Earth’s Origin, Structure, and Internal Dynamics
Item 14: Origin and Structure of the Earth
Source of the Solar System
Planetesimal Theory
- Solar Nebula: About 4,600 million years ago, a rotating cloud of dust and gas, larger than the solar system, began to contract.
- Gravitational Collapse: Contraction or collapse to form a central mass and a rotating disk around it.
- Formation of Protosun: The collision of particles in the central mass releases energy, initiating nuclear fusion of hydrogen, marking the birth of a star, the protosun, inside the nebula.
- Formation of Planetesimals: Particles of dust and gas in the rotating disc around the protosun grouped, forming millimeter-sized granules, which through collisions and mergers, originated larger bodies, planetesimals, ranging from hundreds of meters to kilometers in size.
- Formation of Protoplanets: Collisions and accretion of planetesimals formed primitive planets or protoplanets.
Formation of the Earth
According to the planetesimal theory, the Earth’s formation followed this process:
- Formation of Protoplanet: Within the nebular disk surrounding the protosun, the accretion of planetesimals formed the original Earth. Its increasing size also increased its gravitational field, favoring further accretion. During this stage, the Earth’s temperature rose due to planetesimal impacts.
- Differentiation by Density: The primordial Earth was partially molten, allowing for a distribution of its components according to density. Iron occupied deeper areas, forming the Earth’s core, a process known as the iron catastrophe. Simultaneously, planetary degassing occurred, with internal gases escaping to form the atmosphere.
- Cooling and Ocean Formation: Once the planetesimal bombardment reduced, the Earth began to cool. The decrease in surface temperature allowed water vapor to condense, forming the oceans in the lowest relief areas.
Origin of the Moon
The most accepted theory, developed by William Hartmann and Donald Davis, suggests that in the early Earth’s existence, a Mars-sized protoplanet collided with Earth. The impact ejected a debris cloud into orbit, which accreted to form the Moon.
Exploring Earth’s Interior
Knowledge of Earth’s interior comes from two methods:
- Direct Methods: Observation of accessible areas and materials from Earth’s interior that reach the surface.
- Indirect Methods: Inferences about the interior’s characteristics from data like the behavior of seismic waves.
Mines and Soundings
Mines are excavations for mineral extraction. Soundings are drilled holes, used, for example, in oil extraction. They reveal that temperature increases with depth, known as the geothermal gradient.
Volcanoes
Volcanic eruptions bring material from the interior to the surface. Most materials are from shallow depths, formed by partial melting of rocks. Sometimes, magma from deeper areas carries rock fragments as inclusions, providing samples of deeper materials.
Earthquakes and Seismic Waves
The seismic method studies earthquakes and seismic wave propagation.
Earthquakes, or seisms, are ground vibrations from the sudden release of energy in rocks under stress. They occur when large rock masses fracture or shift along faults.
- Seismic Focus (Hypocenter): The earthquake’s origin point.
- Epicenter: The surface point nearest to the focus.
- Seismic Waves: Vibrations propagating from the focus.
Types of Seismic Waves
- P-waves (Primary): Faster, longitudinal waves, causing compression and dilation.
- S-waves (Secondary): Slower, transverse waves, causing perpendicular ground motion.
- Surface Waves: Created when S-waves reach the surface.
Registration of Earthquakes
Seismographs record and measure earthquakes, producing seismograms.
Propagation of Seismic Waves
The speed of seismic waves depends on the material they travel through. Variations in speed cause changes in the wave’s direction.