IELTS Reading: Sentence Completion

The theory of plate tectonics, which explains the large-scale structure and behaviour of Earth's outer shell, is arguably the most unifying concept in the earth sciences. It describes how the lithosphere — the rigid outer layer comprising the crust and the uppermost mantle — is divided into a mosaic of tectonic plates that move relative to one another atop the partially molten asthenosphere below, driven by heat escaping from Earth's interior. The idea has a contentious history. Alfred Wegener, a German meteorologist, proposed in 1912 that the continents had once formed a single supercontinent — which he called Pangaea — that had subsequently broken apart and drifted to their current positions. He based this on the complementary shapes of continental coastlines (particularly the fit between South America and Africa), similarities in rock sequences and fossil assemblages on continents now separated by thousands of kilometres of ocean, and evidence of past climates inconsistent with current geographic positions (such as tropical plant fossils in Antarctica). Wegener's hypothesis was widely rejected by the geological establishment of his day, primarily because he could not identify a plausible mechanism by which continents could plough through oceanic crust. The mechanism was eventually identified through oceanographic research in the 1950s and 1960s. Scientists mapping the ocean floor discovered an extensive network of mid-ocean ridges — underwater mountain ranges where new oceanic crust is continuously generated by upwelling magma. Symmetrical patterns of magnetic striping in the rock on either side of these ridges, resulting from periodic reversals of Earth's magnetic field recorded in cooling lava, provided conclusive evidence that the ocean floor was spreading. At the same time, deep-sea trenches were identified as sites where old oceanic crust was being subducted — pulled back into the mantle beneath continental margins. The three types of plate boundaries correspond to different geological events. Convergent boundaries, where plates collide, produce mountain ranges (when two continental plates collide) or volcanoes and deep trenches (when oceanic crust subducts beneath continental crust). Divergent boundaries, where plates pull apart, create mid-ocean ridges and rift valleys. Transform boundaries, where plates slide laterally past each other, generate earthquakes but little volcanism.