By: Sydney Jensen
Introduction to Volcanoes
A volcano is a weak spot in the crust where molten material (magma) comes to the surface. Magma- a molten mixture of rock-forming substances, gases, and water vapor from the mantle. When magma reaches, it is called lava. Most volcanoes occur along diverging plate boundries such as mid-ocean ridge or in the subduction zones around the edges of oceans. Along the ridge, lava pours out of the cracks in the ocean floor. Very rarely do the volcanoes rise above the ocean surface. Some volcanoes result from hot spots. Hot spots are an area where magma from deep within the mantle melts through the crust like a blow torch. Hot dont result from subduction. Hot spots can gradually form a series of volcanic mountains. Hot spots can also form under the continent. Lava begins as magma in the mantle. There magma forms in the atmosphere, which lies beneath the lisphere. Liquid magma is less dense than surounding solid material. It flows up in any rocks above. It keeps rising untill it reaches the surface or gets trapped beneath layers of rocks.
Different types of volcanoes
There are many different types of volcnoes but I am only going to tell you about the three basic ones. The first one is Shield volcanoes. At some places on Earth's surface, thin layers of lava pour out of a vent and harden on top of previous layers. Such lava flows gradually build a wide, gently sloping mountain called a sheil volcano. Shield volcanoes rising from a hot spot on the ocean floor created the Hawaiian Islands.
Now I'm going to tell you about Cinder Cone Volcanoes. A volcano can also be a cinder cone, a steep, cone-shaped hill or mountain. If a volcano's lava is thick and stiff, it may produce ash, cinders and bombs. These materials pile up around the vent in a steep, cone-shaped pile.
Last I'll tell you about Composite volcanoes. Sometimes, lava flows alternate with explosive eruptions of ash, cinder and bombs. The result is a composite volcano. Composite volcanoes are tall, cone-shaped mountains in which layers of lava alternate with layers of ash.
Island Arcs
Many volcanoes occur on islands, near boundries where two ocean plates collide. The older, denser plate dives under the other plate, creating a deep ocean trench. The lower plate sinks beneath the deep-ocean trench into the atmosphere. There it begins to melt, forming magma. Because it is less dense than the surrounding rock, the magma seeps upward through cracks in the crust. Eventually, the magma breaks through the ocean floor, creating volcanoes. The resulting volcanoes create a string of islands called a island arc. The curve of an island arc echoes the curve of its deep-ocean trench.