How do we Classify Volcanoes?

How do Volcanoes Form?

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Volcanoes form in areas where magma has made its way to the Earth's surface.  Magma in the asthenosphere has a lower density than the rocks of the Earth's crust.  This difference in density, combined with its extreme heat makes magma rise upwards pushing into the Earth's crust.   

Sometimes, the magma is able to push and melt its way up all the way through the rocks of the Earth's crust and makes it to the surface.  Here the magma cools forming a mound we call a volcano.

 

Hot Spots

HOT SPOTS ARE SUPER HEATED AREAS OF THE MANTLE.  

These areas are hot enough to melt rock and cause hot magma to rise upwards through the asthenosphere and into the crust.  Eventually the superheated magma on a hotspot burns its way up through the crust and creates a volcano.  

IN MANY CASES, HOT SPOTS OCCUR IN THE OCEAN, AND THE VOLCANO THEY PRODUCE BECOMES AN ISLAND.  OVER TIME CONTINENTAL DRIFT MOVES THE VOLCANIC ISLAND AWAY FROM THE HOT SPOT.  WHEN THIS HAPPENS THE VOLCANO IS CUT OFF FROM THE SUPPLY OF MAGMA SO IT GOES EXTINCT, AND A NEW VOLCANIC ISLAND WILL FORM OVER THE HOT SPOT.

THIS IS HOW MANY ISLAND CHAINS, LIKE HAWAII FORM!

 

Types of Volcanoes

There are 3 major classifications of volcanoes based on their shape and the explosiveness of their eruptions: 

Shield Cone Volcanoes

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SHIELD CONE VOLCANOES FORM AS LOW MOUNTAINS WITH BROAD GENTLY SLOPING SIDES, AND NON EXPLOSIVE ERUPTIONS THAT FORM NEAR HOT SPOTS AND DIVERGING BOUNDARIES.

They get their name from the fact that they look like a shield laid flat on the ground.  They form into this shape because of the way the erupt.  Shield cone eruptions are not explosive.  Instead the lave steadily flows quickly out of the volcano and down the sides.  

They form near diverging boundaries and hot spots.   A famous example is Mt. Kilauea in Hawaii, the longest continuously erupting volcano on Earth.  

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Cinder Cone Volcanoes

CINDER CONE VOLCANOES FORM AS VERY STEEP SIDED MOUNTAINS WITH VERY EXPLOSIVE ERUPTIONS THAT FORM NEAR SUBDUCTION BOUNDARIES.

 The explosiveness of their eruptions is due to all of the gasses like Carbon Dioxide, Sulfur Oxides, and Water Vapor, that are trapped inside of the magma.  When the magma reaches the surface the trapped gasses are suddenly released and they explode.  IN FACT, VOLCANIC ACTIVITY CONTRIBUTED A GREAT DEAL TO THE FORMATION OF OUR ATMOSPHERE.  

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These explosions create huge amounts of ash and unique rocks.  The smallest fragments to come out of a volcano are called ash.  Lapilli are the largest fragments that are ejected by the eruption.  

Blocks are a type of lapilli that are erupted as solid chunks of rock.  If liquid magma is ejected, then cools in midair to become a rock we call it a bomb.

 

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Composite Volcanoes

COMPOSITE VOLCANOES ARE THE LARGEST VOLCANOES.  THEY FORM FROM REPEATED ALTERNATING EXPLOSIVE AND NON-EXPLOSIVE ERUPTIONS, THAT MAKE THEM BOTH WIDE AND HIGH.

 

 

 

Plutonic Activity

As magma burns its way up through the crust some of it cools and hardens to form igneous intrusions.  These intrusions come in several varieties, called plutonic structures. 

 

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Dikes

DIKES ARE PLUTONIC INTRUSIONS THAT CUT PERPENDICULARLY THROUGH THE LAYESR THEY INTRUDE.

 
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Sills

SILLS ARE PLUTONIC INTRUSIONS THAT RUN PARALLEL TO THE SEDIMENTARY ROCK LAYERS THEY INTRUDE. 

 
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Laccolith

LACOLITHS ARE UPWARD FACING DOME LIKE BULGES OF IGNEOUS ROCK.

 
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Conic Neck

CONIC NECKS FORM WHEN MAGMA ON ITS WAY UP THE VOLCANO COOLS AND HARDENS INTO IGNEOUS ROCK.  

AFTER THE VOLCANO GOES EXTINCT THE SIDES OF THE VOLCANO ERODE AWAY LEAVING ONLY THE HARDENED CONIC NECK BEHIND.