Plate Tectonics — Set 3

Correct Answer: D. Alfred Wegener

• **Alfred Wegener** = the German meteorologist who proposed Continental Drift in 1912, suggesting all continents were once joined as Pangea based on geological and fossil evidence. • **1912** — the year Wegener published 'The Origin of Continents and Oceans', though his theory was initially rejected and only accepted decades after his death in 1930. • 💡 Wrong-option analysis: Charles Darwin: naturalist known for evolution theory, not continental movement; Max Planck: physicist known for quantum theory, not geology; Albert Einstein: physicist known for relativity, not continental drift.

Correct Answer: C. Divergent boundary

• **Divergent boundary** = called 'constructive' because new lithosphere is created here as plates pull apart and magma rises to form new oceanic crust. • **2.5 cm per year** — the average spreading rate of the Mid-Atlantic Ridge, where new basaltic crust is continuously added to the ocean floor. • 💡 Wrong-option analysis: Subduction boundary: also called destructive as crust is consumed; Convergent boundary: plates collide and crust is crumpled or destroyed, the opposite of constructive; Transform boundary: crust is neither created nor destroyed, called a conservative boundary.

Correct Answer: A. Convergent boundary

• **Convergent boundary** = called 'destructive' because Earth's crust is consumed when plates collide — either subducted into the mantle or crumpled into fold mountains. • **80 km depth** — where subducting oceanic crust begins to melt in the mantle at destructive boundaries, generating magma that fuels volcanic arcs. • 💡 Wrong-option analysis: Transform boundary: called conservative as no crust is created or destroyed; Hotspot: volcanic feature unrelated to plate boundaries, not a boundary type; Divergent boundary: called constructive as new crust is made here.

Correct Answer: C. Africa and South America

• **Africa and South America** = fossils of the freshwater reptile Mesosaurus were found only in these two continents, suggesting they were once connected since it could not cross the salty Atlantic. • **300 million years ago** — when Mesosaurus lived, at which time Africa and South America were still joined in Gondwana, explaining its shared distribution. • 💡 Wrong-option analysis: Australia and Africa: Mesosaurus fossils are not found in Australia; Glossopteris plant fossils span more continents; Asia and Europe: these landmasses were already joined, so shared fossils would not prove drift; North America and Asia: Mesosaurus fossils have not been found across this combination.

Correct Answer: D. Asthenosphere

• **Asthenosphere** = the ductile part of the upper mantle just below the lithosphere, behaving plastically and allowing tectonic plates to glide across it over geological time. • **100-350 km depth** — the depth range of the asthenosphere, where high temperatures keep rock close to its melting point, enabling slow plastic flow. • 💡 Wrong-option analysis: Lithosphere: the rigid outer layer that forms the plates themselves, not the layer they move on; Crust: the outermost solid layer, part of the lithosphere; Inner Core: solid iron at Earth's centre, unrelated to plate movement.

Correct Answer: D. Oceanic-Oceanic convergence

• **Oceanic-Oceanic convergence** = island arcs form when one oceanic plate subducts beneath another, melting to produce magma that rises as underwater volcanoes, eventually breaking the sea surface. • **2,000 km** — the length of the Aleutian Island arc off Alaska, formed by the Pacific Plate subducting beneath the smaller North American oceanic plate portion. • 💡 Wrong-option analysis: Oceanic-Continental convergence: produces coastal volcanic mountain ranges like the Andes, not island arcs; Divergent boundary: creates new ocean floor and rift valleys, not volcanic arcs; Continental-Continental convergence: creates fold mountains like the Himalayas with no subduction-driven volcanism.

Correct Answer: D. A few centimeters

• **A few centimeters per year** = tectonic plates typically move 2-10 cm per year, roughly comparable to the speed at which human fingernails grow. • **10 cm per year** — the fastest measured plate movement, that of the Australian Plate, which has moved significantly northward over millions of years. • 💡 Wrong-option analysis: Several meters: would cause catastrophic annual changes in geography, far too fast for tectonic plates; Less than a millimeter: too slow; some plates move several centimeters; Several kilometers: physically impossible given the forces involved in plate movement.

Correct Answer: B. Pacific Plate

• **Pacific Plate** = the largest tectonic plate on Earth, almost exclusively oceanic, submerged beneath the Pacific Ocean with very little continental landmass. • **103 million km²** — the vast area of the Pacific Plate, most of which lies beneath ocean water, surrounded by the seismically active Ring of Fire. • 💡 Wrong-option analysis: African Plate: contains the entire African continent plus surrounding oceanic crust; North American Plate: contains the whole of North America plus parts of the Atlantic and Pacific; Eurasian Plate: carries the entirety of Eurasia, a massive continental landmass.

Correct Answer: A. The point where three tectonic plates meet

• **Triple Junction** = a geographic point where the boundaries of three tectonic plates meet, creating complex zones with combinations of ridges, trenches, and faults. • **Afar Triple Junction** — the most notable example in Africa, where the African, Arabian, and Somali plates meet, slowly splitting the continent. • 💡 Wrong-option analysis: Where three different rock types meet: describes a geological contact, not a tectonic feature; Where three ocean currents collide: an oceanographic term, not a plate tectonic concept; A place with three active volcanoes: describes a volcanic field, unrelated to plate boundary junctions.

Correct Answer: B. Nazca Plate

• **Nazca Plate** = considered a minor or intermediate tectonic plate, not one of the seven or eight recognized primary major plates, despite its significant subduction activity. • **7-8 major plates** — the number of primary tectonic plates recognized globally (Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American). • 💡 Wrong-option analysis: North American Plate: carries all of North America, one of the seven major plates; Antarctic Plate: surrounds Antarctica and is one of the major plates; Indo-Australian Plate: carries India and Australia, one of the major plates.