Theories of the Origin of Earth & Geological Timescale

Early Theories of Origin of Earth

The major early theories of the Earth’s origin are discussed below.

1. Nebular Hypothesis – This theory was developed by Immanuel Kant and modified in 1796 by Pierre Laplace. According to this hypothesis, the planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
2. In 1900, Chamberlain and Moulton considered that a wandering star approached the sun which resulted in the formation of a cigar-shaped extension of material that got separated from the solar surface. This separated material continued to revolve around the sun and slowly got condensed into planets.
3. The binary theories considered a companion to be coexisting with the sun.
4. In 1950, the Nebular Hypothesis was revised by Otto Schmidt (in Russia) and Carl Weizascar (in Germany). According to them, the sun was surrounded by a solar nebula consisting mainly of hydrogen and helium along with dust. The friction and collision of particles led to the formation of a disk-shaped cloud and planets were formed through the process of accretion.

Origin of Earth Modern Theories

Big Bang Theory

• The Big Bang Theory explains the origin of the universe. It is also called the expanding universe hypothesis.
• In 1927, Abbe Georges Lemaitre, a Belgian astronomer was the first to provide a theory on the origin of the Universe. It was Edwin Hubble who provided evidence that the universe is expanding.
• According to this theory, all matter that formed the universe existed in one point (tiny ball) called singularity having an unimaginable small volume, infinite temperature and infinite density.
• The great event of the big bang happened some 13.7 billion years ago. The tiny ball exploded which led to a huge expansion and this expansion continues even today.
• There was rapid expansion within fractions of a second after the bang. Thereafter, the expansion slowed down. With the expansion some of the energy was converted into matter. Within the first three minutes from the big bang event, the first atom began to form.
• Within 300,000 years from the big bang, temperature dropped down to 4500 K and gave rise to atomic matter. The majority of atoms formed were hydrogen, along with helium and traces of lithium. Huge clouds of these elements fused through gravity to form stars and galaxies.
• Once there were two theories for explaining the origin of the universe – the Big Bang theory and the Hoyle’s concept of steady state.
• The steady state theory considered the universe to be roughly the same at any point of time. However, with greater evidence about the expanding universe, the Big Bang theory was confirmed which proposes that the universe originated from a single explosion of a very minute amount (tiny ball) of matter of high density and temperature.

Formation of Stars

Star formation is the process by which dense regions within molecular clouds in interstellar space called star-forming regions or stellar nurseries collapse under their own gravitational attraction and form stars. The formation of stars is believed to have taken place some 5-6 billion years ago.

Stages in the Formation of Stars:

• Nebula – It is a cloud of gas (mainly hydrogen and helium) and dust in space. It is a star’s birthplace.
• Protostar – It is an early stage of a star formation where nuclear fusion is yet to begin. It looks like a star but its core is not yet hot enough for nuclear fusion to take place.
• T Tauri Star – It represents an intermediate stage between a protostar and a low-mass main sequence star like the sun. It is a young, low-weight star, less than 10 million years old that is still undergoing gravitational contraction.
• Main Sequence Star – At this stage, the core temperature is enough to start the fusion reactions i.e., fusing hydrogen atoms to form helium atoms. The sun is the main sequence star.
• Red Giant – A red giant is formed during the later stages of its evolution as the star runs out of hydrogen fuel at its centre. However, it still fuses hydrogen into helium in a shell surrounding a hot, dense degenerate helium core. This fusion of hydrogen into helium around the core releases much greater energy, pushes much harder against gravity, and expands the volume of the star.
• Fusion of Heavier Elements – As the star expands, helium molecules fuse at the core which prevents the core from collapsing. When the fusion of helium ends, the core shrinks and begins fusing carbon. This process repeats until iron appears at the core. The iron fusion reaction absorbs energy, which causes the core to collapse. This implosion transforms massive stars into supernovae and smaller stars (sun) into white dwarfs.
• Supernovae and Planetary Nebulae – A planetary nebula is an outer layer of gas and dust that is lost when the star changes from Red Giant to White Dwarf. This white dwarf becomes black dwarf when it stops emitting light.

Supernova is the explosive death of a bigger star and it obtains the brightness of 100 million suns for a short time. Neutron stars are produced after a supernova (protons and electrons combine to produce neutron stars).

Our Solar System

Our solar system consists of the sun (the star), eight planets, 293 moons, asteroids, comets, and huge amounts of dust-grains and gases. The solar system is believed to have been formed about 5 – 5.6 billion years ago and the planets were formed about 4.6 billion years ago. The eight planets namely

• Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune revolve around the sun in fixed elliptical orbits. Mercury, Venus, Earth and Mars are called the inner planets and also terrestrial planets, which means earth-like as they are made up of rocks and metals. The other four planets – Jupiter, Saturn, Uranus, and Neptune are called outer planets as well as jovian (Jupiter-like) or Gas Giant planets. They are mostly larger than terrestrial planets and have a thick atmosphere mainly of helium and hydrogen.
• Pluto was earlier considered a planet; however, it is now believed to be a “dwarf planet”. Dwarf planets are tiny planets in our solar system. Any celestial body orbiting around the sun, weighing for self-gravity and nearly round in shape is called a dwarf planet.
• Number of satellites of planets:

Mercury – zero

Venus – zero

Earth – 1

Mars – 2

Jupiter – 95

Saturn – 146

Uranus – 28

Neptune – 16

Moon

• The moon is the only natural satellite of the earth. The word satellite means “companion”. The satellites move around a planet from west to east. They do not have their own light but reflect the light of the sun.
• The moon takes 27 days 7 hours and 43 minutes for both its rotation and revolution around the earth.
• It is the fifth largest natural satellite of the solar system. It is believed that the formation of the moon is a result of a giant impact called a ‘big splat’. A large body (somewhat one to three times the size of Mars) collided with the Earth just after it was formed.
• Due to this heavy impact, a large part of the earth got separated. This portion of blasted material continued to revolve around the earth and eventually formed the present moon (4.44 billion years ago).

Geologic Time

Geologic time refers to the vast expanse of time over which the Earth has formed and evolved, spanning billions of years. It’s the scale used by geologists, paleontologists, and other Earth scientists to understand the history of the Earth and the events that have shaped it. Geologic time is typically divided into a hierarchy of time units, each representing different intervals of time. These units range from the smallest increments, such as seconds or minutes, to the largest, such as eons. The most commonly used divisions of geologic time are:

• Eon: The largest division of geologic time, representing the longest intervals in Earth’s history. The two primary eons are the Archean and Proterozoic, which together comprise the Precambrian eon, and the Phanerozoic eon, which includes the last 541 million years up to the present.
• Era: Eons are subdivided into eras. For example, the Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. Each era represents a significant period characterized by distinctive geological and biological events.
• Period: Eras are further divided into periods. For instance, the Paleozoic era consists of the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods. Each period is characterized by particular geological features, fossil assemblages, and major events in Earth’s history.
• Epoch: Periods are divided into epochs, representing shorter intervals of time. Epochs are defined by specific geological and biological characteristics, such as changes in climate, sea levels, and species diversity.
• Age: Epochs are sometimes further subdivided into ages, though these subdivisions are less commonly used and may vary depending on the specific geological context.

The primary geochemical differentiation of Earth, which led to the formation of its core, mantle, crust, atmosphere, and hydrosphere, occurred during the early stages of the planet’s formation, known as planetary accretion and differentiation. Here’s a breakdown of how each component formed:

• Core: The Earth’s core is composed mainly of iron and nickel, with smaller amounts of other elements. During the early stages of Earth’s formation, denser materials such as iron and nickel sank towards the center due to their greater mass and gravitational attraction. This process is known as planetary differentiation. The heat generated by impacts and radioactive decay further contributed to the melting and segregation of these dense materials into the core.
• Mantle: Surrounding the core is the mantle, which is composed of silicate minerals rich in magnesium and iron. As the Earth differentiated, lighter materials, particularly silicates, remained closer to the surface. The mantle formed from the silicate-rich materials that surrounded the iron-nickel core. The mantle is divided into several layers based on differences in composition and physical properties.
• Crust: The Earth’s crust is the outermost solid shell of the planet. It is primarily composed of silicate minerals, including feldspar and quartz, along with other elements such as aluminum, calcium, and oxygen. The crust formed through processes such as volcanic activity, solidification of molten rock (magma), and differentiation of materials during the early stages of Earth’s history. The crust is divided into two types: continental crust, which is thicker and less dense, and oceanic crust, which is thinner and denser.
• Atmosphere: The Earth’s early atmosphere likely formed through the release of gases from volcanic activity and the outgassing of volatile compounds from the interior during planetary accretion. Initially, the atmosphere was composed mainly of hydrogen, helium, methane, and ammonia. Over time, the Earth’s atmosphere underwent significant changes through processes such as photosynthesis, which added oxygen, and chemical reactions involving volcanic emissions and surface interactions.
• Hydrosphere: Water on Earth likely originated from cometary impacts during the planet’s early history. As the Earth cooled and its atmosphere formed, water vapor condensed and accumulated on the surface, forming oceans, lakes, and other bodies of water. The hydrosphere also includes water stored in ice caps, glaciers, and groundwater.

Minerals and Rocks

About 98% of the total crust of the earth is composed of eight elements namely, oxygen, sodium, calcium, iron, magnesium, silicon, aluminium and potassium, and the rest is constituted by titanium, hydrogen, phosphorus, manganese, carbon, sulfur, nickel and other elements. These elements combine with other elements to form substances called minerals.

Definition of a mineral – A mineral is defined as a naturally occurring organic and inorganic substance, having an orderly arrangement of atoms and a definite composition and physical properties.

• The elements found in the lithosphere combine in different ways to form minerals of various types. About 2000 minerals have been found in the earth’s crust, but most of the commonly occurring ones are related to six major mineral groups that are called major rock-forming minerals.
• The hot magma in the interior of the earth is the main source of all minerals. The magma cools and crystals of minerals get formed. A systemic series of minerals are formed in sequence to solidify so as to form rocks.

Physical Characteristics of Minerals.

External crystal form – External shape of a mineral is determined by its internal arrangement of molecules. It can be cubic, tetrahedral, tabular, hexagonal, etc.

Cleavage – The property of breaking along specific planes is called cleavage. Crystals have one plane along which the bonding between the atoms is weaker than along other planes.

Fracture – When the internal molecular arrangement is so complex that there are no planes of molecules, the crystal will break in an irregular manner.

Lustre – Lustre describes the shining quality of an object. Each mineral has a distinctive lustre like metallic, glossy, silky, etc.

Colour – The colour of a mineral depends on its molecular structure. Some minerals show specific colors like malachite (green), azurite (blue), chalcopyrite (golden yellow), etc. Some minerals due to the presence of different impurities show different colors, such as quartz – it can be red, white, green, etc.

Streak – It is the color of the ground powder of any mineral. It may be of the same color as the mineral or different. For example, Malachite is green and gives a green streak, Fluorite is purple or green but gives a white streak, and chromite and magnetite are almost black and can be distinguished by their streaks – brown for chromite and black for magnetite.

Specific gravity – It is the ratio between the weight of a given mineral and the weight of an equal amount of water. Since it is a ratio, it has no units. For instance, the specific gravity of quartz is 2.65.

Hardness – The hardness of a mineral is measured by its ability to resist scratching. To have a standard method of expressing the hardness of minerals, a standard scale called the Mohs scale is commonly adopted. In the sequence of increasing hardness from 1 – 10, the following minerals are used as the standard of comparison – talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, and diamond. Compared to this, a fingernail is 2.5 and glass or knife blade is 5.5.

Structure – It refers to the particular arrangement of the individual crystals.

Transparency – Transparent, when the light rays pass through so that the objects can be seen. Translucent, when light rays pass through but get diffused and the objects cannot be seen. Opaque, when the light will not pass through. Know more about the physical characteristics of minerals in the linked article.

Classification of Minerals

Broadly, minerals can be classified into metallic and non-metallic minerals.

Metallic Minerals: These minerals are composed of metals and can be divided into three subtypes –

• Precious metals – Platinum, gold, silver, etc. Ferrous metals – Iron mixed with other metals.
• Non-ferrous metals – Metals other than iron like copper, aluminium, lead, zinc, tin, etc. Metallic minerals are generally obtained from igneous rocks, and are malleable and ductile.

Non-Metallic Minerals: These minerals are composed of non-metals like sulphur, silicon, phosphorus. For example, cement is a mixture of non-metallic minerals. Non-metallic minerals are generally obtained from sedimentary rocks, lacking malleability and ductility.

Rocks

Rock is the solid mineral material forming the surface of the earth. A rock is composed of one or more minerals. Petrology is the science of rocks which includes the studying of mineral composition, structure, texture, origin, occurrence, alteration and relationship with other rocks. The age of a rock is determined based on carbon-14 dating.

Classification of Rocks Based on the origin, rocks are of three types – igneous, sedimentary and metamorphic.

Igneous Rocks

• “Ignis” in Latin means ‘fire’. Igneous rocks are formed out of magma and lava from the interior of the earth. When magma in its upward movement cools and turns into solid form, it is called igneous rock.
• There are two types of igneous rocks – intrusive rocks e.g., granite, and extrusive rocks e.g., basalt Deccan Traps. Intrusive rocks are formed when magma rises and cools within the crust which gives rise to various forms like batholiths, laccoliths, dyke, etc. Extrusive rocks are formed when cooling and solidification take place on the surface of the earth.
• Igneous rocks are also classified based on the texture, size, and arrangement of grains or other physical conditions of the materials. If the magma cools slowly at great depths, mineral grains increase in their size. Sudden cooling at the surface results in small and smooth grains.
• The igneous rocks are the oldest of all the rocks. Pegmatite, gabbro, granite, basalt, tuff are some of the examples of igneous rocks.

Sedimentary Rocks:

• Sedimentary rocks are also called detrital rocks. The word ‘sedimentary’ is derived from the Latin word sedimentum, which means settling.
• Rocks of the earth’s surface undergo denudation and are broken into various fragments. These fragments are transported by different exogenous forces and deposited.
• These deposits through compaction turn into sedimentary rocks. The process is called lithification.
• Sedimentary rocks occupy only 5% of the earth.
• They are layered or stratified of varying thickness. Sedimentary rocks are of three types depending upon the mode of formation –
• Mechanically formed sedimentary rocks – For example, conglomerate, loess, limestone, sandstone, etc.
• Chemically formed – For example, potash, halite, etc.
• Organically formed – For example, chalk, coal, limestone, geyserites, etc.

Metamorphic Rocks:

• The word metamorphic means ‘change of form’. The metamorphic rocks form under the action of pressure, volume, and temperature (PVT change).
• Metamorphism is a process by which the already consolidated rocks undergo recrystallization and reorganization of materials within the original rocks. The igneous and metamorphic rocks together account for 95% of the earth.
• The breaking and crushing of the original minerals within rocks without any significant chemical changes is called dynamic metamorphism.
• When the materials of the rocks alter chemically and recrystallize, the process is known as thermal metamorphism. Thermal metamorphism is of two types – contact metamorphism and regional metamorphism.
• Contact Metamorphism – In this case, the rocks come in contact with hot magma and lava as a result of which rock materials recrystallize under high temperatures. Generally, new materials form when lava/magma interacts with the rocks.
• Regional Metamorphism – Due to deformation caused by tectonic shearing together with high temperature or pressure or both, rocks undergo recrystallization which is known as regional metamorphism.
• Sometimes rock grains or minerals form layers or lines during the process of metamorphism. Such an arrangement in metamorphic rocks is called foliation or alienation.
• Sometimes minerals of different nature form alternating arrangements of thin and thick layers that appear in light and dark shades. Such an arrangement in metamorphic rocks is called banding and such rocks are called banded rocks.
• Slate, diamond, marble, quartzite, schist, gneiss are some examples of metamorphic rocks.

Rock Cycle:

The Rock Cycle is a continuous process through which old rocks are transformed into new ones. Igneous rocks are primary rocks and other rocks, sedimentary and metamorphic form from these igneous rocks. These primary rocks under the influence of high pressure/temperature transform into metamorphic rocks. The igneous and metamorphic rocks can break into fragments and these fragments can be the source of sedimentary rocks. The crustal rocks – igneous, sedimentary, and metamorphic once formed may be carried down into the mantle (interior of the earth) through the subduction process, and the same melt and turn into magma which is the source of igneous rocks. In this way, the rock cycle is a continuous process.

Controls on formation of landforms – tectonic including plate tectonic and climatic 

The formation of landforms is influenced by various factors, including tectonic processes (such as plate tectonics) and climatic conditions.

Tectonic Processes:

• Plate Tectonics: Plate tectonics is a fundamental geological process that shapes the Earth’s surface. It involves the movement and interaction of lithospheric plates, which float on the semi-fluid asthenosphere beneath them. Plate boundaries are dynamic zones where tectonic activity occurs, leading to the formation of various landforms:
  • Divergent Boundaries: At divergent boundaries, lithospheric plates move away from each other, often resulting in the formation of rift valleys and mid-ocean ridges. For example, the East African Rift Valley and the Mid-Atlantic Ridge are formed at divergent boundaries.
  • Convergent Boundaries: At convergent boundaries, lithospheric plates collide or move toward each other. Depending on the types of plates involved (oceanic-oceanic, oceanic-continental, or continental-continental), various landforms can be created, including mountain ranges, volcanic arcs, trenches, and folded mountain belts. Examples include the Andes Mountains (formed by the convergence of the South American and Nazca plates) and the Himalayas (formed by the collision of the Indian and Eurasian plates).
  • Transform Boundaries: At transform boundaries, lithospheric plates slide past each other horizontally. This can lead to the formation of linear features such as strike-slip faults and transform fault zones. The San Andreas Fault in California is a well-known example of a transform boundary.

• Faulting and Folding: Tectonic forces can cause rocks to deform through faulting (movement along fractures) and folding (bending of rock layers). These processes contribute to the formation of various landforms, including fault-block mountains, rift valleys, and anticlines/synclines.

Climatic Conditions:

a. Erosion and Weathering: Climatic factors such as temperature, precipitation, and wind influence the rates and types of erosion and weathering processes that shape landforms. For example, chemical weathering is more prevalent in warm and humid climates, leading to the formation of karst landscapes characterized by features like caves, sinkholes, and limestone pavements. Physical weathering processes such as frost action and exfoliation are more pronounced in cold and/or arid climates.

b. Glacial Processes: Glacial activity, influenced by climatic conditions, can shape landforms through processes like erosion, transportation, and deposition. Glaciers carve out U-shaped valleys, create moraines, and deposit sediment as they advance and retreat, forming features like drumlins, eskers, and kettle lakes.

c. Coastal Processes: Coastal landforms are shaped by a combination of tectonic and climatic factors, including wave action, tidal currents, and sea level changes. Coastal erosion, sediment transport, and deposition create features such as sea cliffs, beaches, spits, and barrier islands.