Category The World Around us

Continental drift is still happening, and the continents will continue to move in the future. They are unlikely to return to the shape of Pangaea, but a map of the world 150 million years from now could look significantly different from today’s.

Many times in Earth’s past the continents have been dispersed across the globe, kept apart by spreading oceans. But eventually oceans begin to close, and far-flung lands are drawn inexorably together. They fuse in crunching collisions, welding themselves into single vast terrains: supercontinents.

Continents are short-lived unions. Stirred by hot currents below, these great continental collages are destined to break up and once again go their separate ways. It’s the planet’s version of a family Christmas. Except rather than return every year, Earth’s Continent boom-and-bust cycles last 500 million years. Lost worlds litter our planet’s past – the ancestral supercontinents of Ur, Kenorland, Nuna, Rhodinia, and Pannotia.

Earth’s most recent grand union was 250 million years ago, when a continental mashup brought Pangea together. The giant landmass survived a mere 50 million years. It was undone by splits that tugged its American margins free from its African centre, broke apart the antipodean lands and then cleaved an Atlantic rift northward to release the conjoined bulk of Europe and Asia.

Neighbouring landmasses set off on different trajectories. India, originally snug with Madagascar, sped northwards to plough into Asia, thrusting ancient seafloor up into Himalayan peaks. The divorce of Australia and Antarctica left one to drift off into drier desert latitudes while the other languished in polar isolation. As these vast crustal rafts drifted across the globe, so landscapes and life adjusted. Each continent has been fashioned by that escape from Pangea.

But the continents are starting to come together again. North Africa is advancing into Mediterranean Europe, and over the next few tens of millions of years its shores will crumple into a chain of snowy peaks. Australia – the fastest-moving continent – is already beginning to sweep up New Guinea and the Indonesian archipelago en route to a messy pile-up with Asia. Pangea is slowly reassembling. Give the planet a couple of hundred millions years and we’ll have another supercontinent. Geologists even have a name for it: Pangea Ultima.

A glance at a modern map of the world makes it easy to see that all the continents were once joined together. Perhaps the clearest example is the east coast of South America and the west coast of Africa. Their shapes suggest that they would fit closely if brought together.

In the beginning, more than 4.6-billion years ago, the world was a ball of burning gas, spinning through space. At first, super-heated gases were able to escape into outer space, but as the Earth cooled, they were held by gravity to form the early atmosphere.

Clouds began to develop as water vapour collected in the air … And then it began to pour with rain, causing the early oceans to rise up.It took hundreds of millions of years for the first land masses to emerge.

About 250-million years ago, long, long after the Earth had formed, all the continents of the time had joined together to form a super-continent called Pangaea.

This super-continent broke up about 200-million years ago to form two giant continents, Gondwana and Laurasia. Gondwana comprised what is now Africa, South America, Australia, Antarctica and India. The Indian sub-continent lay off the east coast of Africa, before it broke off and moved north rapidly.

It collided with Asia, creating one of the world’s greatest mountain ranges, which extends for more than 2,500 kilometres – the Himalayas. By now, our world had started to look like something we would recognise.

The amazing process of plate tectonics, in which the Earth’s land masses move slowly across the Earth’s crust, is still continuing. Far in the future, some scientists have predicted that the present continents will converge again, to form a new supercontinent.

Picture Credit : Google

There are a number of theories about the causes of continental drift. One puts forward the idea that hot rocks rise through ocean ridges, cool down and then drag the plates downwards. Another theory suggests that the heat from inside the Earth creates movement in the mantle. The resulting currents then shift the plates around. The third idea is the simplest. At the ocean ridges, the plates are higher than elsewhere, resulting in the force of gravity pulling the plates downwards.

The Earth is in a constant state of change. Earth’s crust, called the lithosphere, consists of 15 to 20 moving tectonic plates. The plates can be thought of like pieces of a cracked shell that rest on the hot, molten rock of Earth’s mantle and fit snugly against one another. The heat from radioactive processes within the planet’s interior causes the plates to move, sometimes toward and sometimes away from each other. This movement is called plate motion, or tectonic shift.

Our planet looks very different from the way it did 250 million years ago, when there was only one continent, called Pangaea, and one ocean, called Panthalassa. As Earth’s mantle heated and cooled over many millennia, the outer crust broke up and commenced the plate motion that continues today.

The huge continent eventually broke apart, creating new and ever-changing land masses and oceans. Have you ever noticed how the east coast of South America looks like it would fit neatly into the west coast of Africa? That’s because it did, millions of years before tectonic shift separated the two great continents.

Earth’s land masses move toward and away from each other at an average rate of about 0.6 inch a year. That’s about the rate that human toenails grow! Some regions, such as coastal California, move quite fast in geological terms — almost two inches a year — relative to the more stable interior of the continental United States. At the “seams” where tectonic plates come in contact, the crustal rocks may grind violently against each other, causing earthquakes and volcano eruptions. The relatively fast movement of the tectonic plates under California explains the frequent earthquakes that occur there.

Picture Credit : Google

The earth’s crust is divided into enormous slabs of rock called tectonic plates. There are about 15 major plates, covering both the land masses and the ocean floor. They fit together like a huge jigsaw puzzle and, due to continental drift; their boundaries are either colliding with or pulling away from each other.

A tectonic plate (also called lithospheric plate) is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plate size can vary greatly, from a few hundred to thousands of kilometers across; the Pacific and Antarctic Plates are among the largest. Plate thickness also varies greatly, ranging from less than 15 km for young oceanic lithosphere to about 200 km or more for ancient continental lithosphere (for example, the interior parts of North and South America).

How do these massive slabs of solid rock float despite their tremendous weight? The answer lies in the composition of the rocks. Continental crust is composed of granitic rocks which are made up of relatively lightweight minerals such as quartz and feldspar. By contrast, oceanic crust is composed of basaltic rocks, which are much denser and heavier. The variations in plate thickness are nature’s way of partly compensating for the imbalance in the weight and density of the two types of crust. Because continental rocks are much lighter, the crust under the continents is much thicker (as much as 100 km) whereas the crust under the oceans is generally only about 5 km thick. Like icebergs, only the tips of which are visible above water, continents have deep “roots” to support their elevations.

Most of the boundaries between individual plates cannot be seen, because they are hidden beneath the oceans. Yet oceanic plate boundaries can be mapped accurately from outer space by measurements from GEOSAT satellites. Earthquake and volcanic activity is concentrated near these boundaries. Tectonic plates probably developed very early in the Earth’s 4.6-billion-year history, and they have been drifting about on the surface ever since-like slow-moving bumper cars repeatedly clustering together and then separating.

Like many features on the Earth’s surface, plates change over time. Those composed partly or entirely of oceanic lithosphere can sink under another plate, usually a lighter, mostly continental plate, and eventually disappear completely. This process is happening now off the coast of Oregon and Washington. The small Juan de Fuca Plate, a remnant of the formerly much larger oceanic Farallon Plate, will someday be entirely consumed as it continues to sink beneath the North American Plate.

Picture Credit : Google

It may not be apparent to us, but the major land masses of the Earth, the seven continents, are not in fixed positions. They are constantly shifted around by forces deep within the Earth. Around 250 million years ago, the land on Earth was made up of one huge continent known today as Pangaea. Over time, this broke up into the continents we know today. This continual movement of the land is known as continental drift.

Wegener thought all the continents were once joined together in an “Urkontinent” before breaking up and drifting to their current positions. But geologists soundly denounced Wegener’s theory of continental drift after he published the details in a 1915 book called “The Origin of Continents and Oceans.” Part of the opposition was because Wegener didn’t have a good model to explain how the continents moved apart. 

Though most of Wegener’s observations about fossils and rocks were correct, he was outlandishly wrong on a couple of key points. For instance, Wegener thought the continents might have plowed through the ocean crust like icebreakers smashing through ice. 

“There’s an irony that the key objection to continent drift was that there is no mechanism, and plate tectonics was accepted without a mechanism,” to move the continents, said Henry Frankel, an emeritus professor at the University of Missouri-Kansas City and author of the four volume “The Continental Drift Controversy”.

Although Wegener’s “continental drift” theory was discarded, it did introduce the idea of moving continents to geoscience. And decades later, scientists would confirm some of Wegener’s ideas, such as the past existence of a supercontinent joining all the world’s landmasses as one. Pangaea was a supercontinent that formed roughly 200 to 250 million years ago, according to the U.S. Geological Survey (USGS) and was responsible for the fossil and rock clues that led Wegener to his theory.

Picture Credit : Google

Most of the earth is made of various solid rocks. The 2000km- (1240-mile-) thick outer core is the only part of the Earth that exists in an entirely liquid form. Iron, nickel and other materials are liquefied by the extremely high temperatures. Molten rock is found in parts of the mantle, some of which comes to the surface as lava.

The Earth’s interior is composed of four layers, three solid and one liquid—not magma but molten metal, nearly as hot as the surface of the sun.

The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannotmelt.

The iron isn’t pure—scientists believe it contains sulfur and nickel, plus smaller amounts of other elements. Estimates of its temperature vary, but it is probably somewhere between 9,000 and 13,000 degrees Fahrenheit (5,000 and 7,000 degrees Celsius).

Above the inner core is the outer core, a shell of liquid iron. This layer is cooler but still very hot, perhaps 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius). It too is composed mostly of iron, plus substantial amounts of sulfur and nickel. It creates the Earth’s magnetic field and is about 1,400 miles (2,300 kilometers) thick.

The next layer is the mantle. Many people think of this as lava, but it’s actually rock. The rock is so hot, however, that it flows under pressure, like road tar. This creates very slow-moving currents as hot rock rises from the depths and cooler rock descends.

The mantle is about 1,800 miles (2,900 kilometers) thick and appears to be divided into two layers: the upper mantle and the lower mantle. The boundary between the two lies about 465 miles (750 kilometers) beneath the Earth’s surface.

The crust is the outermost layer of the Earth. It is the familiar landscape on which we live: rocks, soil, and seabed. It ranges from about five miles (eight kilometers) thick beneath the oceans to an average of 25 miles (40 kilometers) thick beneath the continents.

Currents within the mantle have broken the crust into blocks, called plates, which slowly move around, colliding to build mountains or rifting apart to form new seafloor.

Continents are composed of relatively light blocks that float high on the mantle, like gigantic, slow-moving icebergs. Seafloor is made of a denser rock called basalt, which presses deeper into the mantle, producing basins that can fill with water.

Except in the crust, the interior of the Earth cannot be studied by drilling holes to take samples. Instead, scientists map the interior by watching how seismic waves from earthquakes are bent, reflected, sped up, or delayed by the various layers.

Picture Credit : Google