Practice Tests
IELTS Reading (AC) Task 1 #116

A. The theory of plate tectonics is the unifying framework of modern geology, explaining how the Earth''s surface features are formed and how they change over time. It posits that the planet''s outer layer, the lithosphere, is not a single, solid shell but is broken into a number of large and small rigid plates. These plates are in constant, slow motion, floating on the semi-molten layer beneath them known as the asthenosphere. The movement, interaction, and collision of these plates are responsible for some of the most dramatic geological phenomena on Earth, including earthquakes, volcanic eruptions, and the formation of mountain ranges.

B. The concept of plate tectonics evolved from the earlier idea of continental drift, proposed by Alfred Wegener in the early 20th century. Wegener presented evidence that the continents were once joined together in a single supercontinent he called Pangaea, citing the jigsaw-like fit of continental coastlines, fossil similarities across oceans, and geological formations that matched up on separate continents. However, his theory was initially rejected by the scientific community because he could not propose a plausible mechanism to explain how the continents moved.

C. The crucial evidence that validated Wegener''s ideas and led to the development of plate tectonics emerged in the mid-20th century from oceanographic research. The mapping of the ocean floor revealed the existence of mid-ocean ridges, vast underwater mountain ranges with deep rift valleys at their crests. It was discovered that new oceanic crust is formed at these ridges as magma rises from the mantle and solidifies, pushing the older crust away on either side. This process, known as seafloor spreading, provided the powerful driving mechanism that Wegener''s theory had lacked.

D. There are three main types of plate boundaries where interactions occur. At divergent boundaries, plates move apart, as seen at mid-ocean ridges. At convergent boundaries, plates collide, which can result in one plate sliding beneath another (subduction), leading to volcanic arcs and deep ocean trenches, or the crumpling of crust to form massive mountain ranges like the Himalayas. At transform boundaries, plates slide horizontally past each other, often causing powerful earthquakes, such as along the San Andreas Fault in California. This comprehensive theory has revolutionised our understanding of the dynamic and ever-changing nature of our planet.