The outer layer of the Earth is divided into different parts, called tectonic plates. For instance, North America and some of the ocean all the way to the center of the Atlantic ocean is part of the North American plate. These plates ride on another layer of the Earth, the asthenosphere, which is much weaker and allows the plates to move around because it is so much weaker than the plates are.

Volcanoes usually form at the edge of plates. Eruptions can happen where plates move apart, like in Tanzania. One tectonic plate moving bellow another can cause volcanic eruptions when plates move towards each other. Less often, hot blobs of material from deeper in the Earth can rise, like in a lava lamp, and hit the middle of a tectonic plate, which happens in places like Yellowstone.

So magma and volcanoes form at the edges of tectonic plates, and areas in the center aren't in as much danger.

Underneath the top layer of the earth, the crust, there are tectonic plates. These plates move just under the crust, and that is what moves the continents. All the action happens along the edges of the plates -- when plates converge (collision between two or more plates moving towards each other), diverge (two plates separate as they move away from each other) or slide past each other (transform boundary). Volcanoes (and earthquakes) are most common at the edge of tectonic plates, because there is so much happening. In the middle of a tectonic plate, which we call intraplate areas, there is usually minimal volcanic activity.

Examples of intraplate areas include in the middle of the North American continent, the middle of the South American continent, and the middle of the African continent.

The farther you are from a hotspot or plate boundary, there is less risk of volcanoes. I am not aware of any volcanoes on the eastern coast of the United States, for example, because it is far from any plate boundary.