Endogenic processes: internal energy, diastrophism, volcanism and isostasy

Detail

What "endogenic" means

• Endogenic processes are forces that start inside the earth. They build up and break the crust. They work against the exogenic processes (weathering, erosion, deposition), which wear the surface down.
• Geomorphic process: any physical or chemical action that creates or changes the shape of the earth's surface.

Energy behind endogenic processes

• Three sources of the earth's internal heat:
• Radioactivity: radioactive elements in the rocks break down slowly and give off heat.
• Rotational and tidal friction: the earth spins, and the Moon and Sun pull on it (tides). Rubbing between layers turns this movement into heat.

• Primordial heat: heat left over from when the earth formed and was very hot.

• How the heat acts:

• Temperature rises with depth. This is the geothermal gradient (how fast temperature increases as you go deeper).
• Heat moves from the hot interior towards the cooler surface. This is heat flow.

• This heat drives diastrophism and volcanism (both defined below).

• Why the result is uneven:

• Geothermal gradients and heat flow differ from place to place.
• Crustal thickness and crustal strength also differ.

• So endogenic forces push harder in some places than in others → the first surface they shape (the tectonically controlled original surface) is uneven → exogenic agents then work on this uneven surface.

• Class 9 NCERT asks this as a basic question: what are the sources of the earth's internal heat?

Diastrophism

• Diastrophism: all the processes that move, raise or build up parts of the earth's crust. It has four forms:

Form	What it is	Scale and style	Examples
Orogeny	Mountain building by severe folding	Long, narrow belts; strong, complex deformation	Himalayas, Alps, Andes
Epeirogeny	Broad uplift or gentle warping (bending) of the crust	Very large areas; simple deformation	Continent-building; large plateaus and shields rising or sinking
Earthquakes	Sudden local movement along faults	Local, fairly minor	Himalayan belt, Andaman–Nicobar arc
Plate tectonics	Sideways (horizontal) movement of crustal plates	Global	Indian plate moving north into the Eurasian plate

• Orogeny vs epeirogeny (NCERT asks for the differences):
• Orogeny → mountain-building. Epeirogeny → continent-building.
• Orogeny → narrow belts. Epeirogeny → wide areas.
• Orogeny → severe folding and thrusting. Epeirogeny → simple up-and-down warping, with few folds.

• Orogeny is mostly horizontal squeezing at plate edges. Epeirogeny is mostly vertical movement.

• What all four forms do to rocks:

• They can fault (break and shift) and fracture (crack) the crust.
• They cause changes in pressure, volume and temperature (PVT).
• These PVT changes cause metamorphism: the rock's minerals and texture change without melting (e.g. limestone → marble, shale → slate).

The Himalayas: orogeny happening now

• The cause:
• About 80 million years ago, India lay about 6,400 km south of Asia [2].

• Before the collision, India moved north at about 9 m per century [2].

• The process:

• India hit Eurasia about 40–50 million years ago [2].
• Both landmasses have similar rock density. Neither could sink (be subducted) under the other [2].
• So the crust was pushed upward and stacked up. This formed the Himalayas and the Tibetan Plateau [2].

• After the collision, India's speed dropped by about half [2].

• The result today:

• The Himalayas still rise by more than 1 cm a year. That would be 10 km in a million years [2].
• Peaks such as Mt Everest have risen to more than 9 km in the last 50 million years [2].

• Erosion and gravity stop the range from growing forever. The Eurasian plate may now be stretching out, which lets some areas sink [2].

• Ongoing plate movement explains why the Himalayan belt and North-East India are high earthquake zones.

Volcanism

• Volcanism: the movement of molten rock (magma) onto the earth's surface or towards it. It forms two kinds of features:
• Intrusive forms: magma cools and hardens inside the crust (batholiths, laccoliths, sills, dykes). Example: granite batholiths.

• Extrusive forms: magma reaches the surface as lava and cools there (lava plateaus, volcanic cones). Example: the Deccan Traps, a basalt lava plateau in Maharashtra and nearby states.

• Process vs landform (a common trap):

• Volcanism = the process.

• Volcano = the vent (opening) that lava, ash and gases come out of, and the landform (cone) built around it.

• Indian example: Barren Island in the Andaman Sea is India's only active volcano.

Isostasy (beyond NCERT)

• Isostasy: the crust "floats" on the denser mantle below it, like wood floating on water.
• A thick or light block floats higher, like a big log. A thin or heavy block floats lower.
• So high mountains have deep "roots" pushing down into the mantle.

• When a load is added (ice, sediment), the crust sinks. When a load is removed (ice melts, erosion), the crust slowly rises.

• Two classic models (both 1855). They came from puzzles in the Great Trigonometrical Survey of India near the Himalayas:

Feature	Airy (1855)	Pratt (1855)
Crustal density	Same everywhere	Changes from place to place
Crustal thickness	Changes from place to place	Same depth of base (level of compensation)
Why mountains are high	Thicker crust with deep roots	Lighter (less dense) rock columns
Simple picture	Icebergs of different sizes floating in water	Blocks of different metals floating in mercury

• Post-glacial rebound (glacial isostatic adjustment, GIA): land rises after a heavy ice sheet melts.
• Cause: during the last ice age, thick ice sheets covered Scandinavia and northern North America. The weight pushed the crust down into the mantle.
• Process: the ice melted. The weight was gone, so the crust began to rise slowly back towards balance. Mantle rock flows back in very slowly, so the rise is still going on thousands of years later.

• Result now:

  • Near Hudson Bay (Canada) the land is rising by about 10 mm a year. This is where the ice was thickest at the last glacial maximum. Data come from about 360 GPS sites in Canada and the USA (study published 2007) [3].
  • Uplift gets smaller with distance from Hudson Bay. South of the Great Lakes the land is sinking by about 1–2 mm a year [3].
  • A "hinge line" separates rising land from sinking land. The northern shores of the Great Lakes are rising and the southern shores are sinking [3].
  • The land also moves sideways, outward from Hudson Bay [3].
  • In the past 20,000 years, ice melting and GIA together caused up to several hundred metres of relative sea-level change in parts of North America [3].
  • Scandinavia (the area of the old Fennoscandian ice sheet, around the Gulf of Bothnia) is also still rising.

• Erosion and uplift: when rivers and glaciers remove rock from a mountain range, the range becomes lighter → its roots push it up slowly → the mountain keeps standing for much longer than erosion alone would allow.

• Testing isostasy:
• Gravity anomalies (differences between the gravity actually measured and the gravity expected) show whether an area is in balance.
• A strong anomaly means the area is still adjusting.
• GIA rates also help scientists estimate how thick and sticky (viscous) the mantle is. Mantle viscosity matters for understanding the mantle flow that moves the plates [3].

Prelims Hooks

• Sources of internal heat: radioactivity + rotational and tidal friction + primordial heat. Solar energy is not a source. It drives exogenic processes.
• Orogeny = mountain-building, narrow belts, severe folding. Epeirogeny = continent-building, broad uplift, simple deformation. Expect "match the pair" questions.
• The four forms of diastrophism: orogeny, epeirogeny, earthquakes, plate tectonics. Volcanism is a separate endogenic process, not one of the four.
• Volcanism = process. Volcano = vent and landform.
• PVT changes from diastrophism → metamorphism (not melting; melting leads to magma and igneous rocks).
• Airy = same density, different thickness (mountain roots). Pratt = different density, same base. Both are from 1855 and grew out of surveys in India.
• India collided with Eurasia 40–50 million years ago. The Himalayas still rise by >1 cm a year [2].
• Hudson Bay is rising by about 10 mm a year. The land south of the Great Lakes is sinking by 1–2 mm a year. Both are caused by glacial isostatic adjustment [3].
• Post-glacial rebound examples: Scandinavia, Hudson Bay (Canada). Trap: the Himalayas are rising because of plate collision, not glacial rebound.

Mains Points

• Endogenic and exogenic forces in balance (GS-I):
• Uneven heat flow and crustal strength create uneven relief.
• Exogenic agents then wear this relief down.

• Landforms show which force is stronger at a given place and time. Example: the young, rising Himalayas vs the worn-down, stable Peninsular plateau.

• Ongoing orogeny as a hazard (GS-III, disasters):

• The Indian plate keeps pushing into Eurasia [2].
• Strain builds up along Himalayan faults → great earthquakes, landslides and glacial-lake floods.

• This needs seismic zoning, building codes and NDMA-style preparedness in the Himalayan states.

• Isostasy and sea level (GS-I/III):

• Land moves up or down at the same time as the sea rises. Rising land (Hudson Bay, Scandinavia) can cancel some of the sea-level rise. Sinking land (south of the Great Lakes hinge line) makes it worse [3].

• So coastal-risk planning must measure land movement as well as sea level.

• Epeirogeny and continent-building: broad uplift and warping explain large plateaus, raised beaches and drowned coasts. They also shape drainage, for example rivers cutting deeper into rising land.