Question

what causes an eletric spark?

Answer 1

There can be many causes for electric spark please specify your frame of reference but one major cause is because of high density of Electric field over a specified area in a specified medium say air(it's about 30000 kV/cm^2)..The voltage at which a medium(sometimes referred to as dielectric)...It's called dielectric breakdown voltage

@noluthando

Answer 2

Adjax is absolutely correct... to put in simple words :

When charges collect at places so we can assume that there is a "field" created by them around themselves where effect of charge can be observed "well".
*the strength of this field decreases as the distance from charge increases.
when charges keep on collecting at some place and we have some substance surrounding the object with that lot-of-charge so the "field" (electric field) becomes so string that it can force electrons to leave the atom and convert them to ions.
when these ions regain electrons so they release energy and we see glow as sparks.

We can also see sparks when we take a pointed object near a charged one because when objects are pointed some charge that is "induced" on them can create denser field and the same process happens again and we see sparks.