Asthenosphere

Topic: Interior of the Earth: Earthquakes, Volcanoes and the Layered Earth · NCERT: Class 9, Ch 2 "Shaping of the Earth's Surface"; Class 11, Ch 3 "Interior of the Earth"; Class 11, Ch 4 "Distribution of Oceans and Continents"

Meaning

The asthenosphere ("astheno" = weak) is a weak, plastic, partly molten zone in the upper mantle. It lies just below the rigid lithosphere and reaches down to about 400 km (NCERT).

• It matters because it is the main source of magma for volcanoes.
• It is also the soft layer that the rigid tectonic plates slide over. Without it, plate movement, most earthquakes and fold mountains could not be explained.

Explanation

What "weak, plastic and partly molten" means

• Solid, but it can flow. The asthenosphere is mostly solid rock. Over very long periods it can flow slowly, like soft toffee or putty. This behaviour is called plastic.
• Partly molten. A small share of its rock is melted. Most of it is not.
• Weak, not liquid. It is "weak" because it is softer than the rigid layer above it. It is not a sea of liquid rock. This is a common mistake.
• Material. Like the rest of the mantle, it is made of silicate rock rich in iron and magnesium. It is denser (heavier for its size) than the crust.

Where it sits in the layered earth

• The earth can be divided in two ways:
• By rock type (chemistry): crust → Moho → mantle.

• By strength (how rigid or soft the rock is): lithosphere (rigid) → asthenosphere (weak and plastic).

• The two ways do not match up. The asthenosphere is not a separate rock type. It is a soft part of the upper mantle.

• Order from the surface downward:

• crust → Moho → uppermost mantle (rigid, part of the lithosphere) → asthenosphere → lower mantle → core boundary at 2,900 km.

• Its top depends on how thick the lithosphere is above it:

• Under the oceans, the lithosphere is thin (5-100 km), so the asthenosphere starts at a shallow depth.
• Under the continents, the lithosphere is thick (about 200 km), so the asthenosphere starts deeper.

• Under mid-ocean ridges, the rigid layer is so thin that the Moho itself forms the lithosphere-asthenosphere boundary [2].

• Below it is the lower mantle, which is solid again. The very high pressure there keeps the rock from melting.

How it works: magma source and plate movement

• Source of magma
• Some rock in the asthenosphere is already melted.
• This melted rock is lighter than the rock around it, so it rises through cracks and weak zones.

• It feeds volcanoes, mainly where plates pull apart (mid-ocean ridges) or where one plate sinks under another (subduction zones).

• A soft layer for the plates to move over

• The lithosphere (crust + uppermost mantle) is rigid and broken into tectonic plates.
• These plates rest on the soft asthenosphere and move slowly over it.

• Slow movements of heat in the mantle, called convection currents (hot rock rises, cools and sinks again), are the textbook reason the plates are dragged along.

• What happens at the surface

• Where plates push together, pull apart or slide past each other, the brittle rock above bends until it suddenly breaks. This causes earthquakes.
• Where plates move apart or one dives under another, magma from below rises. This builds volcanoes.
• Key point: earthquakes start in the brittle lithosphere, not in the plastic asthenosphere. Plastic rock bends and flows. It does not snap.

Where it occurs

• Everywhere under the plates. The asthenosphere lies under every tectonic plate. Examples are the Indian, Eurasian, Pacific, Arabian and Anatolian plates.
• India and the Himalayas. The Indian plate moves over the asthenosphere and keeps pushing into the Eurasian plate.
• This has given the Himalayas a crust about 70 km thick and a lithosphere under constant stress.

• Result: the Himalayan belt and North-East India are among the most quake-prone parts of the country.

• Bhuj (Gujarat), 2001. This quake happened inside the Indian plate, far from any plate edge. The moving plate over the asthenosphere builds up stress that can break the brittle lithosphere anywhere.

• Pacific "Ring of Fire". Its volcanoes are fed by magma that rises from the asthenosphere where plates sink.
• Mid-ocean ridges. These are underwater mountain chains where plates move apart.
• Magma from the asthenosphere rises and cools to make new basalt sea floor.

• The asthenosphere comes closest to the surface here [2].

• Tohoku (Japan), 2011. The Pacific plate sinks under the plate carrying northern Japan. This is a subduction zone. The quake set off a huge tsunami.

• Turkey-Syria, 2023. The Anatolian and Arabian plates slide past each other along the East Anatolian Fault. Both plates move over the asthenosphere.

Don't confuse with

• Lithosphere: rigid and brittle, made of crust + uppermost mantle, 10-200 km thick. It is broken into plates and earthquakes start in it. The asthenosphere lies below it and is weak and plastic.
• Lower mantle: also part of the mantle, but solid because very high pressure stops the rock from melting. The asthenosphere is the partly molten zone above it, in the upper mantle.
• Moho (Mohorovicic discontinuity): the crust-mantle boundary, found by Andrija Mohorovicic in 1909. It is not the top of the asthenosphere. The Moho lies almost entirely inside the lithosphere and meets the asthenosphere only beneath mid-ocean ridges [2].
• Outer core: the truly liquid layer, below 2,900 km. The asthenosphere is only partly molten and lies in the upper mantle. It is not a liquid layer.

Prelims Hooks

• Asthenosphere = "weak sphere". It is a weak, plastic, partly molten zone in the upper mantle, reaching about 400 km (NCERT).
• It is the main source of magma for volcanoes. Examples: the Pacific Ring of Fire and mid-ocean ridges.
• Order from the surface downward: crust → Moho → lithospheric mantle → asthenosphere → lower mantle → core boundary at 2,900 km.
• Trap: the lithosphere = crust + uppermost mantle (not crust + core). The rigid plates of the lithosphere move over the asthenosphere.
• Trap: the asthenosphere belongs to the mantle. It is defined by strength (soft), not by rock type, so it is not part of the crust.
• Trap: the Moho is not the lithosphere-asthenosphere boundary, except beneath mid-ocean ridges [2].

Mains Points

• Plate tectonics depends on the asthenosphere. A rigid lithosphere broken into plates slides over the weak, partly molten asthenosphere. This explains where earthquakes and volcanoes happen:
• subduction quake and tsunami at Tohoku 2011;
• plates sliding past each other in Turkey-Syria 2023;

• quake inside the Indian plate at Bhuj 2001.

• India's disaster risk (GS-III). The Indian plate keeps pushing into the Eurasian plate as it moves over the asthenosphere.

• This keeps the Himalayan lithosphere under constant stress.
• Result: the Himalayas and North-East India face high earthquake risk, and even "stable" peninsular areas can have strong quakes (Bhuj 2001).

• Response: earthquake-safe building rules, seismic zoning and a good network of earthquake-recording stations.

• Seismic waves are our only window into this layer. No one can drill or dig to the mantle. Changes in the speed of seismic waves show where the rigid lithosphere ends and the soft asthenosphere begins. This is why the layered model of the earth rests on earthquake records.

Related concepts

• Crust
• Oceanic crust
• Continental crust
• Conrad discontinuity
• Mohorovicic discontinuity
• Mantle
• Lithosphere

Read more

• Structure of the earth I: crust, mantle, lithosphere and asthenosphere