Distribution of Oceans and Continents
Have you ever looked at a world map and noticed how the coast of South America seems like it could fit snugly against the coast of Africa? You're not the first! This observation is the starting point for the theory of Continental Drift.
While a Dutch map maker named Abraham Ortelius first suggested this possibility in 1596, it was a German meteorologist, Alfred Wegener, who developed a complete theory in 1912.
Wegener's theory proposed that about 200 million years ago, all the continents were joined together into a single, massive supercontinent. He named this supercontinent PANGAEA, which means "all earth." This landmass was surrounded by a single, vast ocean called PANTHALASSA, meaning "all water."
According to Wegener, Pangaea began to break apart. It first split into two large landmasses:
These two landmasses then continued to break apart and drift over millions of years into the continents we know today.
Wegener didn't just base his theory on the shape of the continents; he gathered several pieces of evidence to support his claims.
The most obvious evidence is the remarkable match between the coastlines of South America and Africa. This fit is not just approximate. In 1964, a scientist named Bullard used a computer program to match the continents not at their current coastlines, but at the edge of their continental shelves (the 1,000-fathom line), and found the fit to be nearly perfect.
Using modern radiometric dating, scientists have found that the belt of ancient rocks from the Brazil coast, dated at 2,000 million years old, perfectly matches a similar belt of rocks in western Africa. This suggests these rock formations were created at the same time and in the same place, before the continents split.
Tillite is a type of sedimentary rock formed from the deposits left by glaciers. Geologists have found ancient tillite from the same glacial period in several different parts of the Southern Hemisphere, including India, Africa, Falkland Island, Madagascar, Antarctica, and Australia. The presence of the same glacial deposits on these now-separate continents strongly suggests they were once a single landmass covered by the same ice sheet.
An interesting puzzle exists on the coast of Ghana in Africa, which has rich deposits of gold. However, the source rocks for this gold are nowhere to be found in Ghana. The source, gold-bearing veins, is actually found in Brazil. This makes perfect sense if the two continents were once joined—the gold would have eroded from the Brazilian plateau and been deposited in what is now Ghana.
Scientists have found fossils of identical plants and animals, which lived on land or in fresh water, on continents that are now separated by vast oceans.
While Wegener provided strong evidence that the continents drifted, his explanation for why they drifted was less convincing to other scientists. He proposed two main forces:
Most scientists at the time considered these forces far too weak to move entire continents, and his theory was largely dismissed for this reason.
For many years, the idea of continental drift was on the fringe of science. However, after World War II, new technologies allowed for detailed studies of the ocean floor, which revealed surprising information and revived interest in the idea that the Earth's surface is dynamic.
In the 1930s, long before the ocean floor was mapped, a scientist named Arthur Holmes proposed a possible mechanism for continental drift. He suggested that the Earth's mantle contains convection currents.
Detailed mapping revealed that the ocean floor is not a flat, featureless plain. It has immense underwater mountain ranges, called mid-oceanic ridges, and extremely deep trenches. Key findings from this research were:
The ocean floor can be divided into three main parts: continental margins, abyssal plains, and mid-oceanic ridges.
This is the transition zone between the continents and the deep ocean floor. It includes the continental shelf, slope, rise, and deep-oceanic trenches. The trenches are particularly important as they are the deepest parts of the oceans.
These are vast, flat plains that lie between the continental margins and the mid-oceanic ridges. They are covered with sediments that have washed off the continents.
This is a massive, interconnected chain of underwater mountains, forming the longest mountain range on Earth. The central part of the ridge, called the crest, has a rift system and is a zone of intense volcanic activity.
When maps of earthquake and volcano locations are studied, a clear pattern emerges.
The new information from ocean floor mapping led a scientist named Hess to propose the theory of sea floor spreading in 1961. This theory explained the observations that Wegener's theory could not.
The key points of sea floor spreading are:
The ideas of continental drift and sea floor spreading were combined in 1967 into the modern, comprehensive theory of Plate Tectonics, developed by McKenzie, Parker, and Morgan.
This theory states that the Earth's outer layer, the lithosphere (which includes the crust and the top part of the mantle), is not one solid piece. Instead, it is broken into massive, irregularly-shaped slabs of rock called tectonic plates. These plates "float" and move horizontally on the hotter, more fluid layer of the mantle below, known as the asthenosphere.
There are seven major plates and several minor ones.
Some important minor plates include the Cocos plate, Nazca plate, Arabian plate, Philippine plate, and Caroline plate.
The most geologically active areas on Earth are the boundaries where these plates meet. There are three types of plate boundaries.
This is where two plates pull away from each other. Magma from the mantle rises to fill the gap, creating new crust. These are also known as spreading sites. [!example] The Mid-Atlantic Ridge is a classic example of a divergent boundary, where the North American and Eurasian plates are moving apart.
This is where two plates collide. The result depends on the types of plates colliding, but crust is always destroyed as one plate dives under the other in a process called subduction.
This is where two plates slide horizontally past each other. Crust is neither created nor destroyed. These boundaries are often marked by large faults.
Scientists can determine the speed of plate movement by studying the magnetic stripes on the ocean floor. The rates vary widely:
The driving force behind plate movement is the convection cells in the mantle, as first proposed by Arthur Holmes. The slow, circular movement of hot, softened rock in the mantle drags the rigid plates on the surface along with it. The heat for this process comes from two main sources: radioactive decay within the Earth and residual heat left over from the planet's formation.
The theory of plate tectonics beautifully explains the geography of India and the formation of the Himalayas.
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