How did fossil evidence support the theory of continental drift?

Scientists discovered the fossils of many identical-appearing species on widely separated continents. They had two theories for this. First, they theorized that separate species developed identically across the far-flung continents, a notion that was highly unlikely. The second theory was that the continents had been in contact with each other millions of years ago, and had somehow drifted apart.

For example, fossils found in South America were related to those in Australia and Antarctica. These landmasses were in contact sometime in the past, allowing species to roam freely, die, be buried, and become fossilized across these continents. The fossilized remains in the rock layers of the continents drifted with the landmasses, leading to widely separated but nearly identical fossils.

When did scientists determine the jigsaw puzzle fit of the continents?

The actual connection between the continental fit (the idea that continents fit together like the pieces of a jigsaw puzzle) was first proposed in 1858 by Antonio Snider-Pellegrini (1802-1885), Other scientists mentioned this idea for years afterward, but it was not until 1912 that German meteorologist and geologist Alfred Wegener (1880-1930) expanded the theory, suggesting that the continents at one time formed a supercontinent he called Pangea (or Pangaea). Wegeners theory was not taken seriously until about the 1960s, when scientists believed they had finally worked out a mechanism (plate tectonics) for the movement of the continental plates.

Who discovered seafloor spreading?

Harry Hess (1906-1969), an American geologist and professor of geology at Princeton University, discovered seafloor spreading. Based on material brought up from the ocean floor during a drilling project, he determined that rocks on the ocean floor were younger than those on the continental landmasses. He also discovered rocks on the ocean floor varied in age: there were older rocks farther from the midocean ridges and younger rocks around mid-ocean ridges. Hess proposed that the seafloor was spreading as magma erupted from Earths interior along the oceans mid-ocean ridges. The newly created seafloor slowly spreads away from the ridges, and later sinks back into Earths interior around deep-sea trenches.

What magnetic evidence did scientists use to verify seafloor spreading?

When molten lava is expelled from mid-ocean ridges, it cools, creating new ocean floor. And as the rock cools, specific minerals with magnetic properties line up with the prevailing magnetic field of Earth. This preserves a record of the magnetic field orientation at that particular point in time. Changes in the rocks magnetic field records, called magnetic anomalies, happen when Earths field reverses or when the northern and southern magnetic poles change places usually over hundreds of thousands of years. Scientists still do not know what causes these magnetic reversals, but it may have something to do with the giant convection currents in Earths interior.

The theory of seafloor spreading was confirmed by measuring such magnetic anomalies in rocks on the ocean floor. Scientists discovered a symmetrical, striped pattern of magnetic anomalies on the ocean floor, spreading out on either side of the Mid-Atlantic Ridge. This ridge is a long volcanic mountain range that runs down the Atlantic Ocean seafloor between the continents of North America, Europe, Africa, and South America. The pattern and distribution of these stripes showed that the magnetic fields had reversed many times over millions of years and only could have formed if the seafloor had been spreading apart over those millions of years.

 
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