Meghalayan age

Topic: Origin and Evolution of the Earth and the Solar System · NCERT: Beyond NCERT

Meaning

The Meghalayan age is the latest of the three ages of the Holocene epoch. It began about 4,200 years ago with a sudden global cooling and a long drought, and it continues today. Its reference point is a stalagmite from Mawmluh Cave, Meghalaya. It matters because it is the first time an Indian site has been used to define a global time unit. It also links a change in the Indian monsoon to the decline of early civilisations, including the Harappan civilisation.

Explanation

Where it sits on the geological time scale

• The geological time scale is a calendar of the Earth's 4.6 billion years. It is built from rock layers and the fossils in them.
• The units, from largest to smallest: Eon → Era → Period → Epoch → Age.
• The Meghalayan age's place on this scale:
• Eon: Phanerozoic
• Era: Cenozoic
• Period: Quaternary
• Epoch: Holocene (the present warm epoch, which began 11,700 years ago)

• Age: Meghalayan

• Who decided it: In June 2018, the International Union of Geological Sciences (IUGS) and the International Commission on Stratigraphy (ICS) split the Holocene into three ages [1].

• The ICS keeps the official time scale, called the International Chronostratigraphic Chart.
• Stratigraphy means the study of rock layers and the order in which they formed.

Age (oldest → youngest)	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, and still going	Stalagmite from Mawmluh Cave, Meghalaya

What started it: the 4.2 ka event

• Cause: about 4,200 years ago, the world suddenly cooled and dried. This is called the 4.2 ka event. "ka" means thousand years [1].
• Process: the Indian Summer Monsoon weakened over a long period, and the drought lasted about 200 years [1].
• Result: early farming civilisations suffered.
• The Harappan civilisation in India was hit.
• So were the civilisations of Egypt and Mesopotamia.

How a cave stalagmite records it

• Stalagmite: a column of limestone that grows up from a cave floor. Water drips onto the floor and leaves minerals behind, layer after layer.
• Speleothem: the general name for any mineral deposit in a cave, such as a stalagmite or a stalactite.
• How the record works:
• Each layer keeps the oxygen chemistry of the rainwater that fell at that time.
• A shift in this chemistry, called the oxygen-isotope ratio (δ18O), marks the point where the monsoon became weaker [1].
• So the stalagmite works like a diary of past rainfall.

The golden spike (GSSP)

• GSSP (Global Boundary Stratotype Section and Point), also called the "golden spike": one reference point in rock or ice that marks exactly where a time unit begins.
• The Mawmluh speleothem is the GSSP for the Meghalayan Stage [1][2].
• The other two Holocene ages are marked by ice cores from Greenland. The Meghalayan is the only one of the three marked by a cave deposit.

Where it occurs

• Mawmluh Cave: near Sohra (Cherrapunji), East Khasi Hills district, Meghalaya, in north-east India. The reference stalagmite comes from here.
• "Meghalayan Age Caves": on UNESCO's Tentative List for World Heritage status [2].
• Indian subcontinent: the weaker Indian Summer Monsoon during the 4.2 ka event [1]. The Harappan civilisation was hit at this time.
• Rest of the world: the cooling and drought were global. The civilisations of Egypt and Mesopotamia also suffered.
• Greenland: source of the ice cores that mark the two earlier ages, the Greenlandian and the Northgrippian.
• Today: the whole world still lives in the Holocene epoch, Meghalayan age. The Anthropocene was rejected in March 2024 [3].

Don't confuse with

• Holocene: the Holocene is the epoch, which began 11,700 years ago. The Meghalayan is a smaller unit, an age, inside it. It began about 4,200 years ago.
• Northgrippian and Greenlandian: these are the two earlier Holocene ages. Both are marked by ice cores from Greenland. Only the Meghalayan is marked by a cave stalagmite in India.
• Anthropocene: a proposed epoch marked by nuclear fallout, plastics and CO₂, with its proposed site at Crawford Lake, Canada. The SQS rejected it on 5 March 2024 [3]. It is only an informal term, so it has not replaced the Meghalayan age.
• Stalagmite vs stalactite: both are speleothems. A stalagmite grows up from the cave floor, while a stalactite hangs down from the roof. The Meghalayan GSSP is a stalagmite.

Prelims Hooks

• Order of Holocene ages: Greenlandian (11,700 years ago) → Northgrippian (about 8,200) → Meghalayan (about 4,200 to today).
• Ratified in June 2018 by the IUGS and ICS [1]. It was not done by UNESCO or the IPCC.
• Reference marker: a stalagmite (speleothem) from Mawmluh Cave, near Sohra (Cherrapunji), East Khasi Hills, Meghalaya. This is the first Indian site to define a global time unit.
• Trigger: the 4.2 ka event, a global cooling and drought. It weakened the Indian Summer Monsoon, and the drought lasted about 200 years [1]. It is recorded by a shift in δ18O [1].
• Trap: "We now officially live in the Anthropocene." This is false. The official answer is the Holocene epoch, Meghalayan age, because the Anthropocene was rejected in March 2024 [3].
• "Meghalayan Age Caves" are on UNESCO's Tentative List, not the final World Heritage List [2].

Mains Points

• Climate shocks and civilisations (GS-I):
• The 4.2 ka megadrought weakened the Indian Summer Monsoon for about 200 years [1].
• It is linked to the decline of the Harappan civilisation, and of Egypt and Mesopotamia.

• Lesson: farming that depends on the monsoon is open to long droughts. This matters for India today as the climate changes.

• Speleothems as climate records (GS-I):

• Cave deposits like the Mawmluh stalagmite keep a year-by-year record of past rain through δ18O [1].

• They help scientists rebuild the history of the monsoon. India's limestone caves in the north-east are a valuable science resource.

• Geo-heritage conservation (GS-III):

• Mawmluh is a global GSSP [1][2] and is on UNESCO's Tentative List [2]. Yet it faces pressure from tourism and land use.
• India needs a geo-heritage law and protection led by the GSI (Geological Survey of India) to keep such sites safe.

Related concepts

• Mass extinction
• Chicxulub impact
• Lonar crater
• Anthropocene

Read more

• The geological time scale, impacts, extinctions and the Anthropocene