Volcanoes in our Community Volcanic Landforms and How they are made Science Std: Geo # 3f.

Volcanoes in our Community

Volcanic Landforms and How they are made

Science Std: Geo # 3f

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 mapWhen 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

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

OriginalVent

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 1st

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 LaharIs a mixture of water, mud, and volcanic rock fragments

It looks and acts like wet concreteHeat 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”