Why might a major volcanic eruption lead to cooler temperatures of earth's surface?

When a volcano erupts it releases a large amount of dust, smog, rock, and other materials. Basically, these bits and pieces float to form a metaphorical dust cloud. These clouds can often block out the sun like an ordinary cloud; however, because the cloud is dark, even less sun will be able to get through. So because the sun can't reach the ground, the temperature might decrease.