Earth's Layers — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Why is the inner core solid even though it is the hottest part of Earth?
A) It is made of a special type of rock that never melts.
B) The enormous pressure at the center keeps the metal packed into a solid.
C) It is too far from the mantle's heat to melt.
D) The outer core absorbs all the heat before it reaches the inner core.
Despite extreme heat, the inner core is under such immense pressure that the metal atoms cannot move apart to become liquid.
2. A scientist notices that S-waves from an earthquake disappear when they reach the outer core. What does this tell the scientist?
A) The outer core is made of the same material as the mantle.
B) The outer core must be a gas because waves cannot pass through it.
C) The outer core is liquid because S-waves cannot travel through liquids.
D) The outer core is too thin for waves to pass through.
S-waves (shear waves) cannot propagate through liquids. Their absence in the outer core is direct evidence of liquid metal.
3. Which layer of Earth is responsible for driving the movement of tectonic plates?
A) The crust, because the plates sit on its surface.
B) The inner core, because it is the hottest layer.
C) The mantle, because its slow-flowing convection currents push the plates.
D) The outer core, because its liquid metal creates strong forces.
Convection currents in the semi-fluid mantle transfer heat and create drag on tectonic plates above.
4. Why is the oceanic crust denser than the continental crust?
A) Oceanic crust is older and has been compressed longer.
B) Oceanic crust is made of basalt, which is denser than the granite in continental crust.
C) Oceanic crust is closer to the mantle and absorbs heavy minerals.
D) Oceanic crust contains more water, which makes it heavier.
Basalt (oceanic crust) has a density of about 3.0 g/cm³ vs. granite (continental crust) at about 2.7 g/cm³.
Part B: Fill in the Blank
Write the correct answer on each line.
1. As you travel deeper into Earth, both temperature and pressure increase steadily.
Geothermal gradient: temperature rises about 25-30°C per km. Pressure increases due to weight of overlying rock.
2. The mantle behaves like a very slow-moving fluid even though it is made of rock.
Over millions of years, solid rock in the mantle can flow due to extreme heat and pressure — a property called plastic deformation.
3. Earth's layers are classified based on their composition, temperature, and state of matter.
Layers differ by state: crust and mantle are solid (mostly), outer core is liquid, inner core is solid.
4. The boundary between the crust and the mantle is called the Moho.
The Mohorovičić discontinuity (Moho) is the boundary where seismic wave speeds change sharply between crust and mantle.
5. Convection occurs in the mantle because hotter rock rises and cooler rock sinks.
Hot material is less dense and rises; cool material is denser and sinks — this is the convection cycle.