Convection currents, which move the tectonic plates, are found in which layer of the earth?

Correct — B, Mantle. Plate motion is driven by convection in the mantle. Heat from the decay of radioactive isotopes within the mantle, together with heat leaking out of the core, sets up slow cells in which hotter, less dense rock rises and cooler, denser rock sinks. The mantle is solid, but over geological time it deforms plastically, and the weak ductile layer just below the lithosphere — the asthenosphere — is where that flow is easiest. The plates ride on it and are dragged and pushed by it, helped by ridge push and by the pull of a cold slab sinking at a trench.

• (a)Crust — The crust is thin and brittle — as little as five kilometres under the oceans — and it is the layer being moved, not the layer doing the moving.
• (c)Outer core — The outer core is liquid iron and nickel and does convect, vigorously enough to generate the earth's magnetic field. It lies below the mantle, so its flow drives the geodynamo rather than the plates.
• (d)Inner core — The inner core is solid, held that way by immense pressure despite temperatures comparable to the sun's surface. A solid sphere cannot convect.

The earth is layered twice over — chemically into crust, mantle and core, and mechanically into lithosphere, asthenosphere, mesosphere, outer core and inner core. Plate tectonics belongs to the mechanical scheme. The lithosphere is the rigid outer shell, crust plus the uppermost solid mantle, broken into plates; the asthenosphere beneath it is weak enough to flow. Arthur Holmes proposed mantle convection as the engine of continental drift in the late 1920s, long before sea-floor spreading confirmed that something was moving the plates.

The four options are the four layers, so the question is really asking where in the interior a slow, solid-state flow is possible. The core can be dismissed on position — it lies under the mantle and cannot reach the plates — and the crust on rheology, since brittle rock does not convect. The distinction worth carrying is that the outer core does convect and produces the magnetic field, which makes it a plausible-sounding but wrong answer here.

• The mantle extends from the base of the crust to about 2,900 km depth and holds most of the earth's volume.
• The Mohorovicic discontinuity separates the crust from the mantle; the Gutenberg discontinuity separates the mantle from the core.
• The asthenosphere, roughly 100 to 250 km down, is the weak layer on which the plates move.
• Radiogenic heat within the mantle and heat escaping from the core together drive the convection.
• The outer core is liquid and its convection generates the earth's magnetic field, while the inner core is solid.

• Choosing the outer core because it is liquid and known to convect.
• Treating the lithosphere as the same thing as the crust; it includes the uppermost mantle.

A single-fact item on the internal structure, with the layers themselves as the option set.

• practice — not a real PYQ

  The asthenosphere lies within which layer of the earth?

  • (a)Crust
  • (b)Upper mantle
  • (c)Outer core
  • (d)Inner core

  Answer(b) Upper mantle — it is the weak, ductile zone roughly 100 to 250 km down on which the plates move.

• practice — not a real PYQ

  The earth's magnetic field is generated by motion in the

  • (a)Inner core
  • (b)Outer core
  • (c)Lower mantle
  • (d)Continental crust

  Answer(b) Outer core — convection in the liquid iron and nickel there drives the geodynamo.