The eruption of a volcano such as Indonesia's Mount Merapi can unleash powerful forces. Some of the dangers posed by a volcanic eruption:

PYROCLASTIC FLOWS: These clouds of volcanic ash, gas and debris are the biggest threat to those living on the slopes of Mount Merapi. At the bottom of the flow is a layer of coarse rock fragments that bursts forth like a shotgun blast. It hugs the ground, and can splinter forests like toothpicks and destroy buildings. The top layer is a turbulent ash cloud. A pyroclastic cloud is fast, moving at 50 to 100 mph. And it's hot — up to 1,500 degrees. In 1980, Mount St. Helens in Oregon directed a blast of hot material that reached 300 mph.

MAGMA: The boiling magma rumbling and rising to the surface of a volcano and forming lava flows contains dissolved carbon dioxide and other gases. Fresh, foaming magma contains more gas than old magma. If the gas content is high, the explosivity is greater.

ASH: The most likely and widespread danger from any volcano is ash. Initially, ash is blasted 60,000 feet into the atmosphere and winds can carry it for hundreds of miles. The 1883 eruption of Indonesia's Krakatoa volcano, one of the most destructive in history, killed 36,000 people and covered Java with ash. Volcanic ash is not the product of combustion like ash from a wood stove. Gritty and abrasive, it is made up of tiny fragments of rock, natural glass and minerals that get pulverized by earthquakes and internal explosions. The plume can choke engines of passing aircraft and disrupt telecommunications signals. Ash clouds can create gigantic lightning storms, and settled ash clings to itself, forming a concrete-like layer called tuff.

TEPHRA: Tephra is a catchall term for fragments of volcanic rock and lava thrown airborne. Some tephra is nearly four feet wide, but most of it is gravel that acts like shrapnel and shreds whatever is in its path.

LAHAR: Lahar is a catastrophic slurry of water and rock fragments that rushes down the volcano's slopes. It looks like wet concrete and can carry house-sized boulders, trees, even bridges. It follows river valleys, often growing and gaining speed as it consumes the water in the channel.