Mains Marks Booster     4th August 2023        


Origin and evolution

Big Bang Theory or Expanding Universe Hypothesis

  • The universe began with a tiny, dense ball called the "Tiny Ball" or Big Bang.
  • The Big Bang occurred about 13.7 billion years ago and caused a violent explosion.
  • The universe rapidly expanded at first, but the expansion has since slowed down.
  • While space between galaxies is increasing, observations do not support the idea that galaxies themselves are expanding.

Formation of Stars and its Life Cycle

  • Stars formed billions of years ago from growing nebulae, which are dense clouds of gas and dust in space.
  • Galaxies are vast collections of stars spread across immense distances measured in light-years.
  • Constellations are patterns formed by groups of stars, such as Ursa Major or the Big Bear.
  • Stars were used by ancient people for navigation and determining directions at night.
  • The North Star, also known as the Pole Star, remains fixed in the sky and indicates the north direction.

Solar/Stellar Flare

  • Solar flares are sudden brightness surges on stars due to magnetic energy release.
  • They occur near sunspots, often paired with coronal mass ejections.
  • These flares eject clouds of electrons, ions, atoms, and radiation.
  • Strong flares, like those from Proxima Centauri, can strip water and sterilize grounds.
  • If a solar flare is directed at Earth, it can cause auroras.
  • These flares' X-rays and UV rays can disrupt long-range radio communication.
  • They pose significant radiation risks for manned space missions.

Sunspot Cycle

  • The amount of magnetic flux that rises up to the Sun's surface varies with time in a cycle called the solar cycle or sunspot cycle, which lasts approximately 11 years on average, is sometimes referred to as the sunspot cycle.
  • Sunspots are darker, magnetically strong, and cooler areas on the surface of the Sun. Sunspots are not present all over the Sun but are found between 25°-30° latitude.

Understanding the long-term variations of the Sun and its impact on Earth's climate is one of the objectives of the Aditya L-1 Mission.

Dwarf Planet

  • A celestial body that orbits around the Sun. It has sufficient mass for its self-gravity to overcome rigid body forces, resulting in a nearly round shape (hydrostatic equilibrium).
  • It has not cleared the neighborhood around its orbit. It is not a satellite.
  • Prominent Dwarf Planets: Pluto, Ceres, Makemake, Haumea, Eris

Interior of the Earth

Sources of information about the Earth's interior

Direct Sources

Indirect Sources

  • Deep earth mining and drilling: Mponeng gold mine and TauTona gold mine in South Africa are the deepest mines, reaching a depth of 3.9 km. The deepest drilling goes as far as 12 km.
  • Volcanic eruption: Provides direct information about the interior.
  • High Levels of Temperature and Pressure Downwards: Volcanic eruptions, hot springs, and geysers indicate a very hot interior. High temperatures are caused by the disintegration of radioactive substances.
  • Gravitation and the Earth's diameter help estimate deep interior pressures.
  • Evidence from Meteorites: When meteorites fall to Earth, their outer layers burn due to friction, exposing their inner cores. The composition of their cores confirms the similar composition of the Earth's inner core, as both evolved from the same star system in the past.
  • Depth: As depth increases, pressure, density, and temperature also increase due to gravitation.
  • Meteors: Meteors and Earth share a similar internal structure as they originate from the same nebular cloud.
  • Gravitation: The gravitational force (g) varies at different latitudes on the Earth's surface. It is greater near the poles and lesser at the equator due to the distance from the center.
  • Gravity anomalies: Uneven distribution of mass within the Earth influences gravity values, leading to variations known as gravity anomalies. These anomalies provide information about the distribution of mass in the Earth's crust.
  • Magnetic field: The geodynamo effect helps scientists understand the Earth's core. Shifts in the magnetic field offer clues about the inaccessible iron core.
  • Seismic Activity: The most important indirect source of information is seismic activity. Study of seismic waves provides significant understanding of the Earth's internal structure.

Seismic Waves

The study of seismic waves provides a complete picture of earth’s layered interior.

Causes of Earthquakes

Sudden Energy Release along Fault: The abrupt release of energy along a fault generates seismic waves.
Faults in the Earth's Crust: Faults are sharp fractures in the Earth's crustal rock layer.
Opposing Movement of Rocks: Rocks adjacent to a fault tend to move in opposite directions. Friction from overlying rock strata prevents the movement of rocks.
Accumulation of Pressure: Over time, pressure builds up in the rocks due to the hindered movement.
Overcoming Friction and Sudden Movement: Under intense pressure, the rock layer overcomes friction and experiences a sudden movement, leading to the generation of shockwaves.
Focus and Epicenter: The point where energy is released is called the focus or hypocenter. The waves reach the surface. The epicenter is the point on the surface nearest to the focus, directly above it.

Earthquake Waves

Natural Occurrence in the Lithosphere: Natural earthquakes happen within the lithosphere, which encompasses the upper 200 kilometers of the Earth's surface.
Seismographs for Wave Recording: Seismographs are instruments used to record waves reaching the surface during an earthquake.
Classification of Waves: Earthquake waves can be categorized into two types: body waves and surface waves.
Origin and Propagation of Body Waves: Body waves originate from the energy release at the earthquake's focus and propagate in all directions through the Earth's interior.
Interaction and Generation of Surface Waves: Body waves interact with surface rocks, generating surface waves that move along the Earth's surface.
Velocity Variation and Material Elasticity: The velocity of seismic waves changes as they travel through materials with different elasticity or stiffness, and sometimes density.
Directional Changes: Waves can reflect or refract when encountering materials with varying densities, resulting in changes in their direction.
Primary Waves (P-waves) and Secondary Waves (S-waves): P-waves are longitudinal waves that propagate faster and arrive first at the surface, while S-waves are transverse waves that follow with a time lag.

Propagation of Earthquake Waves

Earthquake waves of different types travel in distinct ways, causing vibrations in rocks as they propagate.
P-waves vibrate parallel to the wave's direction, exerting pressure on the material and creating density differences that result in stretching and squeezing.
The other two waves vibrate perpendicular to the propagation direction, with S-waves vibrating perpendicular in the vertical plane, forming troughs and crests in the material they pass through.

Emergence of Shadow Zone

Seismographs located far away record earthquake waves, but there are specific areas where the waves are not detected, known as the "shadow zone."
Each earthquake has its own unique shadow zone, as shown in Figure alongside depicting the shadow zones of P and S-waves.
Seismographs within 105° from the epicenter record the arrival of both P and S-waves, while those beyond 145° only detect P-waves.
Thus, the zone between 105° and 145° from the epicenter is identified as the shadow zone for both wave types, with S-waves not reaching beyond 105°.
The shadow zone for S-waves is larger, covering over 40% of the Earth's surface, while the shadow zone for P-waves appears as a band between 105° and 145° away from the epicenter.

Determining the Earth's Interior through the Characteristics of 'P' and 'S' Waves

Reflection causes waves to rebound, while refraction changes their direction. By observing the record of waves on seismographs, the variations in wave direction can be inferred.
Changes in density significantly affect wave velocity, allowing estimation of the Earth's overall density by observing velocity changes.
The emergence of shadow zones (changes in wave direction) helps identify different layers within the Earth.

Earth's Layers

Earth's layers are identified by studying various direct and indirect sources. The structure of the earth's interior consists of several concentric layers.
Three main layers can be identified: crust, mantle, and core.

Seismic Discontinuities

 Seismic Discontinuities

Earth's Layers Based on Chemical Properties

Earth's Layers Based on Chemical Properties