Volcanoes in our Community Where are the volcanoes and what do they look like? Science Std: Geo # 3f

As we have covered before……  Volcanoes require some special plate conditions to be in place before they can form  Convergent Boundary  Divergent Boundary  Hot Spots  Forms on the upper plate above the convergence  Rift valleys and mid ocean ridges have volcanoes  Localized area in the middle of plates where volcanoes can form

So let’s take a look at some places where volcanoes can form……. Under the Sea ….. There are actually more active volcanoes under the ocean than are on the land above the water Why? All of the ocean basins have a mid-ocean ridge divergent area that is constantly expanding

But we can also have them on land  Volcanoes that erupt on land are much more dangerous than those that erupt underwater  They form volcanic mountain ranges  They form island chains  More chances to hurt people and property  Cascade Mountain range in Washington  The Aleutian Island chain in Alaska  And of course there is always……….

The “Ring Of Fire”

But we can also have them on land  Did you know that……  Why…..?  4/5 of all the world’s volcanoes on land happen where one plate slides beneath another plate  Plate is heated as it is subducted  The heating forces water out of the rocks  This forces the magma to rise up due to lower density  And feed volcanoes on the plates overhead

But we can also have them on land  Where else?  Rift Valleys at divergent plates

But we can also have them on land  Where else? Mount Kilimanjaro is a good example of a volcano formed at a rift valley  Rift Valleys at divergent plates

But we can also have them on land  Where else?  Hot spots The Hawaiian Island chain is a great example of several volcanoes that came from a hot spot

We can look at these volcanoes on maps..

Let’s zoom in on an area of the map…. Maps like this can give you information about where the volcanoes have happened in the past

When a volcano erupts……  It forms one of 2 main types of volcanic cones A Composite Cone A Shield Cone And the type of cone depends on the type of lava that the volcano makes

Types of lava?????……  The type of lava depends on what the magma is made of  Magma is a mixture of:  Liquids (water)  Melted Rock  Dissolved Gasses  Depending on the mixture, the lava behaves differently

Types of lava?????……  The type of lava depends on what the magma is made of  Magma is mostly silicon and oxygen  1 Silicon atom combines with 4 oxygen atoms to make an “silicate ion”  These ions combine with other ions to make most of the minerals in rocks today

Types of lava?????……  The type of lava depends on what the magma is made of  The quantity (%) of silica in magma and affects how easily magma flows  We will look at 3 different silica amounts….. Low (50%) Med (60%) High (70%)

Low Silica Magma…….  Has a low viscosity (flows easily)  Easily forms lava on the surface  Doesn’t erupt with a big bang Low (50%) Med (60%) High (70%)  Oozes up for its eruption  Has to be very hot to melt  Makes a lot of lava  Erupts from a single vent  So, all of these things put together make a….

A Shield Cone The magma oozes out from the top, easily flows and spreads out making a large, flat, gently sloping volcano It is called a shield volcano because it looks like a broad shield that used to be carried by ancient warriors

Medium Silica Magma…….  Has a medium viscosity  Has a harder time forming lava  Sometimes erupts with a big bang Med (60%)  Spurts for its eruption  Isn’t as hot as low silica magma  Makes less lava  Can come from more than one vent  Makes a taller volcano than low silica magma

High Silica Magma…….  Has a high viscosity (hard to flow)  Has a hardest time forming lava that flows  Can easily erupt with a big bang Med (60%)  Squirts like toothpaste for its eruption  Can easily trap gasses in/below the magma flow  Makes the least amount of lava  Can blow out anywhere on an existing volcano  Both the Medium and the High Silica Magma have violent eruptions and can form……. High (70%)

A Composite Volcano The thick, stiff lava piles up and forms the tall volcanoes that we are used to see when we think of a volcano These volcanoes have deep domes and steep sides that can be easily seen on a topographic map

A Composite Volcano The thick, stiff lava piles up and forms the tall volcanoes that we are used to see when we think of a volcano These volcanoes have deep domes and steep sides that can be easily seen on a topographic map When the trapped gasses break free, they can be explosive in a violent eruption that can blast pieces of lava and rock (pycroclastics) into the air

A Composite Volcano The older a composite volcano is, the taller it gets The oldest lava is on the bottom The newer lava piles up on top

A Composite Volcano When the trapped gasses break free, they can be explosive in a violent eruption that can blast pieces of lava and rock (pycroclastics) into the air A composite cone forms after many eruptions. After each initial blast, gooey lava oozes out of the top building up the volcano. After each eruption, the volcano goes quiet until the pressure builds up again and then……

A Composite Volcano and then……

When a large eruption happens The edge of the old cone may fall in making a big crater called a Caldera The caldera may be bigger than the original vent that caused it Original Vent

With an eruption, you can get…. Lava Lava is the molten rock that comes out of a volcano It is magma when it is still in the ground, lava when it comes out of the ground Lava Flow Lava flow is the river of lava that comes out of the volcano It is also what you call the hard rock that is made by the lava flow once it cools off The way the lava flows depends on a couple of different things………

The lava will flow…. Slowly if: It has a high amount of silica in it If it is cooled by rain or other things Faster if: It has a low amount of silica in it Nothing cools it off It is traveling down a steep slope If it is traveling on a level ground or a small slope

The lava will flow…. If the lava starts to crust over It can block the path of the lava for a time It can also form a “lava pipe” or “lava tube” that can carry the lava If the end or the side of the tube breaks open, the lava can come out and continue to flow over the ground at 10 km/hr (7 mph) When a lava tube breaks open, it looks something like this….. While it is traveling in the tube, it can go 45 km/hr (30 mph)

Once it starts flowing…… There isn’t much that can stop it The largest lava flow in history was in Iceland in 1783 and it covered an area equal to 100,000 soccer fields

Other types of flows include…. A Pyroclastic Flow This is a thick mixture of hot ash, rocks and gasses that are thrown out of a volcano with an explosive eruption A pyroclastic flow covered Pompeii in the early 1 st century in a matter of minutes They can move at speeds up to 350 km/hr And it destroys just about everything in its path

Other types of flows include…. A Lahar Is a mixture of water, mud, and volcanic rock fragments It looks and acts like wet concrete Heat from the volcano can melt snow and ice and cause lahars to flow carrying boulders and other large objects Volcanic material can also dam rivers which can break free and flow down hill in a rapid “flash flood”