PCC - Mountain Building - Volcanoes Lesson

 

Mountain Building - Volcanoes

There are several forces that form mountains, and all of them are related in some shape or fashion to plate tectonics. One of the most obvious types of mountain-building occurs through volcanic action, and the type of volcano formed plays a large role in the type of mountain being built.

There are various mineral compositions and temperatures in lava erupting from volcanoes around the world, and these factors can affect how the lava behaves. Aa (pronounced "ah-ah") is more viscous due to its higher silica (SiO2) content and lower temperature (between 1000-1100 degrees Celsius). As it flows along a surface, the outer portion cools and hardens, forming a "skin". However, the inner, liquid mass continues to push forward, causing the crust to crack into pieces (called "clinkers") that are incorporated in the lava flow. Aa is typically very rough in texture because of these characteristics.

The other type of lava is known as pahoehoe (pronounced "pa hoey hoey"). It is hotter than aa, which makes it less viscous. It may also have a lower silica content. As it cools, any cracks that form in it are quickly filled by the liquid lava below, so pahoehoe generally forms smooth, ribbon-like contours across the landscape after it cools.

Cinder-cone volcanoes are formed by explosive eruptions, powered by gases that expand as they rise to the surface from the mantle. The lava is ejected high into the air, where it fragments into small chunks that cool and harden before they fall back to the surface. These rocks accumulate into a cone-shaped structure, generally with a crater at the top. After the majority of the gases have dissipated, lava may then flow over the cinders, solidifying the structure.

Shield volcanoes are formed when lava flows out of a vent with little gas pressure behind it. The fluid lava pours out and quickly flows across the surface, which means that the volcano does not grow in height very quickly. Imagine spilling some water on a surface, freezing it, then pouring some more water on top of it, freezing it, and so on - shield volcanoes spread out horizontally more than they grow vertically. They get their name from the fact that their profile resembles the shape of a warrior's shield laying on the ground.

Composite volcanoes are a combination of the two types of eruptions. They produce cinder cones that grow the mountain vertically, then flow fluid lava over the cinders to cement the shape in place. This is followed by another explosive flow of cinders, which is then cemented in place by more lava. This alternating pattern of cinders and fluid lava allows composite volcanoes to grow to towering heights; Mt. Fuji in Japan, Mt. Vesuvius in Italy, and Mts. Shasta, Hood, St. Helen, and Rainier - all found in the United States - are examples of composite volcanoes.

Watch the video below to learn more:

 

 

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