Directions for teachers: Ask your students to read the online Science News article “How the Earth-shaking theory of plate tectonics was born,” which recounts how plate tectonics became a unifying theory in earth science, and answer the following questions. This story is the first installment of a series that celebrates Science News’ upcoming 100th anniversary by highlighting some of the biggest advances in science over the last century. A version of the story, “Shaking up Earth,” can be found in the January 16, 2021 issue of Science News. Please note that the questions do not cover the “Crucible of life” sidebar. For more on plate tectonics, and to see the rest of the series as it appears, visit Science News’ Century of Science site at www.sciencenews.org/century.

1. What is the theory of plate tectonics? Explain it.

Plate tectonics describes how Earth’s outermost layer, called the lithosphere, is broken into rock slabs (or plates) that float on an inner layer of hot churning fluid. Over eons, the plates collide, diverge and grind past one another to create features such as volcanoes, earthquakes, ocean basins and mountains.

2. Why does the article compare the theory of plate tectonics to Albert Einstein’s general theory of relativity?

Just as Einstein’s general theory of relativity upended our understanding of the universe, plate tectonics revolutionized our understanding of the Earth.

3. What early idea set the stage for plate tectonics? When was this idea proposed and what did it attempt to explain?

In 1912, a German meteorologist proposed that Earth’s continents were on the move — an idea that is now known as continental drift. This idea holds that colliding continents are responsible for geologic formations such as mountains and that at one time, the continents were joined together as one supercontinent dubbed Pangaea. Continental drift would explain why fossils of the same organisms and rocks of the same type and age are found on either side of the Atlantic Ocean.

4. How did continental drift challenge accepted views of Earth? How was the idea initially viewed by other scientists?

Scientists at the time thought the Earth’s crust was rigid and locked in place — a principle called uniformitarianism — and that mountains sprung up as the crust slowly cooled and contracted after its formation. While some scientists found continental drift intriguing, many geologists were skeptical of the idea.

5. What were critics’ main arguments against continental drift?

Critics argued that continental drift contradicted uniformitarianism (which held that Earth’s continents must be immovable), that the idea couldn’t explain how the continents move and that it was a collection of unrelated observations not supported by data.

6. What explanation did English geologist Arthur Holmes come up with for how the continents might move? How did the geology community react to this explanation?

Holmes suggested that the continents might float like rafts atop a layer of thick, partially molten rocks deep inside Earth. Holmes admitted that he lacked data to back up his suggestion, and continental drift was largely shelved by the geology community for decades.

7. What scientific advances helped revive the idea of continental drift beginning in the 1950s? What historical event contributed to these advances?

Sonar mapping developed during World War II revealed the extent of a rift in the ocean floor. Magnetometers revealed alternating stripes of magnetic polarity in seafloor rocks, suggesting each of the stripes formed at different times. A global network of seismograph stations led scientists to discover and measure earthquakes along mid-ocean ridges and beneath trenches. And steel probes inserted into cores drilled into the seafloor revealed that the ridges were much hotter than the surrounding seafloor.

8. What is “seafloor spreading” and how does it connect to continental drift?

Circulating hot rock within the Earth pushes to the surface and forces apart areas of lithosphere. As the lava burbles up between, new seafloor is made. Mid-ocean ridges are where new seafloor is born and deep ocean trenches are where old lithosphere is reabsorbed into Earth’s interior. This cycle is responsible for the growing and shrinking of the seafloor that brings continents together and splits them apart.

9. What sets the unified theory of plate tectonics apart from the idea of continental drift?

Plate tectonics says not only that continents drift, but also explains how and why they drift and connects the details of what is known to be happening within the Earth to the details of lithospheric movement and the features visible on Earth’s surface.

10. How have people benefited from the understanding of plate tectonics?

Understanding how Earth recycles its crust has allowed people to better prepare for earthquakes, tsunamis and volcanoes. It has also shaped scientific research that has led to insights about Earth’s climate and the evolution of life on the planet.

11. What questions do scientists still have about plate tectonics?

Why is Earth the only place in the solar system that seems to have plate tectonics, when and how did plate tectonics begin and when might it end?