Earth's Layers — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. Scientists find identical fossils on continents separated by wide oceans. What does this evidence most strongly support?
A) The fossils were carried across the ocean by strong currents.
B) The continents were once joined together and later moved apart on tectonic plates.
C) Animals swam between the continents long ago.
D) Volcanic eruptions launched the fossils to distant continents.
Identical fossils on separate continents mean those landmasses must have been connected when the animals were alive, then slowly drifted apart on moving tectonic plates.
2. At a divergent plate boundary on the ocean floor, scientists find that rocks closest to the ridge are the youngest. Why?
A) Older rocks sink into the mantle because they are heavier.
B) Ocean currents push the older rocks farther from the ridge.
C) New crust is constantly being created at the ridge as magma rises and cools.
D) Younger rocks float on top of older rocks during underwater eruptions.
Rocks nearest the ridge are youngest because fresh magma continuously rises at the boundary, hardens into new crust, and pushes older rock outward on both sides.
3. Why is the lithosphere described as rigid while the asthenosphere beneath it is described as flowing?
A) The lithosphere is made of metal while the asthenosphere is made of gas.
B) The lithosphere is cooler and more brittle, while the asthenosphere is hot enough that rock moves like thick plastic.
C) The lithosphere has no pressure on it, so it stays hard and stiff.
D) The asthenosphere contains water that keeps the rock soft and wet.
The lithosphere is cool enough to be stiff and breakable, while the deeper asthenosphere is so hot that its rock softens and flows very slowly, almost like thick taffy.
4. The Mariana Trench is the deepest point in the ocean. Which process most likely formed it?
A) A transform boundary caused the seafloor to crack open.
B) An earthquake split the crust into a deep canyon.
C) An oceanic plate subducted beneath another plate, pulling the seafloor down.
D) Volcanic lava built walls on either side, leaving a gap in between.
The Mariana Trench formed where the Pacific Plate dives beneath the Mariana Plate, bending the seafloor sharply downward and creating the deepest point on Earth.
Part B: Fill in the Blank
Write the correct answer on each line.
1. Alfred Wegener proposed that all continents were once joined in a supercontinent called Pangaea.
Pangaea, meaning "all lands" in Greek, was the supercontinent Wegener proposed to explain why coastlines, fossils, and rock types match across today's separated continents.
2. Matching rock formations and fossils on different continents provide evidence that they were once connected.
Finding the same fossils on continents now separated by oceans proves those lands were once joined, because the ancient animals could not have crossed wide oceans.
3. The theory of plate tectonics explains how large pieces of Earth's crust move over the mantle.
Plate tectonics is the scientific theory that Earth's crust is divided into large moving plates, driven by convection currents in the mantle beneath them.
4. Seafloor spreading occurs at mid-ocean ridges where new crust pushes older crust outward.
Seafloor spreading happens when magma rises at a mid-ocean ridge, hardens into new rock, and slowly pushes the older crust away on both sides like a conveyor belt.
5. The coastlines of South America and Africa fit together like puzzle pieces, supporting continental drift.
The eastern coast of South America and the western coast of Africa match up almost perfectly, which was one of Wegener's strongest clues that these continents were once joined.