The lithosphere, mantle plumes and hotspots: the structure and composition of oceanic and continental lithosphere; the principle of isostasy and isostatic adjustment; mantle plumes and hotspots as a cause of intraplate volcanism; hotspot tracks and the age progression of volcanic island chains (for example Hawaii); the basis of the Component 3 geology of the lithosphere option.

What this dot point is asking

Eduqas wants you to describe the structure and composition of oceanic and continental lithosphere, to explain the principle of isostasy and isostatic adjustment, to explain how mantle plumes and hotspots produce intraplate volcanism, to interpret hotspot tracks and the age progression of volcanic island chains, and to see how these ideas form the basis of the Component 3 geology of the lithosphere option.

The answer

Oceanic and continental lithosphere

A tectonic plate is a slab of lithosphere (the crust plus the cool, rigid uppermost mantle), riding on the weak, plastic asthenosphere beneath. The two kinds of lithosphere differ sharply:

• Oceanic lithosphere. A thin (about $5$ to $10\ \mathrm{km}$) basaltic crust over rigid mantle. It is dense, young (continually created at ridges and destroyed at subduction zones), and able to subduct.
• Continental lithosphere. A thick (about $30$ to $70\ \mathrm{km}$) granitic-to-intermediate crust over rigid mantle. It is less dense, far older, and too buoyant to subduct.

The density contrast is the key: dense oceanic lithosphere subducts while buoyant continental lithosphere does not, which is why continental crust survives for billions of years and ocean floor is recycled within about 200 million.

Isostasy

Because the lithosphere floats, it adjusts vertically whenever the load on it changes; this is isostatic adjustment.

• Adding a load (a growing ice sheet, a thick pile of sediment) pushes the crust down into the asthenosphere.
• Removing a load (melting ice, eroding a mountain) lets the crust rise back up (isostatic rebound).

Mountains therefore have deep crustal roots, and regions once buried under thick ice sheets, such as Scandinavia and Scotland, are still slowly rising today because the ice has melted.

Mantle plumes, hotspots and intraplate volcanism

Most volcanism happens at plate margins, but some occurs in the middle of plates. This intraplate volcanism is explained by mantle plumes.

A mantle plume is a narrow column of unusually hot mantle that rises from deep in the mantle and stays roughly fixed in position. Where it reaches the base of the lithosphere it causes melting, producing a hotspot that feeds a volcano at the surface, far from any plate margin.

Hotspot tracks and age progression

Because the plume is fixed but the plate moves over it, a hotspot builds a chain of volcanoes. The volcano sitting over the plume is active; as the plate carries it away, it is cut off from the magma supply and goes extinct, while a new volcano grows over the plume. The result is a track of volcanoes that gets progressively older with distance from the present hotspot.

The classic example is Hawaii: the Big Island is active over the plume today, and the islands and seamounts stretching to the northwest are progressively older and more eroded. The bend in the Hawaiian-Emperor chain even records a change in Pacific Plate motion, and measuring the distance between dated volcanoes gives the plate's speed.

The basis of the Component 3 lithosphere option

The structure, behaviour and intraplate volcanism of the lithosphere are not just background; they are the foundation of the optional geology of the lithosphere topic in Component 3. A secure grasp of oceanic versus continental lithosphere, isostasy and hotspots is what that option builds on.

Examples in context

Example 1. Fennoscandian rebound. Since the ice sheets of the last glaciation melted, the unloaded crust of Scandinavia has been rising by up to about a centimetre a year, a textbook case of isostatic rebound.

Example 2. The Hawaiian-Emperor chain. The line of progressively older, more eroded islands and seamounts trailing northwest from active Hawaii records the Pacific Plate sliding over a fixed plume, and the bend marks a past change in plate direction.

Try this

Q1. State two differences between oceanic and continental lithosphere. [2 marks]

• Cue. Any two of: oceanic lithosphere is thinner, denser, younger and subducts; continental lithosphere is thicker, less dense, older and does not subduct.

Q2. Explain why the land in Scandinavia is still rising long after the ice sheets melted. [3 marks]

• Cue. The ice added a load that pushed the crust down into the asthenosphere; when the ice melted the load was removed, so the crust slowly rises back up (isostatic rebound) until it regains its floating balance, a slow adjustment that is still continuing.

Q3. Explain why a chain of hotspot islands gets older away from the active volcano. [3 marks]

• Cue. The mantle plume and its hotspot stay roughly fixed while the plate moves over it; each volcano is carried off the plume and goes extinct as a new one forms above the plume, so the volcanoes get progressively older with distance from the present hotspot (for example Hawaii).