separated by the fault plane. If the fault plane is not vertical, a distinction is made between a footwall and a hanging wall.
- Fault Plane: The plane of rupture along which the blocks move. It can be vertical, horizontal, or inclined.
- Fault Scarp: The distance one block has moved from the other. It can be measured laterally, horizontally, or vertically.
Types of Faults
- Normal or Direct Fault: The hanging wall is the sunken block.
- Reverse Fault: The hanging wall coincides with the raised block. It occurs due to compressive stresses, decreasing the surface area.
- Strike-Slip Fault: Caused by shear forces.
Faults, like folds, are usually associated with others, causing associations or systems of faults.
Determining the Age of the Earth
Humanity has always wondered about the Earth’s age. Early attempts to calculate it were based on observations of nature and the firmament, along with the religious beliefs of the time. In 1896, Becquerel discovered radioactivity. In 1903, the Curie couple discovered that radioactive elements gave off heat, providing the source of energy that heated the Earth’s interior. This discovery was fundamental for accurately calculating the Earth’s age.
Dating Methods
Events or materials can be dated in two ways:
- Relative Dating: Ordering rocks, fossils, or events from oldest to most modern, without specifying their age.
- Absolute Dating: Assigning a date to materials or events, specifying their age in years.
Principles in Relative Dating
- Fossils, in principle, are of the same age as the strata in which they are found. Strata containing the same type of fossil were deposited at the same time. If we know the relative age of the fossil, we can know the age of the stratum that contains it, even if it is in another place.
- Principle of cross-cutting relationships.
- Uniformitarian Principle: The natural processes that acted in the past are the same as those acting in the present, and with the same intensity.
- Actualism Principle: “The present is the key to the past.”