The atomic radius down a group. Explain why

In general, atomic radius decreases across a period and increases down a group.
Across a period, effective nuclear charge increases as electron shielding remains constant. A higher effective nuclear charge causes greater attractions to the electrons, pulling the electron cloud closer to the nucleus which results in a smaller atomic radius.
Down a group, the number of energy levels (n) increases, so there is a greater distance between the nucleus and the outermost orbital. This results in a larger atomic radius.

That result from subduction of oceanic plates beneath lighter continental plates. most of the earth's volcanoes are located around the pacific ring of fire because that the location of most of the earth's subduction zones.