What is the oxidation number of cl in chlorate ion clo3-

You need the oxidation number for Cl, so we will use only the ClO3 1-. 

Oxygen is not in a peroxide. It is in "chlorate" so the oxidation number of oxygen is 2-. There are 3 oxygens in the ion. 2- x 3 = 6- total. The chlorate ion has a 1- charge so there are 5- charges to be balanced out by positive charges. That means that the Cl (the only other atom there) has to balance out the 5- left over by having a 5 + oxidation number. 

(Cl 5-) + (O3 6-) = ClO3 1-

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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.