Structure of the Universe and Plate Tectonics
Structure of the Universe
Galaxies
Galaxies are grouped in clusters called superclusters, which are arranged in filaments. They are accumulations of cosmic dust, nebulae, and stars. The space between galaxies is called the interstellar medium.
The Milky Way
The Milky Way is a spiral galaxy with nebulae, dust, and 10 to 30 million stars. Our sun is located in one of its arms.
Parts of the Milky Way:
- Medulla or Nucleus: A cluster of old stars, possibly with a black hole in the center.
- Disk: Contains cosmic dust and young stars distributed into 5 arms: Perseus, Orion, Sagittarius, Centaurus, and Cygnus.
- Halo: Spherical, containing old stars clustered in globular clusters or isolated.
Stars
Stars are born in interstellar nebulae. They are huge balls of gas, primarily hydrogen (H) and helium (He). These gases are extremely hot, reaching temperatures high enough to make the interiors of stars act like giant thermonuclear fusion bombs.
Nebulae
Nebulae are clouds of H, He, and heavier chemical elements in powder form, with certain quantities of cosmic and organic compounds.
Formation of a Star Like the Sun
The nebula collapses under its own gravity, and the smallest fragments form protostars. The increased temperature increases collisions of H atoms until they reach 1017 °C, at which point the H atoms start to fuse. This fusion process accumulates in the star’s center, and the star emits a lot of energy (the star is lit). The star’s energy would cause it to explode, but gravity prevents this. The H is turning into helium (He), and the reactions move to the periphery where there is still H. When the H component decreases, the gravitational force increases, and the expansive component becomes a red giant. The He that has accumulated in the nucleus becomes compacted and turns into carbon (C). This releases a lot of energy, making the star unstable. Its outer layers disperse into a ring of gas and dust, known as a planetary nebula. Inside, the nucleus of the former red giant becomes a white dwarf, getting energy from the synthesis of C to He. When this energy runs out, it becomes a black dwarf.
Supernova
A supernova is the explosion and final phase of a star’s life.
Formation of the Solar System
The solar system is immersed in a local bubble. The disk of gas and dust in the center contracted into a ball of H and He. As it warmed, nuclear reactions began, and the sun was lit. The outer regions formed swirls and underwent two processes:
Coagulation: Cosmic dust particles collided and formed larger bodies called planetesimals.
Planetesimal Accretion: Planetesimals impacted each other and grew, forming planets.
Solar System
Planets are celestial bodies that orbit stars. They revolve around the sun and may possess satellites or moons. They are divided into two categories:
Rocky or Inner Planets: These are denser and rocky, including the first four planets.
Gas Giants: These have giant gaseous envelopes and rocky cores, including the outer four planets.
Dwarf Planets: These include Pluto, Ceres, and Eris.
Small Solar System Bodies: These include satellites, comets, and asteroids. They are located in:
Asteroid Belt: Between Mars and Jupiter.
Kuiper Belt: Beyond Neptune and Pluto.
Oort Cloud: At the confines of the solar system. Organic molecules, ice, and dust accumulate here, forming comets.
Plate Tectonics
Early Theories
- Neptunism: All rocks were formed by sedimentation and crystallization in primitive seas.
- Catastrophism: The formation of fossils and the modern form of our planet are due to successive catastrophes.
- Plutonism: Consolidation of sediments and the origin of volcanic and plutonic rocks are due to the cooling of magma.
- Uniformitarianism or Present: Natural forces that acted in the past are the same as those acting today.
- Vertical/Horizontal Mobility: Movement of the Earth’s crust based on horizontal and vertical forces.
Geochemical Model of the Earth’s Crust
The Earth’s layers are divided into the crust, Mohorovicic discontinuity, upper mantle, Repetti discontinuity, lower mantle, Gutenberg discontinuity, outer core, Lehmann discontinuity, and inner core.
Crust
The crust is the outermost layer, extending to the Mohorovicic discontinuity. It is formed by silicates of aluminum (Al), calcium (Ca), sodium (Na), and potassium (K).
Continental Crust: Extends up to 70km in depth. Formed by magmatic, metamorphic, and sedimentary rocks, with abundant andesite and granite.
Oceanic Crust: 6 to 12km thick, formed by basalts and gabbros.
Mantle
The mantle extends from the Mohorovicic discontinuity to the Gutenberg discontinuity. It is formed by peridotite. The first discontinuity appears at 400km depth, where olivine compacts into spinel. At 670km, the second transition occurs when spinel transforms into perovskite.
Core
The core extends from the Gutenberg discontinuity to the Earth’s center. The outer core is liquid and composed of iron (Fe), nickel (Ni), sulfur (S), silicon (Si), and possibly iodine (I). The inner core is solid and composed of Fe and nickel alloy.
Plate Boundaries and Processes
Mid-Ocean Ridges
Mid-ocean ridges are submarine reliefs with intense submarine volcanism. Magma with basaltic rocks is constantly released, creating new oceanic lithosphere. These are called constructive margins and are fragmented by transform faults.
Subduction Zones
Subduction zones are called destructive margins because they destroy oceanic lithosphere. They form elongated depressions called ocean trenches. The Benioff plane is where most seismic foci are located.
Transform Faults
Transform faults are called neutral margins because they slide against each other, generating seismic activity.
Theory of Plate Tectonics
Plate tectonics is a global theory that explains major geological processes. These processes are motivated by a common cause: the Earth’s internal heat, aided by gravitational potential energy.
Earthquakes
Earthquakes occur in subduction zones, mid-ocean ridges, and transform faults. Large masses of rocks collide, causing earthquakes.
Volcanoes
Volcanoes occur in mid-ocean ridges, subduction zones, and hot spots. Magma escapes through cracks in the Earth’s crust.
Formation of Mountains
Mountains are formed when plates collide, pushing sediments that have accumulated in subduction zones. These sediments fold and then rise.
Expansion of the Oceans
Oceanic lithosphere is created continuously on both sides of mid-ocean ridges, causing the oceans to expand.
Continental Drift
Continents, which are part of lithospheric plates, drift and move.
Mineral and Oil Deposits
Plate tectonics allows for the location of mineral and oil deposits.
Volcanism
Volcano Formation
Volcanoes are formed when magma from the mantle rises to the surface through fissures. It cools and releases gas and molten rock (pyroclasts).
Drilling of the Oceanic Lithosphere and Formation of Volcanic Chains
Guyots and atolls are formed when volcanic activity is intense. Volcanic islands form chains, generally leading to volcanic cones.
Origin of Igneous and Basaltic Provinces
Large parts of the world are covered by basaltic lavas.
Thinning of the Continental Lithosphere and Formation of a Rift
A hot spot acts like a torch, heating the lithosphere, causing it to bulge and form a triple junction.
Volcanism in Subduction Zones
Oceanic lithosphere subducting beneath oceanic islands forms arch-shaped archipelagos. Oceanic lithosphere subducting beneath continental lithosphere forms continental volcanic arcs.
Seismos
Earthquakes
Earthquakes occur in mid-ocean ridges, subduction zones, and transform faults. They are caused by the fracturing of rocks at depth.
Seismic Waves
Seismic waves are generated at the focus or hypocenter and are analyzed by seismographs. The epicenter is the surface area above the focus.
Primary (P) Waves: Compression waves that cause rocks to undergo successive compressions and expansions. They are the first to arrive and cause movements in gases, liquids, and solids.
Secondary (S) Waves: Transverse waves that move up and down, perpendicular to the direction of the wave. They arrive second and travel through liquids and solids.
Surface (L) Waves: Arrive after P and S waves. They come in two types: Rayleigh waves (vertical and horizontal movements) and Love waves (horizontal movements).
Earthquake Measurement Scales
Richter Scale: Measures the magnitude of an earthquake, representing the energy released.
MSK Scale: Measures the intensity of an earthquake, ranging from I to XII.
Seafloor Spreading and Subduction
Spreading Ridges
Spreading ridges are responsible for the fragmentation of continents. They cause the expansion of the ocean floor and are called constructive or divergent margins.
Stages of Spreading Ridges:
Stage of Ballooning: The mantle current rises.
Young Ridges: The lithosphere thins.
Middle-Aged Ridges (Red Sea Stage):
Mature Ridges (Atlantic Stage):
Subduction Zones
Subduction zones are called destructive or convergent margins. Oceanic lithosphere is destroyed, causing significant seismic activity.
Oceanic Lithosphere Subducting Beneath Oceanic Lithosphere (e.g., Pacific Plate): Leads to a deep ocean trench. The subducting plate forms magma, resulting in an archipelago.
Oceanic Lithosphere Subducting Beneath Continental Lithosphere (e.g., Andes): Forms orogenies (mountain belts) that extend for hundreds or thousands of miles along convergent plate margins.
Wilson Cycle
The Wilson Cycle is an evolutionary cycle that explains the opening and closing of ocean basins and the distribution of continents over time.