The structure of the Earth and the driving mechanism

Overview

Oceanic and continental crust, ridge push and slab pull.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What are the four main layers of the Earth, working from the outside in?

   Model answer

   The crust (thin and rigid), the mantle (the thickest layer, made of dense silicate rock that can flow slowly), the outer core (liquid iron and nickel) and the inner core (solid iron and nickel, kept solid by immense pressure despite being the hottest part). Temperature and density both increase with depth.

2. 2

   What is the lithosphere, and why is it the lithosphere rather than the crust that forms the plates?

   Model answer

   The lithosphere is the rigid outer shell of the Earth, made up of the crust plus the uppermost, cool and brittle part of the mantle, typically about 100 km thick. The plates are lithospheric because the break between rigid and mobile rock lies inside the mantle, not at the base of the crust, so each plate carries a slab of upper mantle beneath its crust.

3. 3

   What is the asthenosphere, and why does it matter for plate movement?

   Model answer

   The asthenosphere is the layer of upper mantle immediately below the lithosphere, hot enough to be partially molten and to behave plastically over long timescales. Because it can deform and flow, the rigid lithospheric plates above it can slide over it. Without a weak, mobile layer beneath them, the plates could not move.

4. 4

   Distinguish between oceanic and continental crust.

   Model answer

   Oceanic crust is thin (around 6-10 km), young (nowhere older than roughly 200 million years because it is constantly created and destroyed), basaltic in composition and dense (about 3.0 g/cm³). Continental crust is thick (around 30-70 km), ancient (parts exceed 3.5 billion years), granitic in composition and less dense (about 2.7 g/cm³). The density difference decides which plate subducts when two plates converge.

5. 5

   Why can continental crust never be subducted in the way oceanic crust is?

   Model answer

   Continental crust is too buoyant. Its granitic composition makes it significantly less dense than the underlying mantle, so it floats rather than sinking. Oceanic crust is basaltic, denser, and once it has cooled and thickened with age it becomes dense enough to sink into the mantle. This is why the ocean floor is geologically young while the continents preserve rock billions of years old.

6. 6

   How do convection currents in the mantle move the plates?

   Model answer

   Heat from radioactive decay in the mantle and from the core makes deep mantle material hot and less dense, so it rises. Near the base of the lithosphere it spreads sideways, cools, becomes denser and sinks again, forming a slow circulating cell. The horizontal limb of the cell drags on the base of the overlying plate, and rising limbs sit beneath constructive margins while sinking limbs sit beneath destructive margins.

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