As scientists began to explore the ocean floor after World War II, they discovered many new clues to help them solve a mystery that had begun decades earlier – how the continents moved about on the surface of the earth.
Background on the Ground
Alfred Wegener was the developer of a new theory of the earth’s surface called continental drift, and he was an adept investigator, but he passed away in 1930 with his theories generally dismissed. New evidence, though, would excite a whole new generation of geologic investigators and inject new credibility into his ideas. This evidence eventually led to a mechanism for how continental movement would work. It also was a great example of how new evidence from investigations can cause an existing theory to change, which is one of the strengths of science. It led them to a solution for ‘The Mystery of the Drifting Continents.’
New Evidence from Below
In 1948, a scientist was exploring some islands in the Atlantic Ocean, and he found that they actually were the highest points along a submerged mountain range made of surprisingly young volcanic rock. This was a key clue in solving the mystery. This became known as the Mid-Atlantic Ridge.
After World War II, there also was a heightened interest in the mapping of the ocean floor due to the increase in submarines and their use in battle. There was a keen interest in this new field of battle where soldiers now traversed. They sent ships across the ocean to create sonar maps of the ocean floor. Their findings contained more critical clues to help solve ‘The Mystery of the Drifting Continents.’
As they mapped the ocean, they discovered that the Mid-Atlantic Ridge actually ran around the whole earth through the ocean floor like the seams of a baseball. What did this clue mean? They also found that there was a steep valley at the center of the mid-ocean ridge (more questions for investigators).
Deep ocean trenches were found in specific locations on the ocean floor. They wondered if they had anything to do with solving the mystery.
Cracks in the earth’s crust, or lithosphere, as it is also known, were discovered. This was a whole new clue since it was thought that the continental crust (the crust that the continents are made up of) sat on top of the oceanic crust (that’s crust under the ocean), and the surface was all one piece. This clue showed these investigators that both kinds of crust were often different parts of the same piece of crust that they named plates. Plates are giant pieces of lithosphere that all fit together like a big jigsaw puzzle.
Investigation into Clues from the Ocean
As scientists researched the ocean floor more fully, there were several more clues that were found which finally unraveled ‘The Mystery of the Drifting Continents.’
The first was magnetic reversals. As magma cooled to form new lithosphere, the natural iron oxides in the rock act as tiny magnets and align themselves with the magnetic field of the earth. Other scientists knew that meant the magnetic poles of the earth shift, so the location of the pole can be discovered by examining the rock. In the 1950s, scientists started taking measurements of the ocean floor and discovered a pattern of stripes where magnetic orientation switched.
They found this pattern on either side of the mid-ocean ridges. They concluded that the only way for these identical patterns to occur would be if they formed at the same time and moved in opposite directions. This led to the idea of seafloor spreading.
The center of the steep valley on the mid-ocean ridges was extremely hot. Magma was found to be leaking out of the cracks in the ocean floor, pushing aside existing ocean crust and making it wider. This became known as ‘seafloor spreading,’ and this was the first physical evidence that the lithosphere actually moved on the surface of the earth. This was a critical discovery into solving the mystery.
This can be seen in Iceland, where the spreading actually splits the island, making new land in a violent, volcanic way.
As scientists investigated further, they also discovered that all these ideas were confirmed by studying the age of volcanic rocks in the oceanic crust. Rocks nearer to the mid-ocean ridge were much younger than the similar rocks farther away from the ridge.
Further data in the case surrounding the mystery of the moving continents came from accumulated earthquake data. As scientists mapped where earthquakes occurred, they found that they were focused in specific locations and patterns. These patterns happened to coincide with the locations of cracks they discovered, or faults, as they are known, in the lithosphere.
The Mystery Solved
These discoveries also led to the realization that the continents didn’t move through the ocean crust like other scientists thought, but that the continental crust was part of a larger piece of crust called a plate. These geologic investigators discovered that the plates were what was moving, and the continents rode along as part of the whole plate together. After the mystery was solved, this new data was combined together to form a new theory called plate tectonics.
To recap the ‘Mystery of the Drifting Continents,’ scientists had accumulated a lot of data to make the case that the continents were at one time all together in a giant supercontinent called Pangea. As people mapped the ocean floor after World War II, new evidence was discovered that led to a new understanding of how the earth’s crust was put together. This included the discovery of a mid-ocean ridge, underwater volcanoes, magnetic stripes in volcanic rock and seafloor spreading. This led scientists to understand that the crust is a jigsaw puzzle of giant pieces of lithosphere, or crust, that move around, causing the continents to move with them. The only thing left to discover was the cause for the plate movement.
After watching this lesson, you should be able to:
- Define lithosphere, plates and faults
- Summarize the evidence in support of the continental drift theory