(PT) Volcanoes Lesson

Volcanoes 

A volcano is a weak spot in the crust where molten material, or magma, comes to the surface.

Watch the following presentation for a quick introduction to volcanoes.

 

When studying stars and constellations you learned how ancient people explained and gave meaning to what they saw. This is how the ancient people developed myths and legends that were passed down with the telling of stories. Often these stories served a purpose beyond explanation and included warnings to enforce desired behavior. Events and processes on Earth have myths as well. In Hawaii, many myths are told about Pele, the fire goddess of volcanoes. These myths characterize Pele as the creator and the destroyer of the Hawaiian islands. Pele is said to have lived in the flaming depths of active, erupting volcanoes. According to legend, when Pele becomes angry she stirs the fires of Earth and releases those fires through openings on the mountainside.

New rock forms after a volcanic eruption. The photo here is an example of rock formed by lava called Pele's hair.

This rock was formed when lava sprayed out of the ground, much like water would spray from a fountain. When the rock cooled the lava stretched and hardened into thin strands.

We have learned that air, mass, water, and substances that are less dense will tend to rise.

Think about it! 

If magma is less dense than the material around it, how will the magma most likely react?

Magma

Magma forms in the asthenosphere where all materials are under tremendous pressure.

Magma bodies that cool slowly before they reach Earth's surface are called "Batholiths."

Extremely high temperature and pressure can cause the lower mantle to melt and become liquid rock, or magma. Liquid magma is less dense than the surrounding solid material. Magma flows upward filling in any cracks in the rock found above. The magma will continue to rise until it is stopped by layers of rock or until it reaches the surface. As magma rises to the surface, pressure in the volcano decreases.

A volcano erupts when an opening develops in a weak part of rock on the surface. During an active volcanic eruption, gases that were dissolved in the magma will rush out along with that magma. Remember magma that has reached the surface is now called lava.

View the following video clips below. Take notes as necessary to assist you in the assessment for this topic.

Volcanoes an Overview

Types of Volcanoes

Where are volcanoes located?

Volcanic belts form along the boundaries of Earth's plates.  One major volcanic belt is the Ring of Fire, formed by the many volcanoes that rim the Pacific Ocean.  Volcanoes form along mid-ocean ridges, which mark diverging plate boundaries.  Volcanoes also form along diverging plate boundaries on land.  Many volcanoes form near converging plate boundaries where oceanic plates return to the mantle.

The Ring of Fire

The Mid-Ocean Ridge

The mid-ocean ridge consists of the longest chain of underwater volcanic mountains in the world. It forms at divergent plate boundaries where new oceanic crust forms.

Sea-floor Spreading

Sea-floor spreading happens at the Mid-Oceanic Ridge.

Inside a Volcano

Magma collects in a pocket called a magma chamber beneath a volcano.  The magma moves upward through a pipe, a long tube in the ground that connects the magma chamber to Earth's surface.  Molten rock and gas leave the volcano through an opening called a vent.  Often, there is one central vent at the top of a volcano.  However,  many volcanoes have other vents that open on the volcano's sides.  A lava flow is the area covered by lava as it pours out of a vent.  A crater is a bowl-shaped area that may form at the top of the volcano around the central vent.

The Life of a volcano

It may come as a surprise but Volcanoes have a lifetime , they are not permanent feature on the Earth's surface.

Humans experience life in phases. We are babies, toddlers, pre-teens, teens, adults and then senior adults. Volcanoes experience a liftime in phases too.

Volcanoes are described according to the phase they are in. The three phases are active, dormant, and extinct.

Active volcanoes

An active volcano is the most vigorous kind of volcano. Active volcanoes are erupting or have erupted recently, and are expected to erupt again in the near future. However, volcanic activity during the life of a volcano doesn't last forever. Eventually, the conditions that make a volcano active change and the volcano becomes dormant.

Dormant volcanoes

A dormant volcano is a quiet volcano. "Dormant" means sleeping. Dormant volcanoes are not active now, but may become active again in the future. Most of the volcanoes along the northern Pacific coast of North America are dormant.

Extinct volcanoes

An extinct volcano is at the end of its life and is no longer able to erupt. As the volcano erodes, a core of now-solid magma, called a volcanic neck, may be exposed.

Putting it all together

Most of the world's active volcanoes are located along or near the boundaries between the shifting tectonic plates that make up the earth's surface. These plate-boundary volcanoes include those along the edges of the Pacific Ocean in the "Ring of Fire."

The tectonic plates are constantly shifting. If they collide, one plate may be squeezed down into the earth's mantle (the layer just below the crust). When this happens, rocks in the plate are pushed towards the interior of the earth where they become very hot and rise up as magma. If the pressure continues to increase, this boiling magma will shoot out of its underground chamber through volcano vents and erupt as lava.

Volcanoes are also located where plates are spreading apart. A ridge forms, and as the two plates separate, the mantle rock from below the surface flows up into the empty spaces between the plates. The mantle rock will melt, forming magma. As the magma flows out, it cools, hardening to form new crust. This fills in the gap created by the plates separating. Scientists have discovered this type of activity in the Atlantic Ocean along the Mid-Atlantic Ridge, where Iceland's volcanoes are found.

Hydrothermal Vents and Geysers

A hydrothermal vent is a geyser on the seafloor. It continuously spews super hot, mineral-rich water that helps support a diverse community of organisms. They are generally found at least 7,000 feet (2,134 meters) below the ocean surface in both the Atlantic and the Pacific Oceans. They spew hot water for the same reasons that land-geysers do.

 

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