Plate Tectonics — Set 4

Correct Answer: C. 1960s

Plate Tectonics became the unifying theory of geology during the 1960s. This followed the discovery of sea-floor spreading and magnetic striping on the ocean floor. It successfully explained earthquakes, volcanoes, and mountain building.

Correct Answer: C. 30-50 km

Continental crust is generally 30 to 50 km thick, reaching up to 70 km under high mountain ranges. It is significantly thicker and less dense than the oceanic crust. Its lower density prevents it from subducting easily into the mantle.

Correct Answer: A. Rift valley

When continental crust pulls apart at a divergent boundary, a rift valley is created. The crust thins and sinks, often leading to volcanic activity and the formation of lakes. Over time, this rift can evolve into a new sea or ocean.

Correct Answer: D. Subduction zones

Deep-focus earthquakes occur at depths of 300 to 700 km within subduction zones. As the cold plate sinks into the mantle, it remains brittle enough to fracture at great depths. Other boundary types usually produce shallow-focus earthquakes.

Correct Answer: B. Plate Tectonics

Glossopteris was a seed fern whose fossils are found in South America, Africa, India, Australia, and Antarctica. The distribution of this plant suggested these landmasses were once connected in Gondwana. This botanical evidence was crucial for the continental drift and plate tectonics theories.

Correct Answer: D. Nazca Plate

The Nazca Plate is an oceanic plate that is subducting beneath the South American continental plate. This process is responsible for the uplift of the Andes and the frequent earthquakes in the region. It is a prime example of oceanic-continental convergence.

Correct Answer: D. Crust and Mantle

The Moho is the boundary between the Earth's crust and the underlying mantle. It is defined by a distinct change in the speed of seismic waves. It exists within the lithospheric plates themselves.

Correct Answer: B. Ocean basin

The Red Sea is a young ocean basin formed by the divergence of the African and Arabian plates. It began as a rift valley and has since widened enough for seawater to enter. It represents an intermediate stage between continental rifting and a full ocean.

Correct Answer: C. Lithosphere

The lithosphere is the solid, outermost shell of our planet that includes the crust and the upper mantle. It is rigid and brittle, meaning it breaks rather than flows when stressed. This shell is divided into the various tectonic plates.

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

Ridge push is a driving force where cooling, thickening lithosphere slides down the sloping asthenosphere at a ridge. Elevated mid-ocean ridges possess higher gravitational potential energy than deeper ocean floor. This gravity-driven force helps push the plates apart.