1. Volcanoes

2. The Big Idea
Volcanoes are locations where molten rock reaches Earth’s surface, and volcanoes can affect landforms and societies.

3. What You Will Learn
Distinguish between nonexplosive and explosive volcanic eruptions.
• Identify the features of a volcano.
• Explain how the composition of magma affects the type of volcanic eruption that will occur.
• Describe four types of lava and four types of pyroclastic material.

4. Volcanos
The explosive pressure of a volcanic eruption can turn an entire mountain into a billowing cloud of ash and rock in a matter of seconds.
But eruptions are also creative forces—they help form fertile farmland. They also create some of the largest mountains on Earth.

5. Volcanos
During an eruption, molten rock, or magma, is forced to the Earth’s surface. Magma that flows onto the Earth’s surface is called lava.
Volcanoes are areas of Earth’s surface through which magma and volcanic gases pass.

6. Nonexplosive Eruptions
At this moment, volcanic eruptions are occurring around the world—on the ocean floor and on land. Nonexplosive eruptions are the most common type of eruption. These eruptions produce relatively calm flows of lava, such as those shown in Figure 1. Nonexplosive eruptions can release huge amounts of lava.

8. Explosive Eruptions
Explosive eruptions, such as the one shown in Figure 2, are much rarer than nonexplosive eruptions.
However, the effects of explosive eruptions can be incredibly destructive.
During an explosive eruption, clouds of hot debris, ash, and gas rapidly shoot out from a volcano.

9. Instead of producing lava flows, explosive eruptions cause molten rock to be blown into tiny particles that harden in the air.
The dust-sized particles, called ash, can reach the upper atmosphere and can circle the Earth for years.
Larger pieces of debris fall closer to the volcano.

12. What Is Inside a Volcano?
If you could look inside an erupting volcano, you would see these features.
A magma chamber is a body of molten rock deep underground that feeds a volcano.
Magma rises from the magma chamber through cracks in the Earth’s crust to openings called vents.
Magma is released from the vents during an eruption.

14. What Makes Up Magma?
By comparing the composition of magma from different eruptions, scientists have made an important discovery.
The composition of the magma affects how explosive a volcanic eruption is.
The key to whether an eruption will be explosive lies in the silica, water, and gas content of the magma.

15. Water and Magma Are an Explosive Combination
If the water content of magma is high, an explosive eruption is more likely.
Because magma is underground, it is under intense pressure and water stays dissolved in the magma.
If the magma quickly moves to the surface, the pressure suddenly decreases and the water and other compounds, such as carbon dioxide, become gases.
As the gases expand rapidly, an explosion can result.

16. Water and Explosive Volcanoes
This process is similar to what happens when you shake a can of soda and open it.
When a can of soda is shaken, the CO2 dissolved in the soda is released and pressure builds up.
When the can is opened, the soda shoots out, just as lava shoots out of a volcano during an explosive eruption.
In fact, some lava is so frothy with gas when it reaches the surface it becomes solid, called pumice, can float in water!

17. Silica-Rich Magma Traps Explosive Gases
Magma that has a high silica content also tends to cause explosive eruptions. Silica-rich magma has a stiff consistency.
It flows slowly and tends to harden in a volcano’s vents.
As a result, it plugs the vent. As more magma pushes up from below, pressure increases.
If enough pressure builds up, an explosive eruption takes place.

18. Silica-Rich Magma Traps Explosive Gases - Continued
Stiff magma also prevents water vapor and other gases from easily escaping.
Gas bubbles trapped in magma can expand until they explode.
When they explode, the magma shatters and ash and pumice are blasted from the vent.

19. What Erupts from a Volcano?
Magma erupts as either lava or pyroclastic (PIE roh KLAS tik) material.
Lava is liquid magma that flows from a volcanic vent.
Pyroclastic material forms when magma is blasted into the air and hardens.
Nonexplosive eruptions produce mostly lava. Explosive eruptions produce mostly pyroclastic material.

20. Types of Lava
The viscosity of lava, or how lava flows, varies greatly.
To understand viscosity, remember that a milkshake has high viscosity and a glass of milk has low viscosity.
Lava that has high viscosity is stiff.
Lava that has low viscosity is more fluid.

21. Lava
5. Blocky lava and pahoehoe (puh HOY HOY) have a high viscosity and flow slowly.
Other types of lava flows, such as aa (AH AH) and pillow lava, have lower viscosities and flow more quickly.

23. Types of Pyroclastic Material
Pyroclastic material forms when magma explodes from a volcano and solidifies in the air.
This material also forms when powerful eruptions shatter existing rock.
The size of pyroclastic material ranges from boulders that are the size of houses to tiny particles that can remain suspended in the atmosphere for years.