Plate Tectonics — Set 2

Correct Answer: A. Plate subduction

• **Plate subduction** = the Marianas Trench is formed by the Pacific Plate subducting beneath the smaller Mariana Plate at a convergent boundary, creating a deep oceanic trench. • **11,034 metres** — the depth of the Challenger Deep within the Marianas Trench, the deepest known point on Earth's surface. • 💡 Wrong-option analysis: Continental rifting: creates rift valleys and new ocean basins, not deep trenches; Seafloor spreading: creates new crust at ridges, opposite of trench formation; Transform faulting: creates strike-slip faults, not deep trenches.

Correct Answer: A. Magnetite

• **Magnetite** = crystals in lava align with Earth's magnetic field as they cool, creating symmetrical magnetic stripes on the ocean floor that prove sea floor spreading. • **Every 200,000-300,000 years** — the average frequency of Earth's magnetic field reversals recorded in magnetite stripes, providing a timeline of seafloor formation. • 💡 Wrong-option analysis: Feldspar: common rock mineral but not magnetic, cannot record field reversals; Quartz: non-magnetic silicate mineral, leaves no magnetic record; Mica: contains some iron but not significantly magnetic enough to record field reversals.

Correct Answer: A. Denser sinking plates pulling the rest of the plate

• **Slab Pull** = the force where the weight of a cold, dense subducting plate pulls the rest of the plate into the mantle, considered the most significant driver of plate motion. • **3-4 times** — the estimated magnitude by which slab pull is stronger than ridge push in driving plate movement, according to geological studies. • 💡 Wrong-option analysis: Rising magma pushing plates apart: describes ridge push, a different and weaker driving force; Centrifugal force from Earth's rotation: too weak and acts outward, not inward toward subduction zones; Friction between two sliding plates: describes forces at transform faults, resists plate motion rather than driving it.

Correct Answer: C. South American Plate

• **South American Plate** = the Nazca Plate subducts beneath it along the western coast of South America, forming the Andes Mountains and intense volcanic activity. • **7 cm per year** — the rate at which the Nazca Plate is subducting under the South American Plate, one of the fastest subduction rates globally. • 💡 Wrong-option analysis: North American Plate: bordered by the Juan de Fuca Plate in the west, not the Nazca; Antarctic Plate: borders the Nazca to the south but is not the main subduction target; African Plate: on the opposite side of the Atlantic, not in contact with the Nazca Plate.

Correct Answer: D. Basalt

• **Basalt** = the primary volcanic rock forming oceanic crust, denser than continental granite, which causes it to sink during plate collisions. • **3 g/cm³** — the approximate density of basaltic oceanic crust, compared to 2.7 g/cm³ for granite, explaining why oceanic plates subduct under continental ones. • 💡 Wrong-option analysis: Limestone: a sedimentary rock formed from marine shells, not the primary constituent of oceanic crust; Granite: makes up continental crust, lighter and less dense than basalt; Sandstone: a sedimentary rock found in continental settings, not oceanic crust.

Correct Answer: D. Pacific Plate

• **Pacific Plate** = the largest tectonic plate on Earth, almost entirely oceanic, covering a vast portion of the Pacific Ocean and surrounded by the Ring of Fire. • **103 million km²** — the approximate area of the Pacific Plate, making it significantly larger than the second-largest plate. • 💡 Wrong-option analysis: African Plate: large but contains mostly continental crust, smaller than Pacific; Eurasian Plate: the largest plate with significant continental crust, but smaller than Pacific overall; Antarctic Plate: surrounds Antarctica and is large, but smaller than the Pacific Plate.

Correct Answer: C. Strike-slip fault

• **Strike-slip fault** = the San Andreas Fault is this type, where the Pacific Plate and the North American Plate slide horizontally past each other, causing frequent earthquakes. • **1,300 km** — the length of the San Andreas Fault, which has produced earthquakes as large as magnitude 7.9, including the devastating 1906 San Francisco earthquake. • 💡 Wrong-option analysis: Normal fault: rock moves down a dip, associated with extensional tectonics not transform boundaries; Thrust fault: one block rides up over another, associated with compression at convergent boundaries; Reverse fault: similar to thrust, formed by compressional forces, not lateral sliding.

Correct Answer: D. Continental Drift

• **Continental Drift** = the predecessor theory proposed by Alfred Wegener in 1912, suggesting continents were once joined as Pangea and have since drifted apart. • **1912** — the year Wegener published his theory, supported by continental fit, fossil evidence, and matching rock formations across oceans. • 💡 Wrong-option analysis: Isostasy: the concept of crustal equilibrium and floating, not movement of continents; Catastrophism: the idea that sudden catastrophes shaped Earth's geology; Uniformitarianism: the principle that geological processes are slow and uniform over time.

Correct Answer: B. Volcanic activity far from plate boundaries

• **Volcanic activity far from plate boundaries** = a hotspot is where a plume of hot mantle material rises independently, creating volcanoes far from any plate boundary. • **5 cm per year** — the rate at which the Pacific Plate moves over the Hawaiian hotspot, creating a chain of volcanic islands of progressively older ages. • 💡 Wrong-option analysis: Excessive earthquake frequency: earthquakes occur at plate boundaries, not at interior hotspots; Plate boundary friction: describes forces at transform faults, not hotspot characteristics; Cold mantle upwelling: hotspots involve HOT mantle plumes rising upward, the opposite of cold upwelling.

Correct Answer: A. India and Eurasia

• **India and Eurasia** = the Tethys Sea was the ancient ocean between these two landmasses; as India moved north, it was subducted and closed, and its sediments were uplifted to form the Himalayas. • **55 million years ago** — when India began colliding with Asia, completely closing the Tethys Sea and beginning Himalayan uplift. • 💡 Wrong-option analysis: North America and Europe: separated by the Atlantic Ocean, which formed by divergence not subduction; Africa and South America: separated by the South Atlantic, which opened up as Gondwana split; Australia and Antarctica: separated by the Southern Ocean, formed by divergence of Gondwana fragments.