The first two photographs below are part of a Pravda photoset from a joint US-South Korea exercise :

South Korean and U.S. forces conducted a joint drill Wednesday, the first South Korea-led exercise as the allies prepare for the planned transfer of Seoul’s wartime operational command. More than 800 troops from South Korea’s tank battalion and two U.S. squadrons engaged in mobile training and a free-fighting training at the Mugeonri military training field, north of Seoul.

Having seen the same explosive effects before in military drills (see third image below), I am wondering just what exactly causes those torus shaped explosions and what purpose they are used for (other than providing aesthetically pleasing news photographs of course).

Any military or explosives experts out there who care to comment?