Volcanoes

• Science
• 10th Grade

Volcanoes

Earth is geologically active​

• The ash that spews from some volcanoes can form billowy clouds that travel around the world before raining back down.
  
• In the last 10,000 years, more than 1500 different volcanoes have erupted.​

How magma forms​

• ​Magma is a mixture of molten rock, suspended mineral grains, and dissolved gases that fuels all volcanoes.
  
• Magma forms when temperatures are high enough to melt the rocks involved, usually between 1500 degrees F and 2200 degrees F.
  
• Such temperatures exist at the base of the lithosphere and in the asthenosphere.​

How magma forms​

• ​Pressure, which increases with depth, is one factor that determines whether rocks will melt to form magma.
  
• As pressure increases, the temperature required to melt rock also increases.
  
• Due to the effects of pressure, most of the rocks in Earth's lower crust and upper mantle (lithosphere) do not melt to form magma.
  
• The presence of water also influences whether a rock will melt.
  
• At any given pressure, a wet mineral will melt at a lower temperature than a dry one.​

Magma composition

• A number of factors determine the composition of magma.
  
• Viscosity - internal resistance to flow (how thick or runny it is) - higher viscosity means thicker magma
  
• ​Hotter magma means lower viscosity
  
• Lower silica means lower viscosity​

Types of magma

• ​There are three major types of magma, named after extrusive igneous rocks: basaltic magma, andesitic magma, and rhyolitic magma.
  
• ​Basaltic - Hawaiian Islands, Surtsey (south of Iceland) - same composition as basalt - upper mantle - low viscosity (less thick), least explosive
  
• Andesitic - Mt St Helens, Tambora (Indonesia) - same composition as andesite - oceanic crust and sediments - medium viscosity, intermediate explosiveness
  
• Rhyolitic - Yellowstone​ - same composition as granite - continental crust - high viscosity, extremely explosive

Magmas are named after __________ igneous rocks.

Basaltic magma forms when rocks in the upper __________ melt. This magma contains small amounts of silica and has a low __________. Basaltic magma fuels relatively quiet eruptions.

Andesitic magma forms from oceanic crust and __________. This magma contains about 60% silica and has an intermediate viscosity. __________ magma fuels volcanoes with intermediate eruptions.

Rhyolitic magma forms deep beneath __________ crust. This magma has the highest __________ content of the three types of magma. It has the same composition as __________, has a high viscosity, and flows __________. __________ magma produces very explosive volcanoes. 

extrusive

mantle

viscosity

sediments

Andesitic

continental

silica

granite

slowly

Rhyolitic

Intrusive activity​

• ​Magma, because it is molten, is less dense than surrounding rocks.
  
• This density difference forces magma to move upward and eventually intrude into the overlying crust.​

Intrusive activity

• Intruding magma can affect the crust in several ways.
  
• A. Magma can force the overlying rock apart and enter the newly formed fissures.
  
• B. Magma can also cause blocks of rock to break off and sink into the magma, where the rocks may eventually melt.
  
• C. Magma can melt the rock into which it intrudes.​

Plutons

• When magma cools, minerals form and over a long period of time, they will combine to form intrusive igneous rock bodies.
  
• PLUTONS are intrusive igneous rock bodies that can be exposed at Earth's surface as a result of uplift and erosion and are classified based on their size, shape, and relationship to surrounding rocks.​

Plutons

Plutons​

• ​BATHOLITH - largest - irregularly shaped masses of coarse-grained igneous rocks - at least 100 sq km - millions of years to form
  
• STOCK - similar to batholith but smaller
  
• Both cut across older rocks and form 10-30 km below Earth's surface
  
• LACCOLITH - mushroom-shaped with round top and flat bottom - magma intrusion into parallel rock layers close to Earth's surface - at most up to 16 km wide (smaller than batholiths and stocks)​

Plutons​

• ​SILL - few cm to hundreds of meters in thickness - magma intrudes into parallel layers of rock
  
• DIKE - few cm to several meters wide, up to tens of km long - cuts across pre-existing rocks
  
• Textures of sills and dikes vary, many are coarse-grained - formed deep in Earth's crust and cooled slowly
  
• ​Many plutons (esp batholiths) are formed as the result of mountain-building processes (orogeny).
  
• The plutons that form deep beneath Earth's surface represent the majority of igneous activity on Earth.​

Intrusive igneous rock body

Largest pluton

Irregularly shaped pluton that is similar to a batholith but smaller in size

Mushroom-shaped pluton

Pluton that is parallel to the rock it intrudes

Pluton that cuts across preexisting rocks

Process responsible for the formation of many plutons

Magma that has reached the Earth's surface

Pluton

Batholith

Stock

Laccolith

Sill

Dike

Mountain Building

Lava

Types of volcanoes​

​The appearance of a volcano depends on two factors:​

• The type of material that forms the volcano
  
• The type of eruptions that occur​

Based on these two criteria, three major types of volcanoes have been identified:​

• Shield volcanoes
  
• Cinder-cone volcanoes
  
• Composite volcanoes​

Shield volcanoes

Cinder-cone volcano​

Types of volcanoes​

Types of volcanoes​

Shield

Cinder-cone

Composite

Most dangerous / destructive

high amounts of silica

Vesuvius and Yellowstone

rhyolitic lava, highly explosive

Gently sloping sides, nearly circular base

Mauna Kei in Hawaii

low amounts of silica

basaltic lava, nonexplosive eruptions

Small volcano with steep sides

moderate amounts of silica

Mt St Helens and Mt Rainier

andesitic lava

Anatomy of a volcano​

• ​VENT - where lava erupts through a crustal opening
  
• As lava flows out onto the surface, it cools and solidifies around the vent, eventually accumulating to form a mountain known as a VOLCANO.
  
• CRATER - bowl-shaped depression at the top of a volcano, usually less than 1 km in diameter, that is connected to the magma chamber by a vent
  
• CALDERAS​ - large depressions up to 50 km in diameter than can form when the summit or side of a volcano collapses into the magma chamber that once fueled it

Volcanic material​

• ​TEPHRA are rock fragments thrown into the air during a volcanic eruption. They are classified by size.
  
• DUST and ASH are the smallest tephra.
  
• Somewhat larger fragments are called LAPILLI (little stones).
  
• Largest tephra include angular volcanic BLOCKS and rounded or streamlined volcanic BOMBS, both of which can be the size of a house or larger.​

Volcanic material​

• ​Some tephra cause tremendous damage and kill thousands of people.
  
• A PYROCLASTIC FLOW is a cloud of volcanic gas, dust, and other tephra traveling at speeds of nearly ​125 mph.
  
• The temperature at the center of pyroclastic flow can exceed ​1300 degrees F.

Lava erupts through an opening in Earth's crust called a _____.

1. vent
2. crater
3. caldera
4. volcano

A bowl-shaped depression that forms around the vent of a volcano is a _____.

1. magma chamber
2. vent
3. crater
4. sill

Rock fragments thrown into the air during a volcanic eruption are called _____.

1. dikes
2. sills
3. calderas
4. tephra

The smallest tephra are _____.

1. lapilli
2. dust
3. volcanic bombs
4. volcanic blocks

Which of the following forms when the top or side of a volcano collapses into the magma chamber?

1. dike
2. pyroclastic flow
3. caldera
4. vent

Fast-moving clouds of gas, ash, and other tephra are _____.

1. calderas
2. pyroclastic flows
3. volcanic blocks
4. volcanic bombs

Where do volcanoes occur?​

• ​Most volcanoes form at plate boundaries.
  
• About 80% of all volcanoes are found along convergent boundaries.
  
• About 15% are found along divergent boundaries.
  
• Only about 5% of extrusive igneous activity occurs far from plate boundaries.​

Where do volcanoes occur?​

Where do volcanoes occur?​

• Convergence involving oceanic plates creates subduction zones, and the magma generated is forced upward through the overlying plate and forms volcanoes when it reaches the surface.
  
• The volcanoes associated with convergent plate boundaries form two major belts:
  
• - The larger belt, the Circum-Pacific Belt, is also called the Pacific Ring of Fire.
  
• - The smaller belt is called the Mediterranean Belt.​

Where do volcanoes occur?​

• ​At divergent plate boundaries, magma is forced upward into the fractures and faults that form as the plates separate.
  
• These areas of major faults and fractures are called rift zones.
  
• Most of the world's rift volcanism occurs under water along ocean ridges, but there are two places on continental land where they occur - Iceland and ​Kenya (Africa).

Where do volcanoes occur?​

• ​Some volcanoes are located far from plate boundaries and form as the result of hot spots.
  
• HOT SPOTS are unusually hot regions of Earth's mantle where high-temperature plumes of mantle material rise toward the surface.
  
• A plume does not move laterally (horizontally), which results in a trail of progressively older volcanoes that formed as a plate moved over a hot spot.
  
• ​The Hawaiian Islands continue to rise above the ocean floor as the Pacific Plate moves over a hot spot.

Where do volcanoes occur?​

Most of the world's volcanoes form along __________ plate boundaries, where oceanic crust descends into the mantle at __________ zones, then magma is forced upward toward the surface. A smaller percentage form along __________ plate boundaries as magma is forced upward into fractures and faults that form as plates spread apart. Only about 5% of volcanoes form far from plate boundaries over __________, which are unusually hot regions of Earth's mantle. The magma that forms moves upward toward the __________ and melts the crust to form a volcano. As a tectonic plate moves over a hot spot, a string of __________ forms. The __________ Islands are forming as the result of a hot spot.

convergent

subduction

divergent

hotspots

crust

volcanoes

Hawaiian