Plate Tectonics — Set 1

Correct Answer: D. Transform boundary

• **Transform boundary** = forms when two tectonic plates slide horizontally past one another, causing earthquakes without creating or destroying crust. • **San Andreas Fault** — the most famous transform boundary, marking where the Pacific Plate slides past the North American Plate in California. • 💡 Wrong-option analysis: Convergent boundary: plates move toward each other, not sideways; Subduction zone: one plate sinks beneath another; Divergent boundary: plates move apart, creating new crust.

Correct Answer: C. Harry Hess

• **Harry Hess** = proposed the theory of sea floor spreading in the early 1960s, explaining that new oceanic crust forms at mid-ocean ridges and spreads outward. • **1960** — the year Hess published his landmark paper 'History of Ocean Basins', providing the key mechanism for plate tectonics. • 💡 Wrong-option analysis: Arthur Holmes: proposed mantle convection earlier but not seafloor spreading specifically; Alfred Wegener: proposed Continental Drift in 1912, a different concept; Abraham Ortelius: 16th-century cartographer who noted continental fit, not seafloor spreading.

Correct Answer: C. Convection currents in the mantle

• **Convection currents in the mantle** = the primary driver of plate movement, where heat from the core causes magma to rise, cool, and sink in a continuous cycle. • **2,900 km** — the depth of the mantle where these convection currents operate, exerting a dragging force on the lithospheric plates above. • 💡 Wrong-option analysis: Oceanic tides: caused by lunar gravity, too weak to move plates; Lunar gravity: affects tides but is not the primary plate driver; Earth's rotation: contributes minimally compared to mantle convection.

Correct Answer: B. The Himalayas

• **The Himalayas** = formed by the convergent collision of the Indian Plate and the Eurasian Plate, pushing the crust upward in the world's highest mountain range. • **50 million years ago** — when the Indian subcontinent began colliding with Eurasia, and the mountains are still rising at about 5 mm per year. • 💡 Wrong-option analysis: The Rockies: formed by subduction of oceanic plates under North America; The Andes: formed by Nazca Plate subducting under South America; The Alps: formed by African Plate colliding with Eurasian Plate.

Correct Answer: D. Pacific Ocean

• **Pacific Ocean** = the Ring of Fire is a large horseshoe-shaped zone around this ocean, associated with continuous oceanic trenches, volcanic arcs, and intense seismic activity. • **90%** — of the world's earthquakes and about 75% of active volcanoes are located along the Ring of Fire. • 💡 Wrong-option analysis: Indian Ocean: has the mid-Indian ridge but not the Ring of Fire; Arctic Ocean: tectonically less active, not associated with this zone; Atlantic Ocean: has Mid-Atlantic Ridge but no Ring of Fire.

Correct Answer: A. Lithosphere

• **Lithosphere** = the rigid outer layer of Earth that is broken into tectonic plates, including the crust and the uppermost part of the mantle. • **100 km** — the average thickness of the lithosphere, which floats and moves on top of the semi-fluid asthenosphere beneath it. • 💡 Wrong-option analysis: Asthenosphere: the semi-fluid layer the plates move ON, not the plates themselves; Mesosphere: deeper mantle layer below the asthenosphere; Outer Core: liquid iron-nickel layer far below, not related to plates.

Correct Answer: C. One plate sinks beneath another

• **One plate sinks beneath another** = at a subduction zone, a denser tectonic plate descends into the mantle under a less dense plate, melting and forming volcanic arcs. • **11,000 metres** — the depth of the Marianas Trench, the deepest subduction-formed feature on Earth, where the Pacific Plate sinks beneath the Mariana Plate. • 💡 Wrong-option analysis: Plates slide past each other: describes transform/conservative boundaries; New crust is created: happens at divergent boundaries, opposite of subduction; Plates move apart: also describes divergent boundaries.

Correct Answer: C. Divergent boundary

• **Divergent boundary** = the Mid-Atlantic Ridge is where the North American and Eurasian plates move apart, with magma rising to form new oceanic crust. • **65,000 km** — the total length of the Mid-Atlantic Ridge, making it the longest mountain range in the world (mostly underwater). • 💡 Wrong-option analysis: Transform boundary: plates slide sideways, not apart; Convergent boundary: plates move toward each other, not apart; Conservative boundary: another name for transform boundary where crust is neither created nor destroyed.

Correct Answer: B. Pangea

• **Pangea** = a massive supercontinent that incorporated almost all Earth's landmasses, which began to break apart about 200 million years ago into current continents. • **300 million years ago** — the approximate age of Pangea, proposed by Alfred Wegener in 1912 using continental fit and fossil evidence. • 💡 Wrong-option analysis: Laurasia: the northern part that broke off from Pangea later, not the original supercontinent; Rodinia: an even older supercontinent from about 1.1 billion years ago; Gondwana: the southern portion of Pangea after it split, not the whole supercontinent.

Correct Answer: B. Rift valley

• **Rift valley** = formed on land when continental crust pulls apart at a divergent boundary, creating a sunken valley with volcanic activity and lakes. • **6,000 km** — the length of the East African Rift, a modern example stretching from the Afar Triangle to Mozambique, slowly splitting the African continent. • 💡 Wrong-option analysis: Deep-sea trench: forms at subduction zones, not divergent boundaries; Fold mountains: form at convergent boundaries where crust crumples; Volcanic island arc: forms at oceanic-oceanic subduction zones, not divergent boundaries.