Which layer of Earth is liquid?
A. Crust
B. Inner core
C. Mantle
D. Outer core
Show answer and reasoning
Correct answer: D. Outer core
Correct. The outer core is molten iron and nickel. Seismic S waves, which cannot pass through liquid, are not transmitted through it, which is the main evidence for its liquid state.
- A: The crust is the thin, solid, rocky outer layer.
- B: The inner core is solid iron and nickel, kept solid by the enormous pressure at Earth's center despite its high temperature.
- C: The mantle is solid rock that flows very slowly over long time scales; it is not a liquid.
Source: Praxis 5005: Earth Science, I.A structure of the Earth
Which of Earth's layers makes up the largest share of the planet's volume?
A. Outer core
B. Crust
C. Mantle
D. Inner core
Show answer and reasoning
Correct answer: C. Mantle
Correct. The mantle extends from the base of the crust to a depth of about 2,900 km and makes up roughly 84 percent of Earth's volume.
- A: The outer core is a substantial layer but smaller than the mantle.
- B: The crust is the thinnest layer, only about 5 to 70 km thick.
- D: The inner core is a sphere about 1,200 km in radius, the smallest of the layers.
Source: Praxis 5005: Earth Science, I.A structure of the Earth
The rigid plates that move across Earth's surface are made of which material?
A. The entire mantle
B. The crust only
C. The crust plus the uppermost solid mantle (the lithosphere)
D. The liquid outer core
Show answer and reasoning
Correct answer: C. The crust plus the uppermost solid mantle (the lithosphere)
Correct. Tectonic plates are pieces of the lithosphere, which includes the crust and the rigid upper part of the mantle. They ride on the weaker, slowly flowing asthenosphere beneath.
- A: Most of the mantle lies beneath the plates and flows slowly rather than moving as rigid pieces.
- B: The crust alone is too thin to form the plates; the rigid upper mantle is part of each plate.
- D: The outer core is far below the plates and is liquid, not rigid.
Source: Praxis 5005: Earth Science, I.A structure of the Earth
The Mid-Atlantic Ridge is an undersea mountain chain where new ocean floor forms. Which type of plate boundary is it?
A. Convergent
B. Subduction
C. Divergent
D. Transform
Show answer and reasoning
Correct answer: C. Divergent
Correct. At a divergent boundary plates move apart, magma rises to fill the gap, and new crust forms. Mid-ocean ridges are the classic example.
- A: Convergent boundaries are where plates collide; they produce trenches and mountain ranges, not new ocean floor.
- B: Subduction happens at convergent boundaries where one plate sinks beneath another; it destroys crust rather than creating it.
- D: Transform boundaries are where plates slide past each other, such as the San Andreas Fault, with no new crust created.
Source: Praxis 5005: Earth Science, I.B Earth processes
The Himalayas are still rising as the Indian plate pushes into the Eurasian plate. What kind of boundary produces this mountain range?
A. A convergent boundary where oceanic crust subducts beneath a continent
B. A divergent boundary
C. A convergent boundary between two continental plates
D. A transform boundary
Show answer and reasoning
Correct answer: C. A convergent boundary between two continental plates
Correct. When two continental plates collide, neither sinks easily because both are buoyant, so the crust crumples and thickens into high mountains.
- A: Ocean-continent subduction builds volcanic mountain chains such as the Andes; the Himalayas are not volcanic and formed from two continents colliding.
- B: Divergent boundaries pull plates apart and form rift valleys or ridges, not folded mountain ranges.
- D: Transform boundaries slide plates sideways and cause earthquakes but do not build large mountain ranges.
Source: Praxis 5005: Earth Science, I.B Earth processes
