The geological time scale, impacts, extinctions and the Anthropocene

Detail

1. The geological time scale: how Earth's history is divided

• Geological time scale: a calendar of the Earth's 4.6 billion years. It is built from rock layers and the fossils in them.
• The official version is the International Chronostratigraphic Chart. The International Commission on Stratigraphy (ICS) keeps it. Stratigraphy is the study of rock layers and their order in time.
• The units, from largest to smallest:
• Eon → Era → Period → Epoch → Age.
• Each one is split into the next smaller unit.

Eon	Approximate span	What happened
Hadean	4.6 – 4.0 billion years ago	Earth forms, molten surface, the Moon forms, the first crust appears
Archean	4.0 – 2.5 billion years ago	Oldest surviving rocks; first simple life (bacteria)
Proterozoic	2.5 billion – 539 million years ago	Oxygen builds up in the air; first cells with a nucleus; first many-celled life
Phanerozoic ("visible life")	539 million years ago – today	Life with hard shells and skeletons; most fossils come from this eon

• Precambrian: an informal name for the Hadean, Archean and Proterozoic together, so everything before 539 million years ago. It covers almost 90% of Earth's history.

Eras of the Phanerozoic

Era	Meaning	Span (million years ago)	Periods (oldest → youngest)	Main life
Palaeozoic	"old life"	539 – 252	Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian	Sea animals, fish, first land plants, amphibians, coal forests
Mesozoic	"middle life"	252 – 66	Triassic, Jurassic, Cretaceous	Dinosaurs, first mammals and birds, flowering plants
Cenozoic	"new life"	66 – today	Palaeogene, Neogene, Quaternary	Mammals spread; humans appear

• The Quaternary period has two epochs. The Pleistocene was the age of the great Ice Ages. The Holocene is the present warm epoch.

2. Key time markers to fix

Event	When	Note
Cambrian explosion	about 539 million years ago	A fast spread of animals with hard parts. It marks the start of the Phanerozoic and the Palaeozoic.
Break-up of Pangaea	about 200 million years ago	Pangaea was the single supercontinent. It split into Laurasia (north) and Gondwanaland (south). See plate-tectonics.
End of non-bird dinosaurs	66 million years ago	The end-Cretaceous extinction. It closes the Mesozoic. Birds are the surviving dinosaur line.
Holocene begins	11,700 years ago	The last Ice Age ends and farming begins.

3. Indian rock systems on the time scale

• India's rocks are grouped into rock systems, and each system fits a slot on the global scale (see minerals-rocks-rock-cycle).

Indian rock system	Global time slot	Examples in India	Economic value
Archaean (incl. Dharwar)	Archean–early Proterozoic	Gneiss and schist of the Peninsula; Dharwar rocks in Karnataka	Iron ore, gold (Kolar, Hutti), manganese
Purana (Cuddapah, Vindhyan)	Proterozoic	Cuddapah basin (Andhra Pradesh); Vindhyan range (Madhya Pradesh)	Limestone, building sandstone, diamonds (Panna)
Dravidian	Palaeozoic	Extra-Peninsular areas such as Kashmir and Spiti	Few minerals
Aryan: Gondwana, Deccan Traps, Tertiary	Late Palaeozoic to Cenozoic	Damodar, Mahanadi and Godavari valleys (Gondwana); Maharashtra plateau (Deccan Traps); Assam and Himalayan foothills (Tertiary)	About 98% of India's coal is Gondwana coal; Tertiary rocks hold coal and petroleum

4. Mass extinctions: the "Big Five"

• Mass extinction: a large share of the world's species dies out in a short geological time, often within a few thousand to a few hundred thousand years.
• Five such events stand out in the fossil record:

#	Event	Approximate time	Likely cause
1	End-Ordovician	about 445 million years ago	Ice age and a fall in sea level
2	Late Devonian	about 372 million years ago	Ocean oxygen loss; a series of crises
3	End-Permian ("Great Dying")	about 252 million years ago	Siberian Traps volcanism
4	End-Triassic	about 201 million years ago	Volcanism as Pangaea split
5	End-Cretaceous (K–Pg)	66 million years ago	Chicxulub impact plus Deccan Traps

End-Permian, the worst event

• Siberian Traps: a huge area of flood basalt in Russia. Flood basalt is lava that pours out of long cracks, layer after layer, over large areas.
• The chain of events:
• Huge eruptions released carbon dioxide and sulphur gases.
• The world warmed, and the oceans turned acidic and lost oxygen.
• About 90% of sea species died out. This event closes the Palaeozoic.

Sixth mass extinction

• Many scientists call today's fast, human-driven loss of species a possible sixth mass extinction (see biodiversity-conservation).

5. Chicxulub impact and the end of the dinosaurs

• Chicxulub impact: an asteroid strike 66 million years ago at the north-western tip of Mexico's Yucatan Peninsula. The crater faces the Gulf of Mexico [6].
• Size: the crater is about 180 km wide and about 900 m deep. It lies under about 1 km of younger rocks and sediments, so it was not detected for decades [7].
• Cause → process → result:
• Cause: an asteroid hits the Earth at very high speed.
• Process: a global cloud of dust and sulphur gases blocks sunlight, and the Earth goes through about a decade of near-freezing temperatures [7].

• Result: photosynthesis collapses and food chains break. The non-bird dinosaurs die out, along with about 75% of all other species [7].

• Deccan Traps as an added stress:

• The Deccan Traps are flood basalts in western and central India, mostly in Maharashtra.
• They erupted around the same time. Their gases added climate stress, but the impact is seen as the main cause (see earth-interior, plate-tectonics).

6. Impact craters in India

Lonar crater (Lonar Lake)

Feature	Detail
Location	Buldhana district, Maharashtra, on the Deccan Plateau
How it formed	A meteorite hit Deccan basalt (dark volcanic rock)
Shape and drainage	Almost circular; an endorheic (closed) basin, which means water flows in but no river flows out [2]
Size	Average diameter about 1,830 m; about 150 m deep; the rim rises about 20 m above the land around it [3]
Age	Estimates vary. One widely cited range is 35,000–50,000 years ago [3].
Proof of impact	Maskelynite, a natural glass that forms only in very high-speed impacts [3]
Water	Saline and alkaline (salty, with a high pH)
Status	Ramsar site no. 2441 [2]. It was listed as a Ramsar site in November 2020. The Ramsar record gives the formal designation date as 22 July 2020 [2]. Also a National Geo-heritage Site [2].

• Why it matters:
• Lonar is one of the few known impact craters in basalt on Earth.

• Basalt is also found on the Moon and Mars, so scientists study Lonar to compare it with craters there [3].

• Ramsar site: a wetland listed as internationally important under the Ramsar Convention (1971, Ramsar, Iran).

Other impact structures in India (check the current lists)

• Dhala in Shivpuri district, Madhya Pradesh, is a large, old and heavily worn crater.
• Ramgarh in Baran district, Rajasthan, is a ring-shaped structure.

7. The Holocene's three ages (ratified 2018)

• In June 2018, the International Union of Geological Sciences (IUGS) and the ICS split the Holocene into three ages [4]:

Age	Began	Reference marker
Greenlandian	11,700 years ago	Ice core from Greenland
Northgrippian	about 8,200 years ago	Ice core from Greenland (a sudden cooling)
Meghalayan	about 4,200 years ago to today	Stalagmite from Mawmluh Cave, Meghalaya

Meghalayan age

• Cause → process → result:
• Cause: a sudden global cooling and drought about 4,200 years ago, called the 4.2 ka event ("ka" means thousand years) [4].
• Process: the Indian Summer Monsoon weakened over a long period, and the drought lasted about 200 years [4].

• Result: early farming civilisations were hit, including the Harappan civilisation in India, as well as civilisations in Egypt and Mesopotamia.

• How the stalagmite records it:

• A stalagmite is a column of limestone that grows up from a cave floor as dripping water leaves minerals behind.

• Its layers hold the oxygen chemistry of the rainwater at the time. A shift in this chemistry (the oxygen-isotope ratio, δ18O) marks the start of the weaker monsoon [4].

• GSSP: the Global Boundary Stratotype Section and Point, or "golden spike". It is a single reference point in rock or ice that defines where a time unit begins.

• The Mawmluh speleothem is the GSSP for the Meghalayan Stage [4][5]. A speleothem is any cave mineral deposit, such as a stalagmite or stalactite.
• It is the first Indian site to define a global time unit.

• Mawmluh Cave is near Sohra (Cherrapunji), East Khasi Hills.

• The "Meghalayan Age Caves" are on UNESCO's Tentative List for World Heritage status [5].

8. The Anthropocene: proposed, then rejected

• Anthropocene ("human-new"): a proposed epoch in which human activity is the main force changing the planet.
• Proposed markers in the rock and sediment layers:
• Radioactive fallout from nuclear bomb tests
• Plastics
• Higher carbon dioxide

• These markers point to the Great Acceleration, the sharp rise in population, economic growth and resource use after World War II [6].

• Proposed reference site: Crawford Lake, Halton Region, Ontario, Canada [6].

• Its bottom mud forms yearly layers. Plutonium from hydrogen-bomb tests shows up in the layer for 1952 [6].

• Outcome:

• The Subcommission on Quaternary Stratigraphy (SQS), part of the ICS, rejected the proposal by a wide margin on 5 March 2024 [6].

• The IUGS upheld the decision. So we still officially live in the Holocene epoch, Meghalayan age.

• The Anthropocene is still widely used as an informal term in science and policy [6].

Prelims Hooks

• Eons, oldest → youngest: Hadean → Archean → Proterozoic → Phanerozoic. Eras: Palaeozoic → Mesozoic → Cenozoic.
• Official keeper of the time scale: the International Commission on Stratigraphy (ICS), working under the IUGS. It is not UNESCO or the IPCC.
• Worst mass extinction: end-Permian (about 252 million years ago), linked to the Siberian Traps. It is not the end-Cretaceous.
• Chicxulub crater: about 180 km wide, on the Yucatan Peninsula, Mexico, dated to 66 million years ago. The Deccan Traps (India) were a contributing stress.
• Lonar Lake: Buldhana, Maharashtra; meteorite impact into basalt; saline and alkaline water; Ramsar site (2020) and National Geo-heritage Site.
• Impact structure → state: Lonar → Maharashtra; Dhala → Madhya Pradesh; Ramgarh → Rajasthan.
• Meghalayan age: began about 4,200 years ago; ratified in 2018; reference marker is a stalagmite from Mawmluh Cave, Meghalaya. Order of Holocene ages: Greenlandian → Northgrippian → Meghalayan.
• Trap: "The Anthropocene is the current official epoch." This is false. The SQS rejected it in March 2024, and Crawford Lake (Canada) was only the proposed site.
• Gondwana rocks (Aryan system) hold about 98% of India's coal. The Dharwar (Archaean) system holds iron ore and gold.

Mains Points

• Volcanism and impacts as drivers of extinction (GS-I): flood basalts (Siberian Traps, Deccan Traps) and asteroid strikes (Chicxulub) changed the climate quickly. They worked through gases, darkness, acid oceans and oxygen loss. This shows how the Earth's interior and outside events shape the history of life.
• Climate shocks and civilisations (GS-I): the 4.2 ka megadrought, recorded at Mawmluh, weakened the monsoon and is linked to the Harappan decline. It is a lesson for today's monsoon-dependent agriculture under climate change.
• Geo-heritage conservation (GS-III): Lonar (a Ramsar and geo-heritage site) and Mawmluh (UNESCO Tentative List) face tourism, sewage and land-use pressure. India needs a geo-heritage law and GSI-led protection of impact craters and fossil sites.
• The Anthropocene debate (GS-III): the formal label was rejected, but the markers in sediments (plutonium, plastics, CO₂) still show deep human impact. The term supports the "sixth mass extinction" view and global action under the CBD and UNFCCC.