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.