The process that continually adds new crust is

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Correct — C, seafloor spreading. New crust is manufactured along the mid-ocean ridges. Where two oceanic plates draw apart, the confining pressure on the mantle beneath them falls; the mantle melts by decompression without needing any extra heat; basaltic magma rises into the widening crack and freezes onto the trailing edge of each plate. Repeat that indefinitely and the ocean floor grows outward, symmetrically, from the ridge axis. Three independent lines of evidence pin it down. The mid-ocean ridge is a single continuous submarine mountain range 65,000 km long — the longest on Earth — running down the middle of the Atlantic, through the Indian Ocean and across the Pacific. The ocean floor is youngest at the ridge crest and grows older with distance from it in both directions. And the basalt, as it cools through the Curie point, locks in the direction of Earth's magnetic field at that moment, so the repeated reversals of that field are printed on the seabed as mirror-image magnetic stripes on either side of the axis — the observation Fred Vine and Drummond Matthews published in Nature in 1963, which converted a hypothesis into settled science. Robert Dietz had named the process in Nature in 1961 and Harry Hess of Princeton had laid out its mechanism, calling his own paper an exercise in 'geopoetry'. Measured spreading is slow but relentless: about 25 mm a year in the North Atlantic, 80 to 145 mm a year along the East Pacific Rise. The word doing the work in the stem is 'continually' — spreading is a standing, ongoing manufacture of crust, not an episode.

Why the other options are wrong — (a) The exact opposite end of the same conveyor. At a convergent margin the denser oceanic plate bends down into a trench — the Mariana Trench reaches nearly 11 km — and is consumed back into the mantle, which is why the oldest large-scale oceanic crust anywhere is only about 180 to 200 million years old while continental rocks run past 4 billion. There is a genuine nuance: melting above a subducting slab builds island arcs and adds some new CONTINENTAL crust over geological time. But on balance subduction destroys crust, and Earth's constant surface area requires that it destroy exactly as much as the ridges create.  (b) An earthquake is the sudden release of elastic strain that has been accumulating along a fault; it is a symptom of plate motion, not a manufacturing process. It can move crust — the 2004 Sumatra-Andaman rupture shifted islands by metres — but it adds no new crustal material at all. It also fails the other half of the stem: earthquakes are discrete events separated by decades or centuries, whereas 'continually' describes something that never stops.  (d) The most sophisticated wrong answer, and the one a well-read candidate reaches for, because slow convection in the mantle is indeed the engine of the whole system — it is the phrase Q24 of this same paper uses to define plate tectonics. But convection is a mantle process, not a crust-forming one. It supplies the driving force and delivers the heat and melt; the actual creation of crust is the emplacement and solidification of that melt at the ridge, which is what 'seafloor spreading' names. Convection is the cause; spreading is the process the stem asks for.

Concept: Earth's crust is not a fixed skin but a conveyor belt with a manufacturing end and a scrapping end, and the two are in balance because the planet's surface area does not change. At divergent boundaries — mid-ocean ridges, and the East African Rift on land — plates separate and new basaltic crust is added by seafloor spreading. At convergent boundaries the denser plate subducts into a trench and is recycled into the mantle, generating deep earthquakes and arc volcanoes on the way down. At transform boundaries plates slide laterally past each other and crust is neither made nor destroyed, only displaced, as along the San Andreas Fault. The two kinds of crust behave very differently in this cycle: oceanic crust is basaltic, only about 6 to 7 km thick and dense at roughly 3.0 g/cm³, so it subducts readily and is constantly renewed; continental crust is granitic, 30 to 70 km thick and lighter at about 2.7 g/cm³, so it resists subduction and survives for billions of years. The repeated assembly and break-up of supercontinents driven by this creation-and-destruction cycle is called the Wilson cycle.

State PCS 2023 Q23 — Geography PYQ with Answer

State PCS 2023 Q23 — Geography PYQ with Answer