3.4 - Earth's Structure & Plate Tectonics

The Earth's structure

The three layers

1. Crust - The thin, solid outer layer of rock, with an average depth of about 20 km.
2. Mantle - The semi-solid layer between the crust and core. It's fairly rigid near the crust, but deeper down it becomes hotter, less rigid, and can flow slowly.
3. Core - Made mainly of iron, taking up just over half the Earth's radius. It has a solid inner core and a molten outer core.

Tectonic plates and how they move

• Radioactive decay inside the Earth releases heat, driving convection currents in the mantle.
• The crust and rigid uppermost mantle are cracked into tectonic plates (about seven major plates and several smaller ones).
• Convection currents carry the plates along, moving them typically a few centimetres a year.
• Sudden plate movements cause earthquakes.
• Earthquakes and volcanoes happen far more often at plate boundaries than anywhere else.

From continental drift to plate tectonics

Early observations

• Coastlines on opposite sides of oceans (such as Africa and South America) fit together like a jigsaw.
• Matching fossils were found on opposite sides of oceans far too wide for those organisms to have crossed.

Wegener's theory of continental drift

• In the early twentieth century, Alfred Wegener proposed that around 300 million years ago all landmasses were joined as one supercontinent, Pangaea, which broke apart and drifted (continental drift).
• Alongside matching coastlines and fossils, he showed that rock layers of the same age and type matched across separated continents.

Why the theory was rejected at first

• Wegener's explanation of how the continents moved wasn't convincing.
• He suggested tidal forces and the Earth's rotation moved them, which geologists showed was physically impossible.
• With no credible mechanism and no way to detect the movement at the time, most scientists rejected the idea.

The evidence that changed opinion

• 1930s - Scientists suggested convection currents in the mantle could move the plates.
• 1940s - Surveys of the Mid-Atlantic Ridge, where magma rose through the sea floor and solidified symmetrically on each side (sea-floor spreading).

1950s - Magnetic patterns in the rock on either side of the ridge matched in mirror-image bands, confirming that new sea floor was spreading outward.

1960s - The accumulated evidence led to Wegener's theory being widely accepted as the modern theory of plate tectonics.

Plate boundaries

• A tectonic plate carries either continental crust (thicker, beneath land) or oceanic crust (thinner and denser, beneath oceans).
• A plate boundary is where two tectonic plates meet.

Constructive boundary - Plates move apart

• Two plates move apart, usually along a mid-ocean ridge such as the Mid-Atlantic Ridge.
• Magma rises from the mantle to fill the gap, cooling and solidifying to form new crust made of igneous rock.
• Produces volcanic activity and earthquakes.

Destructive boundary - Plates move together

• Two plates move towards each other.
• Where oceanic crust meets continental crust, the denser oceanic plate is forced down beneath the continental plate into the mantle (subduction).
• The descending plate melts to form magma, which rises to create volcanoes, while the point where the plate sinks forms a deep ocean trench.
• Produces earthquakes.
• Where two continental plates collide, neither subducts, so the ground folds upward to form mountain ranges.

Conservative boundary - Plates move sideways

• Two plates move sideways past each other, or in the same direction at different speeds.
• No crust is created or destroyed, so no volcanoes form.
• The movement produces powerful earthquakes.