1. VOLCANIC ACTIVITY
Chapter 18

2. MAGMA
Magma is a mixture of molten rock, suspended minerals, and gases.
All volcanoes are fueled by magma.

3. Formation of Magma Magma forms from three factors:
Composition of Rocks (those that need to melt to become magma)
Forms when temperatures are high enough to melt rocks
Most rocks begin melting between 8000 and C
These temperatures exist between the lithosphere and the asthenosphere

4. Pressure
Water
Pressure increases with depth due to the weight of overlying rocks
As pressure increases, temperature at which substance melts also increases
The effect of pressure explains why most of the rocks in the Earth’s lower crust and upper mantle do not melt to form magma, although temperatures are high enough
The presence of water influences whether or not a rock will melt
A wet mineral or rock will melt at a higher temperature than the same mineral or rock under dry conditions

5. Viscosity of Magma
Viscosity is the internal resistance to flow; meaning, the ability to flow rapidly (like smooth liquid) or flow slowly (like a milkshake)
Hotter magma has lower viscosity
Cool magma flows less quickly than hot magma
If magma is low in silica, the viscosity is low

6. Types of Magma Magmas are named after extrusive rocks.
There are three types of magma, each forming from different areas of the Earth.
The three types are:
Basaltic
Andesitic
Rhyolitic

7. Magma Chart Basaltic Andesitic Rhyolitic
Form from rocks in the upper mantle
Low silica amounts
Low Viscosity
Quiet eruptions (flowing)
Andesitic
Form from oceanic crust and sediments
About 60% silica amount (intermediate)
Intermediate Viscosity
Intermediate eruptions
Rhyolitic
Form deep beneath continental crust
Highest amount of silica within this type of magma
High Viscosity
Explosive eruptions (most dangerous)

8. INTRUSIVE ACTIVITY
Plutons are intrusive igneous rock bodies that represent most of the igneous activity on Earth
Mountain building is responsible for the formation of many plutons.
There are five types of plutons:
Batholith
Laccolith
Sill
Stock
Dike

9. Pluton Classification
Batholiths
Pluton Classification
Largest plutons
Irregularly shaped masses
Coarse-grained igneous rocks
Cover at least 100km
Take millions of years to form
Common in interiors of major mountain chains
Stocks
Similar to batholiths but smaller in size
Cuts across older rocks
Forms 10-30km beneath Earth’s surface
El Capitan – large granite batholith in America…
Yosemite National Park

10. Pluton Classification
Laccoliths
Sills
Mushroom shaped pluton with a round top and flat bottom
Forms when magma intrudes parallel rock layers close to Earth’s surface and rocks “bow” upward due to heat and pressure of magma body
Relatively small compared to batholiths and stocks
At most, up to 16km wide
Common in Black Hills of South Dakota
Forms when magma intrudes parallel to layers of rock
Can range from a few centimeters to hundreds of meters in thickness
Palisades Sill, overlooking Hudson River, near NYC

11. Pluton Classification
Dikes
Pluton that cuts across pre-existing rocks
Form when magma invades cracks in surrounding rock bodies
Few cm to several m wide and up to tens of km long
Most are coarse-grained

12. Volcanism produces various features that alter the Earth’s landscape.
18.3 VOLCANOES
Volcanism produces various features that alter the Earth’s landscape.

13. Common Parts of a Volcano
Within a volcano, there is a portion that fuels the eruptions. This is referred to as the magma chamber.
Once the magma chamber is fueled, it erupts through an opening in the crust. This opening is called the vent. Over time, the lava will solidify and accumulate to form a mountain known as a volcano. At the top of the volcano, sits the vent.
The vent is surrounded by a bowl-shaped depression that is referred to as a crater. The crater connects to the magma chamber by way of the vent.
In some instances, the pressure of the volcano becomes too great to support its walls and the side wall (summit) will collapse into the magma chamber, resulting in a caldera.

14. Anatomy of a Volcano

15. Crater Lakes
Sometimes, a crater lake will form as result of too many eruptions.
The top of the magma chamber will become partially empty and collapse, forming a caldera.
The caldera will eventually fill up with water and become a lake, such as the one shown in the picture of Crater Lake National Park (Oregon)

16. 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 this, three major types of volcanoes have been identified:
Shield
Cinder Cone
Composite

17. Shield Volcano Low viscosity Low amounts of gases and silica
Make up the Hawaiian Islands
Mauna Loa – famous example
Broad, gently sloping sides
Nearly circular base
Layered basaltic lava
Non-explosive eruptions

18. Cinder Cone Volcano Steep sided volcano
Material is ejected high into the air and falls back to Earth, around the vent
Generally small (less than 500 m high)
More water and silica than shield volcanoes
Large volume of gases within the magma
More explosive than shield volcanoes
Tephra thrown into the air (rock fragments)
Famous example includes the Izalco Volcano in El Salvador

19. Composite Volcano Largest (in height) of all volcanoes
Highly explosive
Most dangerous to humans
Famous examples include Mount St. Helens and Mount Rainier
Form from alternating layers of volcanic fragments / lava
Large amounts of silica, water and gases

20. Tephra
As noted, tephra refers to rock fragments thrown into the air during volcanic eruptions. Tephra can be one of the following:
Newly cooled / hardened lava
Mineral grains that have crystallized prior to eruption
Pieces of the volcanic cone

21. Tephra Classification by Size
VOLCANIC ROCKS / BOMBS – can be very large. Documented sizes are comparable to a small car or house. When angular, they are called rocks and when rounded, they are called bombs.
DUST – less than .25mm in diameter
ASH – less than 2mm in diameter (but larger than dust)
LAPILLI (little stones) – less than 64mm in diameter (but larger than ash)

22. Pyroclastic Flow
Pyroclastic Flow refers to rapidly moving volcanic material. This flow can travel up to speeds of 200 km/h and may contain hot, poisonous gases. Temperatures of flow can exceed 700 degrees Celsius
To explain damage, one of the most dangerous flows occurred on the island of Martinique, where more than 29,000 people suffocated or were burned to death in 1902 due to the eruption of Mt. Pelee

23. Where do Volcanoes Occur?
Volcano distribution is not random. Most form at plate boundaries.
Convergent Plate Boundaries – make up about 80% of all volcano locations
Divergent Plate Boundaries – make up about 15% of all volcano locations
Away from any plate boundaries – only approximately 5% of all activity

24. Convergent Volcanism
There are two major belts along convergent plate boundaries:
1. Circum-Pacific Belt (Pacific Ring of Fire)
This stretches along the western coasts of North and South America, across the Aleutian Islands, and down the eastern coast of Asia.
2. Mediterranean Belt (Italy)
Includes Mount Etna and Mount Vesuvius

25. Divergent Volcanism
As plates move apart, fractures and faults are created. This results in major separations called rift zones. Most of the world’s rift volcanism occurs under water along deep ocean ridges. This results in a process referred to as seafloor spreading. Rift Volcanism can be observed above water in Iceland, which is part of the Mid-Atlantic Ridge. Several volcanoes are present within this area.

26. Hot Spots
Some volcanoes are far from plate boundaries and form from hot spots (unusually hot regions of Earth’s mantle).
Chains of volcanoes that form over hot spots provide important information about plate motions
Some of Earth’s best known volcanoes form from hot spots under the Pacific Ocean

27. Mt. Kilauea
The worlds most active volcano is currently located over a hot spot on the big island of Hawaii.
Sometimes, hot spots can result in flood basalts. Flood basalts erupt from fissures rather than a central vent, forming flat plains or plateaus
The volume of basalt in these eruptions can be tremendous.
Kilauea has what is called a “fire hose” dumping lava straight off the side of the volcanic cliff into the Pacific Ocean.