Earth Structure and the Development of Plate Theory

Layers of the Earth

The Earth is not a uniform ball of rock. It is divided into a series of concentric layers, each with distinct properties, and understanding their structure is the foundation for understanding why plates move, why volcanoes erupt where they do, and why earthquakes occur along specific zones.

The outermost layer is the crust, which exists in two forms. Oceanic crust is dense, thin (5–10 km), and composed mainly of basalt. Continental crust is less dense, thicker (25–70 km), and composed mainly of granite. This density difference is crucial: when an oceanic plate meets a continental plate, the denser oceanic crust sinks beneath the lighter continental crust — a process called subduction.

Below the crust lies the mantle, which extends to a depth of about 2,900 km and makes up roughly 84% of the Earth’s volume. Although the mantle is solid, it is able to flow extremely slowly over geological timescales — a behaviour called solid-state flow or plasticity. The uppermost part of the mantle is rigid and, together with the crust above it, forms the lithosphere.

Beneath the lithosphere, the upper mantle becomes slightly weaker and more ductile. This zone, called the asthenosphere, is where the solid-state flow is most pronounced. The lithospheric plates effectively float on the asthenosphere, which is why they can move.

At the Earth’s centre is the core, divided into an outer liquid core and an inner solid core, both composed mainly of iron and nickel. The liquid outer core generates the Earth’s magnetic field through convection of molten iron — and that magnetic field is what makes magnetic stripes on the ocean floor possible, as you will see below.

Wegener and Continental Drift

In 1912, Alfred Wegener proposed the hypothesis of continental drift, publishing his full case in 1915. He gathered several lines of evidence. The jigsaw fit of the continents — especially the complementary coastlines of South America and Africa — suggested they were once joined. Fossil distributions supported this: identical fossils of the freshwater reptile Mesosaurus were found in both South America and southern Africa, regions now separated by the Atlantic Ocean. A freshwater animal could not have crossed an open ocean, so the two landmasses must have been connected.

Wegener also noted matching geological formations: the Appalachian Mountains of North America are structurally continuous with ancient mountain belts in Scotland and Scandinavia. He further cited evidence of past climate change: glacial deposits in tropical Africa and India suggested those landmasses were once much closer to the South Pole, as part of a single giant continent he named Pangaea.

Despite this evidence, Wegener’s hypothesis was rejected by most geologists of his day. His fatal weakness was that he could not explain the mechanism — what force was powerful enough to move entire continents through the solid ocean floor? Without a credible mechanism, the scientific community remained sceptical.

Modern Evidence and GPS

Since plate tectonics became established, further lines of evidence have confirmed and refined it. The age of the ocean floor increases with distance from mid-ocean ridges, exactly as the spreading model predicts. Heat flow is highest at ridges (where hot magma is close to the surface) and decreases toward subduction zones. Ocean drilling has retrieved basalt cores whose magnetic signatures confirm the spreading rate calculations derived from magnetic stripes.

Today, GPS measurements directly confirm that plates are moving. The Pacific Plate moves north-west at about 7 cm per year — a rate that is measurable with satellite precision. GPS data also allow geologists to monitor strain accumulating along active faults, helping to assess earthquake hazard.

The driving mechanism of plate motion is described by the specification as “not yet completely understood”. Mantle convection (hot mantle rising, cooler mantle sinking) was long cited as the main driver, but current evidence weights slab pull — the sinking of dense oceanic crust at subduction zones, dragging the attached plate behind it — as the dominant force. Ridge push — the elevated position of mid-ocean ridges causing the plates to slide downhill under gravity — is a secondary driver. Students who write “convection currents alone drive plates” are oversimplifying the evidence.