When plate boundaries converge, volcanoes are a common result. Volcanoes are large, mountain like, structures that spew lava and volcanic rock when they erupt. The spew of volcanic rock and lava is known as tephra. Tephra is broken up into different classifications, according to size. The biggest pieces, being volcanic bombs. Volcanic bombs range in size from 64mm in diameter and up.  Next comes lapilli, ranging from 2 to 64 mm. The smallest form of tephra is known as ash, and is less than 2 mm in diameter. 

   Along with 'solid' objects, volcanoes also release gasses. The most common gasses released from volcanoes include carbon dioxide, hydrogen chloride, nitrogen, and water vapor. Volcanoes release so much water vapor that, most of Earth's water has probably been released from a volcano sometime in the geological past. Some volcanoes emit a gas called sulfur dioxide, which combines with water vapor and oxygen to form sulfuric acid. This acid is then washed out of the atmosphere by rain. Rain that contains sulfuric acid is known as acid rain. Another hazard of volcanoes are lava flows. Lava flows are streams of molten lava that flow down the slope of a volcano. Lava flows destroy most everything in their paths. ** 
   As you can probably tell, volcanoes can be extremely hazardous, and many have caused mass devastation. One major hazard of volcanoes is volcanic ash released when volcanoes erupt. After an eruption, ash will cover anything in sight. In many cases, it lands on roof tops of buildings and houses, potentially causing the roofs to collapse. Ash will also cover plants, blocking the process of photosynthesis. If photosynthesis does not occur, the plants will eventually die off. Volcanic ash will cause breathing problems for lifeforms on Earth too. Also, if the ash is spewed high enough, it can damage aircraft, endangering those on board. Hazards of volcanic ash also include the formation of lahars (yet another hazardous effect of volcanoes). If ash lands on the slope of a volcano rain can mold the loose ash into lahars, which are wet mixtures of water, mud, ad volcanic rock fragments. Upon formation, lahars will slowly move downhill, threatening to cover entire cities with several meters of mud.

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