Insight into Moving Continents
The ground beneath our feet is not as staedy as we may think. In fact,the continents that make up most of the Earth’s land surface are always on the move, shifted around by forces deep inside the Earth.
This movement is known as continental drift. It takes place because the inside of the planet is hot and turbulent. The intense heat generated at the Earth’s core is carried upward where it disturbs the cool, rocky surface. This forces the plates of crust that make up the continents, called tectonic plates, to move.
Each year the continents drift by nearly half an inch (about a centimeter). Some are crunching together, some are splitting apart,
others are grinding past each other. As this happens the Earth’s features are created or changed. Violent earthquakes and volcanoes are dramatic reminders that the plates never stop moving.
Each tectonic plate has a lower layer of solid rock and an upper layer called the crust. The plates ride upon Earth’s slowly moving, mostly solid mantle. Where the crust is thin, the Earth’s surface is low-lying and covered by seas and oceans. continents form where the crust is thicker and stands higher.
As the tectonic plates move, the continents are carried with them and the oceans change shape.
GLOBAL JIGSAW PUZZLE
The plates that form the Earth’s surface fit together like a jigsaw puzzle.This map shows the boundaries of the Earth’s plates and the directions in which the plates are drifting. The pieces slowly change shape as they move. Great mountain ranges have formed along the blue zones where plates are colliding. Lines of volcanoes are dotted along the red zones where one plate is sinking (subducting) below another, causing molten rock to erupt to the surface.
When the German scientist Alfred Wegener stated in 1915 that today’s continents were once part of a single landmass, people
ridiculed him. But Wegener was right. He argued that although ancient plant fossils, such as the Glossopteris fern (right) are found on widely separated continents, they could only have come from one original continent. Today, geologists agree with Wegener that the continents did indeed drift apart.
WHEN PLATES COLLIDE
The Andes Mountains of South America extend along the Pacific coast for about 5,530 miles (8,900 km). They began to form about 170 million years ago when the Nazca Plate collided with (and sank beneath) the South American plate. The foothills (above) show where a folding, or buckling, of the continental crust has occurred. Mountain-building in the Andes slowed down about six million years ago.
The illustration above shows what happens at the boundaries that separate one plate from another. At spreading boundaries, plates are moving apart, and molten rock (magma) rises up to fill the gaps. Transform faults lie along boundaries where plates scrape past one another,generating earthquakes. Where convergent boundaries are found, plates are pushing together to create mountain ranges in a process of folding and uplifting.
WEST OF JAVA
This is Anak Krakatoa in Indonesia, a volcano that first erupted in 1927. It is one of a long string of volcanoes that lies along a boundary where the Indo-Australian plate is sliding below the Eurasian plate. The subsiding plate melts as it is forced downward
into the Earth’s mantle, and squeezes magma to the surface to form volcanoes.
The Mid-Atlantic Ridge is a spreading plate boundary that stretches from the Arctic to the Southern Ocean. Most of it lies beneath the ocean, but at Thingvellir in Iceland (left), it crosses over land. The boundary between the North American plate on the left and the Eurasian plate on the right is clearly visible.
Where the plates have moved apart, the crust in between has collapsed, forming a steep-sided rift valley. The region is very active volcanically.In 1963, a huge underwater eruption occurred 80 miles (130 km) south of Thingvellir. Lava rose to fill the gap in the widening ridge, and cooled to form the new island of Surtsey.