Unit 4 Chapter 13

Volcanism

There are approximately 485 active volcanoes in the world.

Volcanic eruptions are a major source of some of the most dramatic changes to the Earth’s surface. Some eruptions are more powerful than an atomic bomb.

By studying temperature within the earth, scientists can learn more about the volcanic eruptions.

Formation of MagmaFormation of Magma

Magma is molten rock under the earth’s Magma is molten rock under the earth’s surfacesurface

It can form under 3 conditions:It can form under 3 conditions:1.Temperature of the rock rises above the melting 1.Temperature of the rock rises above the melting point point of the material.of the material.2.If enough pressure is removed from the rock, the 2.If enough pressure is removed from the rock, the

melting point will decrease and the rock will melting point will decrease and the rock will melt.melt.3.Addition of fluids such as water may decrease the 3.Addition of fluids such as water may decrease the

melting point of some rock.melting point of some rock.

Volcanism

Volcanism is anything that includes the movement of magma. The melting of the area around the initial movement can melt further adding more material to the magma.

Lava is magma that comes to the surface.

Volcano – an opening or landform in the earth through which molten rock, gases and ash erupt.

1. Magma chamber 2. Country rock 3. Conduit (pipe) 4. Base 5. Sill 6. Branch pipe

7. Layers of ash emitted by the volcano 8. Flank 9. Layers of lava emitted by the volcano

10. Throat 11. Parasitic cone 12. Lava flow 13. Vent 14. Crater 15. Ash cloud

Major Volcanic ZonesMost volcanoes are in the area of convergent and divergent zones. A major zone is the Pacific Ocean. The Ring of Fire encircles this area.

Subduction Zones- they occur at subduction boundaries thick lava (magma) very gassy, therefore very, very explosive with a lot of tephra (pyroclastics)

- they form steep sided volcanoes with a lot of activity

- Most famous volcanoes

Mt St Helen's & Mt Vesuvius

The Aleutian Islands, island arcs, by the North Pacific Ocean,

Mid Ocean Ridges

- They are long narrow fractures in oceans or on land with the largest amount of magma

- They contain basaltic lava with only few gases, smooth easily flowing lava

- They are found in spreading centers (division boundaries)

- Mid Atlantic Ridge

Hot Spots

- It is a volcano not at a plate boundary- They produce smooth basaltic lava - non gassy - very hot- they form large volcanic rises under water until it is big enough to rise above the surface- Hot spots do not move, the plate moves over the hot spot

Ex. Hawaii

Intrusive Activity

Since magma is less dense than rock will push upward and intrude into the overlaying rock. This can cause it to change (Metamorphic), melt or even crack the surrounding area.

Sometimes the magma will cool or solidify without erupting. This will harden within the volcano. After some time, the surrounding sedimentary rock will erode away leaving the igneous intrusion called a pluton.

Section 2 Volcanic EruptionsVolcanoes are windows into Earth’s interior. Lava can provide a look into what is inside the

earth. There are two types of magma/lava, Mafic and Felsic.

Rhyolitic (Felsic)a. high in silicab. Most viscous (slow moving)c. Light colored d. Usually associated with explosionse. Lowest melting point f. by Continental Hot Spots (Yellowstone Caldera)

Basaltic Magma (Mafic)a. Low in silica b. Fast moving (Least viscous)c. Dark colored d. Highest melting pointe. Rarely explosivef. by Rifts, oceanic hot spots (Hawaii)

Types of Eruptions

ViscosityThis is the magma/lava’s resistance to flow. Magma high in silica resists flow, low in silica flow very fast.

Magma contains many gases – H2O, CO2, SO2 , H2 , H2S (Hydrogen Sulfide)

Very gassy - explosive eruption usually Felsic

No gas - mild non-explosive eruption usually Mafic

Quiet EruptionsOceanic eruptions are among the quietest eruptions. That is

because they are made from mafic magma which has a low viscosity. Not explosive.

Lava Flows

Aa - rough, jagged surfaces

Pahoehoe - rope like lava

Blocky lava – cools quickly on top while lava flows underneath

causing the lava on top to chunk and break. Most viscous.

Explosive Eruptions

Pyroclastic materials

It is any explosion that happens during volcanic activity. It can be explosive and it can be mild. It contains super heated gases, ash and other objects that are ejected from the opening.

Pyroclastic flow:

Is a dense, superheated cloud that travels down hill. You can not out run this movement.

Types of Pyroclastic Material

Pyroclastic material is classified by size.

The solid fragments are also called tephra.

1. Ash - less than 2mm2. Blocks & bombs - more than 64mm

Blocks always solidBombs become solid in flight

3. Lapilli’s - anything between 2mm & 64mm4. Lahars is fast moving mud flow from melting snow and ice mixing with loose gravel.

Types of Volcanoes

There are 3 main types of volcanic cones that can be produced by a volcano.

Shield– Broad base– Cooled lava layers because of low viscosity; less explosive– Mauna Loa, Hawaii – 4170ft above sea level, base is 5000ft

below sea level.

Cinder Cone

A cinder cone happens when molten lava spewed from a vent piled up.

It is the smallest of the volcanoes, formed by larger ones.

It has a sloped angle that can be as much as 40o

Composite

A composite volcano is made from alternating layers from previous eruptions piling up around a vent.

They have very violent eruptions

They can lay dormant for a long time until it needs to relieve pressure

again.

Mount St. Helen’s is called a stratovolcano because of its size.

Other unique volcanic features:

Lava Plateau (Dome)

This is not a normal dome mountain. This happens when lava comes up from a crack in the ground. It can spread on the surface making a layer of basalt on top. This can cover large areas.

Columnar jointingAre unique six sided columns that are formed when

basaltic lava cools & cracks, you know, the internal arrangement of its atoms. You can find this anywhere, Iceland, New York, California, etc.

Devils Tower, Wyoming

Calderas

A caldera forms from an eruption. The magma beneath a volcano is released than the top of a volcano collapses in. A large crater is formed which can later fill with water.

Mount Mazama in Oregon filled with water after its collapse and is now called Crater Lake.

Predicting Volcanic EruptionsPredicting Volcanic EruptionsScientists study the eruptions to see if there are patterns to help Scientists study the eruptions to see if there are patterns to help predict eruptions.predict eruptions.

Earthquake ActivityEarthquake ActivityWarning signs of an eruption. Earthquakes usually precede an Warning signs of an eruption. Earthquakes usually precede an

eruption.eruption.

Patterns in ActivityPatterns in ActivityBefore an eruption there may be an upward movement of Before an eruption there may be an upward movement of

magma, causing a swelling of the outer area. Scientists look magma, causing a swelling of the outer area. Scientists look for the history of a volcano to help them try to predict when it for the history of a volcano to help them try to predict when it may erupt again. A dormant volcano can erupt again.may erupt again. A dormant volcano can erupt again.

Other Volcano Information

Extraterrestrial VolcanoesMoon

– Evidence of volcanoes are the dark spots which is mostly basalt– Evidence is also there because we can see volcanic formations

Mars– Has a number of shield volcanoes– Olympus Mons – largest known volcano in the solar system– 1999 Mars Global Surveyor saw lava flows only 20-60 million years old.

Venus – Has about 1600 large volcanoes– Scientist feel that the volcanic activity is causing the heavy thick

atmosphereIo

– Most volcanically active place in the solar system– Very active today

Some of the most powerful ones in our history:

1. Krakatoa - Indonesia 1883- Most violent eruption in historic times- Loudest natural sound ever heard (1900 miles)- blew away 1/2 an island- broke windows 90 miles away- 36,000 people died

2. Vesuvius - 79 AD - Pompeii was buried under 30 feet of ash - everyone died

3. Mt Popocatepetl (Mt. Popo) - current