Volcanoes form through two main processes: the convergence of tectonic plates and hot spots in the mantle. When plates converge, one plate subducts and melts, forming magma that rises to the surface and erupts, eventually building a volcano. Hot spots are areas of high heat in the mantle that melt rock below, causing volcanoes to form over hot spots as plates move. The viscosity of magma depends on its temperature and silica content, determining the type of volcano and eruption: more viscous magma with low heat and high silica becomes stuck and erupts explosively, while less viscous magma flows easily in gentle eruptions.

1. VolcanoesPart 1

2. Volcano History
The word volcano comes from
Vulcan, the Roman god of fire and
metalworking who “lived” under Mt.
Etna.
The Study of Volcanoes is
referred to as volcanology.
Volcano: A landform that
develops where lava, ash, and
gas erupt.

3. How do volcanoes Form?
There are two major ways volcanoes
form:
Convergence of Tectonic Plates
Hot Spots

5. Convergence
When two tectonic plates collide:
1. There is subduction of a plate into the
asthenosphere.
2. The plate begins to melt rock, forming magma.
3. The hot, low density magma rises toward Earth’s
surface.

6. Convergence
When two tectonic plates collide:
4. Pressure pushing up on Earth’s crust
gets higher and higher.
5. Eventually, magma pushing up forms a
crack called a fissure.
6. Magma erupts through the fissure and
onto Earth’s surface.
7. Magma that is above Earth’s surface is
called lava.
After several eruptions, lava builds up and a
rocky hill, a volcano is formed.

9. Hot Spot
 A hot spot can also form a volcano:
1. A hot spot is an area in the lower mantle that is really hot.
2. Since it is so hot, parts of the mantle melt and form magma.
3. Hot magma rises because it is less dense.
4. Pressure is put onto the surface of the Earth, forming a fissure.
5. A volcano is formed over many eruptions.
Examples: Hawaii, Yellowstone
Hot spots are normally found inside of plates, not at boundaries.

10. Hot Spot Example: Hawaii
1. Hawaii sits on the Pacific Plate, which is moving Northwest.
2. There is a hot spot under Hawaii causing magma to rise.
3. The hot spot is located in the mantle, and is not moving.
4. The plate moves over the hot spot, forming a line of volcanoes.
Which islands are the oldest, the northern or the southern islands?

12. The type of volcano and how it erupts
depends on magma’s viscosity:
 Thick / Viscous Magma
 Honey or Molasses
 Does not flow very easily.
 Why is Magma Viscous?
 Low temperatures
 High Silica content
• Thin/ Non-Viscous
Magma
– Water
– Flows very easily.
• Why is Magma Non-
Viscous?
– High temperatures
– Low Silica content
VISCOSITY: How easily a liquid will flow

13. Why Silica?
The more Silica in magma, the thicker and more
viscous it is.
Silica tetrahedrons link together in long chains that
stop magma from flowing easily.

15. Lava Flow

16. What about the Magma?
 Magma with Silica content that is…
HIGH
Granitic Magma
Medium
Andesitic Magma
Low
Basaltic Magma
VISCOUS MAGMA
• Low Temperature
• High Silica
• Called Granitic Magma
Medium Viscous Magma
• Medium Temperature
• Medium Silica
• Called Andesitic Magma
Non-Viscous Magma
• High Temperature
• Low Silica
• Called Basaltic Magma

17. Why is Viscosity so important
anyway?
Thick and Viscous Magma
1. There are gasses in magma, such as Water Vapor and
CO2.
2. Gasses rise up toward Earth’s surface with the magma.
3. If the magma is thick, it does not flow up easily and
GETS STUCK!
4. Rising gasses also get stuck and can not escape.
5. The gasses build up pressure until they explosively erupt.

19. Why is Viscosity so important
anyway?
Thin and Non-Viscous Magma
1. If the magma is thin, it flows up easily and
DOES NOT GET STUCK.
2. Rising gasses also flow up and can easily
escape.
3. Pressure does not build up and magma
gently erupts.

20. GENTLE
ERUPTION
MODERATELY
EXPLOSIVE ERUPTION
EXPLOSIVE
ERUPTION
Volcanoes
 The type of volcano and how it erupts depends on…MAGMA’s
VISCOSITY.
VISCOUS MAGMA
• Low Temperature
• High Silica
• Called Granitic Magma
Medium Viscous Magma
• Medium Temperature
• Medium Silica
• Called Andesitic Magma
Non-Viscous Magma
• High Temperature
• Low Silica
• Called Basaltic Magma

22. Volcanoes
You must know the following:
Type of Volcano
Temperature
Silica Content
Eruption Style