Plate Tectonics — Set 4

Correct Answer: C. 1960s

• **1960s** = plate tectonics became the unifying theory of geology in this decade, following discoveries of seafloor spreading and magnetic striping on the ocean floor. • **1968** — the year the theory was formally codified when J. Tuzo Wilson and others synthesized seafloor spreading, continental drift, and seismic data into modern plate tectonic theory. • 💡 Wrong-option analysis: 1920s: Wegener's Continental Drift was largely rejected in this era; 1940s: wartime ocean mapping began providing evidence but theory was not yet accepted; 1980s: plate tectonics was already well-established by then, not a decade of initial acceptance.

Correct Answer: C. 30-50 km

• **30-50 km** = the average thickness of continental crust, reaching up to 70 km under high mountain ranges like the Himalayas. • **70 km** — the maximum thickness under major mountain belts, compared to oceanic crust which is only 5-10 km thick, explaining why continents resist subduction. • 💡 Wrong-option analysis: 5-10 km: the thickness of oceanic crust, not continental; 500-600 km: far too thick, approaching the depth of the lower mantle; 100-150 km: includes the lithospheric mantle but exceeds average crustal thickness.

Correct Answer: A. Rift valley

• **Rift valley** = formed when continental crust pulls apart at a divergent boundary, thinning and sinking to create a valley often associated with volcanic activity and lakes. • **East African Rift** — a 6,000 km continental rift that will eventually split Africa, with Lake Tanganyika and Lake Malawi forming in its basin. • 💡 Wrong-option analysis: Island arc: forms at oceanic-oceanic subduction zones, not continental divergence; Oceanic trench: forms at subduction zones where oceanic plates sink; Mid-ocean ridge: forms when oceanic plates diverge underwater, not continental plates.

Correct Answer: D. Subduction zones

• **Subduction zones** = deep-focus earthquakes occur here at depths of 300-700 km, as the cold, brittle subducting plate fractures deep within the mantle. • **700 km** — the maximum depth at which deep-focus earthquakes occur, exclusively in subduction zones where cold slab material remains brittle enough to rupture. • 💡 Wrong-option analysis: Transform faults: produce shallow-focus earthquakes near the surface, not deep ones; Continental rifts: generate shallow earthquakes due to extensional faulting near surface; Mid-ocean ridges: produce shallow earthquakes from magma intrusion and normal faulting, not deep-focus events.

Correct Answer: B. Plate Tectonics

• **Plate Tectonics** = Glossopteris fossils found in South America, Africa, India, Australia, and Antarctica suggest these landmasses were once joined in Gondwana, supporting continental movement. • **5 continents** — the number of southern landmasses where Glossopteris fossils are found, an impossible distribution unless they were once connected in a single supercontinent. • 💡 Wrong-option analysis: Oceanic Circulation: explains climate patterns via ocean currents, not fossil distribution across continents; Glacial Advance: explains ice-age deposits, not why a seed fern grew on five separate landmasses; Natural Selection: explains evolution of species, not the geographical distribution of identical fossils on separated continents.

Correct Answer: D. Nazca Plate

• **Nazca Plate** = an oceanic plate subducting beneath the South American continental plate along its western coast, responsible for the uplift of the Andes and regional earthquakes. • **7 cm per year** — the rate at which the Nazca Plate dives under South America, making it one of Earth's most seismically and volcanically active zones. • 💡 Wrong-option analysis: Scotia Plate: a small plate in the southern Atlantic/Antarctic region, not involved in Andes formation; Cocos Plate: subducts under Central America and Mexico, forming the Mexican volcanic belt, not the Andes; Pacific Plate: the largest plate, but the Nazca Plate is the one directly west of the Andes.

Correct Answer: D. Crust and Mantle

• **Crust and Mantle** = the Moho is the boundary between these layers, defined by a distinct increase in seismic wave velocity as waves enter the denser mantle rock. • **Andrija Mohorovičić in 1909** — the Croatian seismologist who discovered this discontinuity by analyzing earthquake waves that sped up at a certain depth. • 💡 Wrong-option analysis: Lithosphere and Asthenosphere: the boundary between these is the lithosphere-asthenosphere boundary (LAB), deeper than the Moho; Upper and Lower Mantle: separated by the 660 km discontinuity, much deeper than the Moho; Inner and Outer Core: the inner-outer core boundary is at 5,100 km depth, far deeper.

Correct Answer: B. Ocean basin

• **Ocean basin** = the Red Sea is a young ocean basin formed by divergence of the African and Arabian plates, representing an intermediate stage between a continental rift and a full ocean. • **30 million years ago** — when the African and Arabian plates began separating to form the Red Sea, which continues to widen at about 1.5 cm per year. • 💡 Wrong-option analysis: Subduction zone: where one plate sinks under another, creating trenches and mountains, not the spreading Red Sea; Transform fault: where plates slide sideways, not apart as in the Red Sea; Fold mountain belt: forms at convergent boundaries, opposite of the divergent Red Sea formation.

Correct Answer: C. Lithosphere

• **Lithosphere** = the rigid, solid outermost shell of Earth comprising the crust and uppermost mantle, broken into tectonic plates that move on the underlying asthenosphere. • **100 km thick** — the average thickness of the lithosphere, which is brittle and breaks under stress rather than flowing, enabling earthquake generation. • 💡 Wrong-option analysis: Atmosphere: the gaseous envelope surrounding Earth, not the solid outer shell; Hydrosphere: encompasses all water on Earth including oceans, rivers, and ice; Biosphere: the zone of life on Earth encompassing all ecosystems, not the rocky outer shell.

Correct Answer: B. Cooling rock at ridges becomes denser and slides down

• **Cooling rock at ridges becomes denser and slides down** = ridge push occurs because elevated mid-ocean ridges have high gravitational potential energy, causing cooling, thickening lithosphere to slide downslope and push the plates apart. • **2-3 km elevation** — the height of mid-ocean ridges above the surrounding ocean floor, providing the gravitational gradient that drives the ridge push force. • 💡 Wrong-option analysis: Ocean currents push against the ridge: currents act on the seafloor surface, far too weak to move entire lithospheric plates; Magnetic forces repel the plates: Earth's magnetic field does not exert forces strong enough to move tectonic plates; The moon's gravity pulls the ridge: lunar gravity causes tides but is too weak and uniform to cause directional plate movement.