Friday, August 15, 2008

Why do Volcanoes appear only in certain places?


There are no volcanoes near New York City or London or Paris- nor are there likely to be any in the future. Yet there are parts of the world where there are several volcanoes quite near each other.

Central America, bordering the Pacific Ocean, is one of the most active volcano areas of the world. In fact, more than two-thrids of the active volcanoes and a lrage numer of those which have been extinct for only a short time are found around the borders of the Pacific Ocean!

The reason is simply this: The earth's crust in these areas must be "weak" or have weak spots in realtion to the earth's crust in other parts of the world. For without a weak spot in the crust of the earth, a volcano couldn't come into being.

Here is how a volcano is born: As you know, the center of the earth is hot. The deeper you go under the surface of the earth, the higher the temperature. At a depth of about 20 miles, it is so hot (1,000 to 1,100 degrees centigrade) that most rocks found there simply melts.

When rock melts, it expands and needs more space. In certain areas of the world, new mountain ranges have recently been formed (new in terms of thousands of years). Under and near these new mountain ranges, the pressure in less than elsewhere. It is a kind of "weak spot" in the earth's solid crust.

So the molten rock, which is called "magma", expands into these parts and a local reservoir of the molten rock is formed. This material rises along the cracks formed by the uplift. When the pressure in the reservoir of molten rock becomes greater than the strengthof the roof over it, it burst forth as a volcano. The eruption lasts until the gas is gone.

The material that comes out of volcano is mainly gaseous, but large quantities of molten rock (which we call "lava") and solid particles that look like cinders and ash are also thrown out. The eruption is really a gas explosion, but some of the lava becomes finely p[owdered and makes the eruption look like black smoke.

Sunday, August 10, 2008

Why do we still have Glaciers today?

The great ice mass that began the Ice Age in North America has been called "a continental glacier" it may have been about 4,500 metres thick in its center. This great glacier probably formed and then melted away at least four times during the Ice Age.
The Ice Age or glacial that took place in other parts of the world still had not had a chance to melt away! For instance, the big island of Greenland is still covered with a continental glacier, expect for a narrow fringe around its edge. In the interior, this glacier often reaches heights of more than 3,000 metres. Antarctica is also covered by a vast contiental glacier which is nearby 4,000 metres high in places!
So the reason we still have glaciers in certain parts of the world is that they have not had a chance a melt away since the Ice Age. But most of the glacier that exist today have been formed in recent times. These glaciers are usually the valley type of glacier.
It starts in a broad, steep-walled valley shaped like a great amphitheatre. Snow is blown into this area or slides in from avalanches from the slopes above. This snow doesn't melt during the summer but gets deeper year by year. Eventually, the increasing pressure from above, together with some melting and refreezing, forces the air out of the lower part of the mass and changes it into solid ice. Further pressure from the weight of ice and snow above eventually squeezes this mass of ice is the valley glacier.
There are more than 1,200 such glaciers in the Alps of Europe! Galciers are also found in the Pyrenees, Carpathian, and Caucasus Mountains of Europe, and in southern Asia. In Southern Alaska, there are tens of thousands of such glaciers, some from 25 to 50 miles long!