Ash and volcanic rocks from the Mount Merapi volcano cover deserted houses in Central Java.

Tarko Sudiarno/AFP/­Getty Images

Is that ash falling from the sky?

When volcanic ash descends back to the Earth, the effects can be mild or devastating. It all depends on how much ash was created by the eruption and your distance from the volcano. Ash may dust an area for as little as 30 minutes or fall for days, blanketing everything with several million tons of heavy powder. The experience can be disorienting, frightening and even deadly. ­

In large quantities, volcanic ash poses a significant threat to the environment. If you were to cover a section of your lawn with a tarp, you'd eventually kill the underlying plants. Likewise, a heavy layer of volcanic ash can deprive plants of sunlight, oxygen and necessary interactions with other organisms. It can even kill microorganisms living in the soil. With large trees, the added weight of volcanic ash can break limbs in the same way an ice storm does. In addition, volcanic ash often carries with it potentially poisonous chemicals from the eruption. High acidity levels in the ash can change soil composition, making it impossible for some species of plants to survive.

­The picture isn't much better for animals. Volcanic ash weighs down and immobilizes insects that have pollen-grabbing hairs on their bodies. Larger animals are susceptible to skin and eye irritation. If ash particles are less than 10 microns in diameter, they're also respirable. This means you can inhale them, resulting in a variety of short-term respiratory problems. Poisonous chemicals in the ash, such as fluoride, frequently pose a threat to livestock, coating not only the animal, but also their food and water supplies.

Massive quantities of volcanic ash inflict a lot of immediate damage, but in the long term, they can greatly enrich soil and ocean beds. This process may take weeks, months or even thousands of years, depending on the exact composition of the ash. But eventually, these particles of exploded magma work their way into our habitats, providing plant life with crucial organic carbon and nitrogen. Volcanic soils also hold a great deal of water, allowing for better-irrigated vegetation.

Large deposits of ash and other pyroclastic material may also eventually form into solid rock. Layers of hot ash often fuse into sheets of welded tuff. Much of the rest of it gradually ­becomes a part of our landscape. As such, the results of volcanic ash fall are all around us.

Meanwhile, inventive humans have found their own uses for the ash, incorporating it into ceramics, building materials, industrial abrasives and even toothpaste.

­Explore the links on the next page to learn more about destructive and live-giving properties of volcanic eruptions.