Volcanoes
A volcano is a cone shaped mountain that spews magma, solids, and gas out of the Earth.
Molten rock material or magma is forced upward by denser surrounding rock. A volcano is an eruption where magma, solids, and gases are spewed out of the Earth forming cone shaped mountains. Magma flowing onto the Earth's surface through a vent or opening is called lava.
Volcanoes have circular holes near their summits (peaks) called craters. Some eruptions throw rock & lava 1000s of meters into the air. Bits of rock and solidified lava dropped from the air is called tephra. Tephra varies in size from volcanic ash to cinders to larger rocks called bombs or blocks. Some volcanoes form because of the collision of plates and some form from plates pulling apart.
Pyroclastic Flow – massive avalanche of hot, glowing, rock flowing on a cushion of intensely hot gases.
Volcanoes add new rock to the Earth’s crust. Some eruptions are violent, while others flow quietly and slowly. Lava with silica is thicker and flows slower (high viscosity) than lava with iron and magnesium, which flows more easily (lower viscosity). Viscosity is a liquids resistance to flow. The slower the liquid flows the higher its viscosity. (Water has a much lower viscosity than honey). The amount of water vapor and gases trapped in the lava also influence how lava erupts. Steam builds pressure in rising magma (like shaking a soda bottle) the pressure is released as magma rises to the surface and eventually erupts.
The type of lava and the gases trapped in the lava determine the type of eruption that occurs.
Types of volcanoes
Shield Volcanoes – lava low in silica ( flows quicker) flows in broad flat layers. The buildup of basaltic layers forms a broad (wide) volcano with gently sloping sides called a shield. These are the largest volcanoes and they form at divergent boundaries (plates moving apart)
Cinder Cone Volcanoes – moderate to violent eruptions throw volcanic ash, cinders, and lava high into the air. The lava cools quickly in midair and the particles of solidified lava, ash, and cinders fall back to Earth (tephra). This tephra forms a small cone of material called a cinder cone volcano. They are usually less than 300 meters (m) high and they often form in groups near other larger volcanoes. We know that eruptions are powered by high gas content. When the gas content decreases eruption stops. Eruptions at these volcanoes don’t usually last long.
Composite Volcanoes – Steep sided mountains composed of alternating layers of lava and tephra are composite volcanoes. They sometimes erupt violently releasing a large amount of ash and gas (which falls back onto the volcano creating a tephra layer); other times their eruptions are quieter which creates a lava layer on the mountain. Composite volcanoes form at subduction zones (where one plate sinks beneath another).
Shield Volcano
Cinder Cone
Composite
These videos show the victims (humans and animals) of the volcano, Mount Vesuvius at Pompeii, a thriving city at the time. It shows how people probably lived at the time, their social lives, and what the city probably looked like. The eruption happened almost 2000 years ago, yet these bodies were entombed by the hot ash and pyroclastic flow. Their bodies were preserved at the time of their deaths. The bodies in this video are in a museum today. WARNING: This is sad and can be hard to look at but this is history and it does show the severity of living near a volcano Pompeii Video from the History Channel