Disturbances at the boundaries of the tectonic plates are one of the main causes of volcanic eruptions, although they can be caused by intraplate volcanism as well. Magma chambers deep beneath the surface of the Earth are giant reservoirs of molten rock. Hot gases and molten rock boil within until eventually they push through the surface and erupt out of a volcano, which is formed by the rise of magma and gases. Accumulation of less dense magma and hot gases force the crust upwards, forming the familiar conical shape of a volcano. However, not all volcanoes take on the signature shape of a volcano, as seen with Yellowstone, which is caused by a giant mantle plume beneath, broiling beneath the surface. 
 
At the points where Earth’s plates meet one another there are also extreme pressures being exerted. There are several types of collisions that occur, including divergent, convergent, and transform boundaries. Earth’s plates are either pulling away from one another (East African Rift), colliding into each other, such as the Andes Mountains where the Nazca Plate is subducting under the South American Plate, or sliding past each other in opposing but parallel directions. The Pacific Plate meets the North American Plate at the San Andreas Fault which is a transform strike-slip type boundary. 
 
Hot spots exist such as the Hawaiian Emperor-Seamount chain which stretches 3,600 miles Northwest from the main island of Hawaii, where eruptions have spewed material to form the various islands going Northwest that are visible by satellite. The further Northwest they go, the older they are. There are several circumstances under which volcanic eruptions can occur, but in all the scenarios pressure from rising magma seeking to force itself through Earth’s crust is what triggers volcanic eruptions.  
 

Ash clouds are ejections of dust, ash and sulfuric acid from volcanoes that pollute the upper layers of the atmosphere. If thick enough, ash clouds can substantially reduce the amount of sunlight Earth receives and can result in a volcanic winter in the most severe cases. During a volcanic winter there is a reduction in global temperatures caused by the volcanic ash and droplets of sulfuric acid and water blocking the Sun’s rays. Wind currents drive ash clouds around the Earth into the upper layers of the atmosphere, eventually lowering temperatures far from where the eruption occurs.  
 
An example of the global impacts a volcanic eruption can have is the Year Without a Summer. In 1815, the massive eruption of Mount Tambora in the Dutch East Indies blackened the skies with ash and sulfuric acid. The ash cloud, carried by wind currents, blanketed large parts of the world including Europe, the Americas, and Asia. The cloud of water, sulfuric acid and ash reflected large amounts of the sun’s radiation, reducing temperatures on Earth and having long reaching effects. It caused widespread crop failures in the Northern Hemisphere and a decrease in global temperature of 0.7–1.3 °F. There were reports from around the world of the sunlight not making it through and it being cold in summer, sometimes even snowing. In one place, frost afflicted crops in June which would have destroyed entire harvests, leading to starvation, which Ireland experienced. There were hundreds of thousands of deaths across Europe and the Americas caused by one volcanic event in present day Indonesia. Ash clouds can be detrimental to the delicate balance of the world and the dependence all life has on sunlight reaching Earth. 
