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Because all of the silica tetrahedrons have the same charge, they mutually repel each other.  With a high number of silica tetrahedrons, the repulsive forces prevent the melt from being pushed together and flow.

A melt rich in silica electrically and mechanically exclude other ions.  These excluded ions (notably iron and calcium) collect along the sides of magma’s passageway and constrict the magma’s flow.

The silica tetrahedrons are much larger than the other ions found in the magma melt.  These large ions impede the movement of the melt.

Silica tetrahedrons tend to form bonds very easily.  The bond formation creates more complex crystal structures which prevents the magma from flowing easily.

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Temperature has no affect on the viscosity.

The viscosity goes up as the temperature goes up.

The viscosity goes down as the temerature increases.

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carbon monoxide

carbon dioxide

water (steam)

sulfur dioxide

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The higher the percentage of volatiles, the more explosive the eruption is likely to be.

A high gas content means that there has been a great deal of magmatic separation of the molten mass.  The resulting volcanic eruption then will be very quiet (non explosive).

The higher the percentage of volatiles, the lower the viscosity of the magma.  This results in very quiet lava flows instead of explosions.

Once exposed to atmospheric oxygen the volatiles combust.  This creates the explosions associated with most eruptions.

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They have been situated over an active hot spot for hundreds of millions of years that has continued to feed the volcano.

They are created by very fluid lava flows that persist for many centuries.

The largest of the Earthly shield cones are found in the oceans.  Without interference from terrestrial landforms (like mountains) these shield cones can continue to grow in size.

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Earth

Jupiter

Mars

Venus

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They are the result of powerful explosions that blast huge holes along the surface of the volcano.

They are the result of rock melting near the volcano’s summit.  The melted rock flows out onto the surface creating the large depression.

Most calderas are the result of a process called “degassing”.  The rocks and molten material near the top of the volcano simply emit gasses.  This reduces the pore pressures in the rock allowing the rocks to collapse to form the caldera.

They are collapse features created when magma moves out away from an area.  The resulting reduction in pressure causes a collapse of the rocks at the surface.

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Shield cone

Domestic cone

Composite cone

Cinder cone

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Cinder cone

Composite cone

Strato cone

Domestic cone

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Composite cone

Cinder cone

Shield cone

Domestic cone

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Cinder cone

Singular cone

Shield cone

Composite cone

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Composite cone

Polynesian cone

Shield cone

Cinder cone

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Shield cone

Composite cone

Cinder cone

Fissure cone

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Composite cone

Crater cone

Shield cone

Singular cone

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Composite cone

Shield cone

Domestic cone

Cinder cone

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Cinder cone

Rift cone

Domestic cone

Shield cone

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Calderas are found only at the summit of the volcano, craters can be found anywhere on the volcano.

Calderas are formed when lava flows into fissures along the flanks of an active volcano and craters are areas where lava flows outwards.

Calderas are associated with very silica rich volcanoes while the crater is produced by more basaltic volcanoes.

The only difference is size.  calderas are generally larger that craters.  Calderas are generally formed by collapse and most craters are produced by explosions.

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The threat is basically limited to the tourism of the area.  An eruption could disrupt the groundwater structure and cause many of the popular thermal features to shut down or become erratic.

An eruption would disrupt the Earth’s balance of atmospheric gases and reduce the ozone levels significantly.

It could erupt and cover much of the western US with volcanic dust and ash.

An eruption of the caldera would produce prodigious amounts of ozone that would greatly influence the Earth’s climate.

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melting glacial ice mixed with the magma to produce the initial explosion, but also cooled the magma so later eruptions were limited

crystallization and cooling of the magma

A magma surge forced the initial violent eruption.  That surge depleted the magma reservoir beneath the surface that resulted in weaker eruptions.

differentiation of the magma body below

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The ring eruption was created by the collapse of the summit to form the caldera that is now occupied by Crater Lake.

The ring eruption blasted the summit of the volcano leaving the depression now occupied by crater lake.

The ring eruption was a series of eruptions around the summit of Mt Mazama.  The buildup of volcanic rocks from these eruptions created a central basin that is now the area of Crater Lake.

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The volcanic rocks around the lake consist of very durable volcanic glass and do not contribute sediment to make the lake muddy.

The lake received its water only from clean groundwater springs.

The water is the result of the cooling of the magma body.  Water escapes the cooling magma and accumulates in the caldera.  Although the lake is very clear, it is also very acidic.

The drainage is, for the most part, away from the lake.  Streams flow down the outer flanks of the volcano instead of into the lake.

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The only difference between the two types of volcanoes is the size.

The lava of the cinder cones is full of volatiles (gases)

The lava of the cinder cones has a great deal more silica.

The lava of the shield cones is much cooler than that of the cinder cones.

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The cinders are used in the manufacture of glass.

They often contain valuable minerals like peridot and sapphires.

They are a source of cinders for construction.

They are a source of geothermal energy.

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It is very hot.

It moves down the flank of the volcano very rapidly.

It contains no oxygen

All of the above.

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obsidian and basalt

granite and a little diorite

any of these choices

volcanic tuff or simply ash and cinders

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The basalt came from a very active downward plunge of the Juan de Fuca Plate off the west coast of North America.

Most likely the basalt was erupted from a series of fissures situated over a mantle plume.

The flows were the result of an asteroid impact.  The heat of the impact created an abundance of molten rock.

A supervolcano in eastern Washington created the flows.

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Dike

Sill

Laccolith

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Sill

Batholith

Stock

Volcanic Pipe or Neck

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Sill

Dike

Stock

Batholith

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Dike 

Laccolith

Batholith

Sill

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Volcanic neck (or pipe)

Dike

Stock

Laccolith

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batholith

monolith

laccolith

pacholith

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The release of volatiles from the lava or magma.

The alignment of platy minerals.

Cooling of the rock and the resulting contraction.

Crystallization forms the huge columns.

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The sill will have a baked layer above and below it while the lava flow will bake only the layer below it.

Lava flows will not develop columnar jointing while sills will.

Lava flows consist of a more solid type of basalt while the sills may contain an abundance of gas pockets.

Sills are always horizontal while lava flows can form at many different angles.

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The pegmatites are zones of weakness as they formed along cracks in the granite.

The problem is only cosmetic.  Close-up views of the sculpted faces show the pegmatite veins and many visitors consider them unattractive.

The pegmatites are avenues for toxic volatiles associated with the parent magma body.  As a result, they pose a risk to park visitors.

They are of economic importance because of the valuable minerals they contain.  The mountain and its pegmatites are threatened by mining companies.