Geological Time Scale – UGC NET Geography – Detailed Notes

The beginning of the cosmic time scale is traced to the existence of extremely hot original matter in the Universe about 15 billion years ago. This primordial matter possessed unimaginably high temperatures. Since the Universe cannot be younger than its own components, it must be older than galaxies, the Earth, and all chemical elements.

Big Bang Expansion and Separation of Galaxies

Evidence suggests that around 10–12 billion years ago, this primordial mass expanded rapidly and exploded. This event led to the separation of galaxies, followed by the creation of stars and their planetary systems. If expansion continues indefinitely, galaxies will drift farther apart until each stands isolated in space.

Formation of Galaxies, Stars, and Planets

Approximately 10 billion years ago, the first-generation galaxies began spiraling and separating. The Milky Way Galaxy emerged around 6 billion years ago. From this mass, the Sun and the Solar System formed roughly 5 billion years ago. More precise estimates place the birth of the Sun and its planets at about 4.56 billion years ago.

Geologic Time Scale

Units of Geologic Time

Geologic time is divided into hierarchical units used to describe Earth’s long history:

• Eon: Time span of half a billion years or more
• Era: Duration of several hundred million years
• Period: Time unit covering hundreds of millions of years
• Epoch: Spanning tens of millions of years
• Age: Spanning millions of years

Major Eons of Earth’s History

The Earth’s geological time is categorized into two major eons:

• Phanerozoic Eon (Visible life)
• Cryptozoic Eon (Hidden or no visible life)

Meaning of Phanerozoic and Cryptozoic

The term Phanerozoic comes from Greek Phaneros (visible) and Zoo (life), referring to the eon in which visible life flourished.
The term Cryptozoic (hidden life) denotes the earlier eon when life was primitive and less visible.

Major Geological Eras

List of the Five Major Eras:

1. Cenozoic Era – 65 mya to present (Recent life)

2. Mesozoic Era – 245–66.4 mya (Age of Reptiles/Ammonoids)

3. Palaeozoic Era – 540–245 mya (Ancient life)

4. Precambrian or Proterozoic Era – 4500–540 mya (Primitive life)

5. Archaean Era – Earliest geological era

The Precambrian Era

Duration and Importance:

The Precambrian Era spans from approximately 4500 mya to 540 mya, covering nearly 88% of Earth’s total geologic time. It encompasses the time between the formation of Earth (about 4600 mya) and the beginning of the Cambrian Period.

Fossil and Geological Characteristics:

Very little is known about this era due to poor fossil records. Fossils such as stromatolites exist but have limited use in biostratigraphy. Tectonic movement and other geologic events of this time remain poorly defined.

Significant Features of the Precambrian:

• First appearance of life
• Existence and movement of tectonic plates
• Gradual oxygen enrichment of the atmosphere
• Climatic transition from dry and cold to warm and moist conditions

Archaean Era (3960–2500 mya)

• Extensive mountain-building events

• Formation of banded iron formations and greenstone belts

• Presence of shallow seas

• Accumulation of free oxygen

• Origin of early life forms including prokaryotes, bacteria, and blue-green algae

Proterozoic Era (2500–540 mya)

• Alternation of dry, cold climate with warm, moist conditions

• Emergence of eukaryotic cells and multicellular life

• Earliest known fossil records, including soft-bodied marine invertebrates

• Origin of sponges, cnidarians (sea anemones), and annelids (segmented worms)

Palaeozoic Era (540–245 mya)

The Palaeozoic Era, known as the era of ancient life, marks the flourishing of invertebrates. It spans from 540 to 245 million years ago and is divided into six geological periods.

Six Periods of the Palaeozoic Era:

1. Permian – 286–245 mya

2. Carboniferous – 360–286 mya

3. Devonian – 408–360 mya (Age of Fish)

4. Silurian – 438–408 mya

5. Ordovician – 505–438 mya (Age of Graptolites)

6. Cambrian – 540–505 mya (Age of Trilobites)