3.5 1.a - Continental drift and the evidence for plate tectonics
Plate tectonics explains the movement of Earth's outer shell and the concentration of many earthquakes and volcanoes. This lesson builds that explanation from Earth's structure and the evidence of fossils, ancient ice, sea-floor ages and magnetism, showing why several independent observations support one theory.
Lithosphere, asthenosphere and plate motion
Earth can be divided by composition or by mechanical behaviour, and these divisions are not interchangeable. Composition gives a thin rocky crust, a much thicker silicate mantle, and a metallic core. The outer core is liquid and the inner core solid. The mantle is predominantly solid rock: its ability to deform slowly does not make it a global ocean of liquid magma. The USGS account of Earth's interior explains how seismic and other observations reveal these divisions.
Mechanically, the lithosphere comprises the crust and the strong uppermost mantle. It is divided into plates, which can include both continental and oceanic regions. Beneath lies the weaker asthenosphere, part of the upper mantle that deforms over geological time. Its behaviour permits motion of the stronger shell above. A boundary between crust and mantle is therefore not the same as the lithosphere–asthenosphere boundary. The USGS teaching companion distinguishes these layers.
Heat from Earth's interior, including radioactive decay and remaining formation heat, sustains mantle circulation. Hotter material can rise as cooler, denser material sinks: convection transfers material and energy. Plates are part of this interacting system, rather than passengers carried at a fixed speed by identical circular conveyor belts.
Gravity also acts directly on plates. Cooling oceanic lithosphere becomes denser; where it subducts, its sinking portion can pull the connected plate behind it: slab pull. Elevated young lithosphere near a ridge can move away under gravity towards lower, older sea floor: ridge push, despite the name, is not simply magma pushing horizontally from a narrow vent. Mantle traction and resistance also affect motion. BGS's explanation of driving forces identifies convection, ridge push and slab pull. Their relative importance varies, so one small convection-cell sketch cannot establish the force balance for every plate.
Continental drift, fossils and ancient ice
Continental drift proposes that continents have changed their relative positions, including separation from earlier joined land masses. Alfred Wegener presented his theory in two articles in 1912, using several geographical and geological matches. The fit of opposing Atlantic continental margins is suggestive, although modern coastlines are altered by erosion, deposition and sea-level change; a continental-margin reconstruction is more meaningful than matching every present beach exactly.
Fossils provide a stronger test than outline alone. Comparable fossils of organisms such as Mesosaurus occur in South America and southern Africa. A freshwater-associated reptile is difficult to explain as repeatedly crossing a wide saline ocean, whereas adjoining habitats before separation provide a coherent explanation. Glossopteris plant fossils across several southern continents add another line of evidence. Fossil evidence must still match age and environment: finding vaguely similar shells on two coasts would be much weaker. USGS's historical account places these arguments in the development of the theory; BGS explains fossils' use in reconstructing past worlds.
Ancient glacial deposits and striations also occur on southern continents now separated by oceans. Striations record abrasion and can indicate former ice movement. When the continents are reconstructed into an adjoining southern land mass, deposits and flow directions can form a more consistent pattern around a former ice-covered region. Their present tropical or subtropical positions alone would give a less plausible climate reconstruction. The inference concerns continental positions and past climate together, not a claim that Earth's climate has always been fixed.
Early drift theory had an important weakness: proposed forces did not adequately explain how continents could move through oceanic crust. Later plate tectonics changed the mechanism. Continents move with lithospheric plates, while oceanic lithosphere is created and recycled. A useful scientific theory must explain processes and make observations mutually consistent, rather than merely assembling attractive resemblances.
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