Title: Volcanism and earth evolution - IRCC

Volcanism and
Earth Evolution
Hetu Sheth
Dept. of Earth Sciences
IIT Bombay
Barren Island (India) in eruption, March 2009

• Earth is ~4.5 billion years
old and has an average
radius of 6,371 km.
• It has three main divisions
or shells:
1. The core (mostly metallic
iron and nickel).
2. The mantle (mostly Fe-Mg
silicate rocks).
3. The crust (various silicates,
carbonates, etc.)

Seven elements that
make up ~97% of the
Earth by mass, and
which form many
diverse minerals and
rocks.

Structure and fabric of the Earth’s ocean floor, and the even more rugged topography of the
continents, as revealed by satellites

Gondwanaland and
continental drift, proposed by
Wegener in 1912

Plate tectonics: Lithosphere is the outer strong shell of the
Earth, consisting of the crust and the uppermost mantle, and
divided into many plates, in perpetual relative motion.
Magma genesis in the Earth is intricately linked to tectonics.

Temperature increases
with depth, but increasing
pressure prevents rocks
from melting. The
geotherm is normally
below the solidus of
mantle peridotite at all
pressures.
The melting temperature of
any substance is directly
proportional to the pressure.

Melting a rock by
decreasing its pressure,
without adding or
removing heat (“adiabatic
decompression”), is very
efficient and significant.
Melting a rock by
supplying heat and
increasing its
temperature is not
efficient or significant.

And, adding a flux (such as H 2 O) to a mineral or a rock at
high pressure greatly reduces its melting temperature.

Two major plate tectonic mechanisms of magma generation:
1. At mid-ocean ridges: Decompression melting of the mantle
2. At island arcs and active continental margins: Reduction of mantle
melting temperature by addition of H 2 O

Continental break-up
between India and
Madagascar, and related
magmatism 88-85 million
years ago, is represented in
southern India.
Coconut Island,
St. Mary’s Islands,
Karnataka

India’s subsequent northward drift:
Deccan volcanism during 68-60 Ma
Mahabaleshwar

Mahabaleshwar
Deccan flood volcanism: ~1.5
million km 3 of lava was erupted in
a few million years.
Tyrannosaurus rex
Major environmental crisis – the
K/T Boundary mass extinctions,
including the dinosaurs.

“Flood basalt” volcanism such as
Deccan volcanism is a major
driver of climate change due to
volatiles released.
Broadly speaking, CO 2 produces
atmospheric warming. SO 2
produces atmospheric cooling.
Estimates for CO 2 and SO 2
emissions:
For a single 1000 km 3 eruption,
~10 Gt each (= 1 Gt/yr each if
eruption lasted 10 years.
Eruption rate is a key variable.)
Pre-eruptive S contents in
flood basalts (direct
measurements in glass
inclusions): 1000-2500 ppm
Vented as SO 2 , converted to
H 2 SO 4 aerosols, acid rain…

Mumbai city and IITB campus
are on rocks of the Deccan
volcanic event.
Basalt, Gilbert Hill, Andheri
Trachyte, Manori Island, Malad

Still later (about 40 million years before the
present), India collided with Eurasia, raising the
Himalaya and the Tibetan Plateau.
Himalayan magmatism involves much remelting of
older crust due to doubling of crustal thickness and
higher heat flow into the crust.

The substructure of
volcanoes (not to scale):
Visible after millions of years
of uplift and erosion of
overlying rocks
Taranga hills, north
Gujarat, are granites
about 800 million
years in age.

Subduction zone magmatism
Mount St. Helens,
USA

1980 Mount St.
Helens ash
cloud during first
13 minutes, and
map view of the
ash deposit
(thickness in
centimetres).

Left: Artist’s conception of
the A. D. 79 eruption of
Vesuvius that destroyed
the cities of Pompeii and
Herculaneum

Andaman subduction zone

Barren Island (~500 m),
India’s only active volcano:
135 km NE of Port Blair
When did it begin?
Narcondam volcano (710 m):
140 km NNE of Barren Island
Extinct? Or simply “sleeping” Examples of huge fissure-type eruptions
(dormant)?
are the famous flood basalt plateaus of
the world, e.g., the Deccan plateau, India.
How old?

Barren Island’s eruption in March 2009
Barren Island’s latest eruption
(March 2009)

Barren Island: Field work and
sample collection
To be followed by
• Radio-isotopic dating
• Geochemical analyses
• Petrogenetic modelling

Dhule area, Maharashtra
Some benefits of volcanism:
1. Xenoliths (samples of deepseated
rocks)
2. Geothermal energy
3. Economic minerals…
Taiwan
South Africa

ACKNOWLEDGEMENTS
• IIT Bombay
• Department of Science and Technology
• Indian Coast Guard
• Photo and figure credits: H. C. Sheth, some from Drs. B. Skinner and J. Winter, and U.S.G.S.
Sunset, Diglipur (North Andaman)