1. Section 1: How and Where Volcanoes Form
Focus Question:
How and where do volcanoes form?
Key Vocabulary:
* volcano * hot spot

2. Magma Formation
A volcanic eruption occurs when magma (molten rock that has formed deep within Earth) rises to the surface.

3. Magma Formation
Conditions at both divergent and convergent boundaries are ideal for magma formation.
Most volcanoes are found along mid-ocean ridges and at subduction boundaries.
Magma rises to the surface because it has a lower density than the solid materials that surround it. The characteristics of a magma and the rates at which it rises depend upon the amount of silica it contains.

4. At Subduction Boundaries
Volcanoes always form on the overriding plate at a subduction boundary.
Volcanic mountains form on continental plates (example: the Cascade Range which extends along the Pacific coast from northern California to British Columbia, Canada).
Volcanic island arcs form on oceanic plates (example: the Mariana Islands in the Pacific Ocean).

5. At Divergent Boundaries
Magma rises through the rift at mid-ocean ridges.
While most divergent boundaries are under the ocean, one place where a mid-ocean ridge rises above sea level is Iceland.
In 1783 the Icelandic volcano Laki erupted. The eruption killed over ¾ of Iceland’s livestock, caused flooding, and destroyed crops, eventually resulting in the deaths of nearly 10,000 people from starvation or disease.

6. Over Hot Spots
Not all volcanic activity occurs at plate boundaries. Hot spots are areas of volcanic activity that result from plumes of hot solid material that have risen from deep within Earth’s mantle.
A hot spot remains in the same place, while the lithospheric plate continues to move across it.

7. Over Hot Spots
The Hawaiian Islands have formed as the Pacific Plate has moved northwest over a hot spot.
Today, the youngest island, Hawaii, lies above the hot spot.
Island formation continues about 30 kilometers off the coast of the island of Hawaii. Here, an underwater volcano, the Loihi Seamount, is slowly growing into the next Hawaiian Island. Loihi is still about 900 meters below sea level.

8. Section 2: Magma and Erupted Materials
Focus Question:
Why are some volcanoes explosive?
Key Vocabulary:
* viscosity * lava
* pahoehoe * aa
* pillow lava * pyroclastic material
* pyroclastic flow

9. Magma and Erupted Materials
Kilauea, a Hawaiian volcano, has spewed molten rock for decades. In contrast, Mount St. Helens exploded violently in 1980 after more than a century of being quiet.
Differences in the volcanic activity at these two sites result partly from differences in the magmas that rise to the surface there.

10. Types of Magma A magma’s viscosity is its resistance to flow.
Silica, the principal ingredient in all magmas, determines the magma’s viscosity.
Magmas high in silica resist flow. Andesitic and rhyolitic magmas contain more silica and are more resistant to flow.
Magmas low in silica flow more easily. Basaltic magmas contain the least silica and flow most easily.

11. Types of Magma
Magmas also contain gases – mainly water vapor and carbon dioxide. The gases, dissolved in the magmas at the depths where they form, may bubble out of solution as the magma rises.
Most gases escape easily from basaltic magmas. When these magmas reach the surface, any remaining gases usually produce relatively harmless fountains and floods.
Gases in more viscous andesitic and rhyolitic magmas can’t escape as easily. When these magmas reach the surface, the gases expand and cause explosive eruptions.

12. Types of Magma Each type of magma tends to form at specific locations.
Basaltic magmas form at rifts and at oceanic hot spots.
Andesitic magmas tend to form at subduction boundaries.
Rhyolitic magmas generally form where hot spots underlie continental plates.

13. Lava Flows Magma that reaches Earth’s surface is called lava.
Like magma, lava is primarily molten rock. However, the composition of lava may differ from magma because materials may be added to or removed from the magma as it rises to the surface.

14. Lava Flows on Land
Basaltic lava flows are usually associated with less-explosive eruptions. As these cool, they form basaltic rock.
Volcanologists use two Hawaiian terms to describe solidified lava flows on land: pahoehoe and aa.
Pahoehoe forms from high temperature lava that cools into smooth, ropelike surfaces.
Aa forms from cooler lava that quickly cools with rough, jagged surfaces.

15. Underwater Lava Flows
Lava that cools underwater have a distinctive shape – a rounded, “pillow-like” form with a hard crust.
Pressure builds up inside the lava until its crust cracks, and more lava pours out, forming yet another pillow shape.
The resulting mass of rounded lumps is called pillow lava.

16. Ash and Rock Fragments
More explosive eruptions usually involve magmas which contain trapped gases.
When these gases are released, solid fragments called pyroclastic material may be ejected.
Pyroclastic materials are classified by size
“Bomb”

17. Pyroclastic Flow
In some violent eruptions, pyroclastic material combines with hot gases to form a pyroclastic flow – a dense, superheated cloud that travels downhill with amazing speed.
The cloud may follow the course of a valley, moving faster than 100 kilometers per hour.
The eruption of Mount Vesuvius in A.D. 79 produced a pyroclastic flow that buried the Roman city of Pompeii under pumice and ash.

18. Section 3: Volcanic Landforms
Focus Question:
What kind of landforms are produced by volcanic eruptions?
Key Vocabulary:
* shield volcano * cinder volcano
* composite volcano * lahar
* caldera * lava plateau

19. Volcanic Landforms
The shape and structure of a volcano are determined by the nature of its eruptions and the materials it ejects.

20. Shield Volcanoes
Because of its low viscosity, basaltic lava tends to flow long distances before hardening.
In some cases, the lava builds up in layers, forming shield volcanoes with broad bases and gently sloping sides.
Shield volcanoes discharge basaltic lava that is less explosive than other types of volcanoes.
Lava flows from shield volcanoes can damage homes, highways, and other property.

21. Example of a Shield Volcano
Kilauea, Hawaii

22. Cinder Cones
Cinder cones, perhaps the simplest type of volcano, forms when molten lava is thrown into the air from a vent. As it falls, the lava breaks into fragments that harden before hitting the ground.
Fragments accumulate, forming a cone-shaped mound with an oval base.
Cinder cones tend to be smaller than other types of volcanoes, typically forming in groups and on the sides of larger volcanoes.

23. Example of a Cinder Volcano
Capulin, New Mexico

24. Composite Volcanoes
Composite volcanoes develop when layers of materials from successive explosive eruptions accumulate around a vent. The materials include hardened lava flows and other pyroclastic material.
After a violent eruption, a composite volcano may remain relatively quiet for a long period of time. Beneath the surface, gas-rich magma may again be building up pressure, eventually leading to another explosive eruption.

25. Example of a Composite Volcano
Mt. Shasta, California

26. Classifying Volcanoes
An active volcano is one that is currently erupting or has erupted recently (in geological terms).
A dormant volcano is one that has not erupted lately but is considered likely to do so in the future.
An extinct volcano is one that has not erupted for a very long time and is considered unlikely to do so in the future.

27. Lahar
Magma, water, and gases from composite volcanoes explode into a massive cloud of superheated ash and stones. Some of the hot ash mixes with snow and ice on the mountain to form fast-moving mudflows called a lahar.

28. Calderas
Sometimes magma beneath a volcano is released after the top of the volcano collapses, forming a large crater-shaped basin called a caldera.
Many active volcanoes have calderas at their summits.
In some cases, a caldera fills with water to form a lake
Crater Lake, Oregon
Some calderas are still active (Yellowstone N.P.).

29. Lava Plateaus
Lava plateaus form as basaltic lava pours out of a fissure (or crack) on the Earth’s surface.
The basaltic lava that formed the Columbia Plateau in the northwestern U.S. is over one kilometer thick in some places and covers and area of about 164,000 square kilometers.