1. Will it flow or will it blow? Types and Products
Volcanic Eruptions
Will it flow or will it blow?
Types and Products

2. Explosive Eruptions Less common More destructive
Mostly produce clouds of hot ash, gas and rock fragments shoot rapidly out of a volcano.
Ash can reach upper atmosphere and stay for years, blocking sunlight
Can blast billions of tons of material
Eg Mount St. Helens

3. Nonexplosive Eruptions (Effusive)
More common
Less destructive (more time)
Mostly lava flowing over the Earth’s surface
Most rock on ocean floor was produce by nonexplosive eruptions (Oceanic Volcanoes)

4. Factors Affecting Eruptions
The type of eruption is determined by the composition of the magma (what it’s made of): water – more water means less viscous, flows easier gas - more gas, more violent silica - more silica means more viscous, slower flowing

5. Gases in Magma Water vapor, Carbon dioxide, Sulfur, Hydrogen
As magma comes to the surface, the gases are released as bubbles, making the eruption more explosive.

6. Silica Content Silica is silicon and oxygen.
Magma with low silica contents is thinner, less viscous, allowing gases to escape more easily.
Magma with high silica content is thick and slow-moving

7. Silica and Gas Content
Silica-rich Magma will flow slowly and may harden inside a volcano’s vents, blocking them.
As more magma pushes up from below, the pressure increases.
Silica-rich magma may be so stiff that water vapor and other gases cannot escape from the magma.
Trapped bubbles of gas may expand until they explode.

8. Lava Composition

9. Mafic/Basaltic Lava Flow
Dark in colour due to high iron and magnesium
Low viscosity due to low silica content,
Rapid (up to 100km/hr), Long distance flow – up to 100s of km
Form much of ocean crust
Mafic magma allows gases to escape more easily, resulting in less explosive eruptions.
(P and a’a)

10. Intermediate/Andesitic Lava Flows
Intermediate to high viscosity (more Silicon than basalt) moderate amounts of silica, Fe and Mg
Mound near vent, flow slowly
Outer crust fractures, creating rubble

11. Felsic or Rhyolitic Lava Flows
Highest silica content, most viscous
Low iron, and magnesium
Rarely flows
Plugs vent as a lava dome
Lower temperature
gases do not escape easily, leading to more explosive eruptions.
Common in continental crust

12. Lava Flows, Pyroclastic debris, volcanic gases
Eruptive Material
Lava Flows, Pyroclastic debris, volcanic gases

14. Types of Lava Flows
Geologists classify lava by the shapes it forms when it cools.
There are two major types of basaltic lava flow: a'a and pahoehoe .

15. Pahoehoe Pahoehoe lava, is fast- flowing and runny.
forms smooth, “ropey” masses.

16. A’a lava A’a lava has a rough jagged surface,
formed when slow-moving, sticky lava cools and breaks up into sharp, blocky shapes. 
More common
Crust deformed rapidly

17. Blocky Lava
Blocky lava is cool, stiff lava that does not travel very far from the volcano forms piles of rocks with sharp edges.
Larger blocks than aa lava, and have smoother surface
Santorini, Greece

18. Pillow Lava Forms when lava flows out of the Earth’s crust underwater.
As it cools, it forms rounded lumps that look like pillows.

19. Types of Pyroclastic Material
Pyroclastic materials are individual eruptive(solid) fragments are called pyroclasts (“fire fragments”).
Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation.

20. Tephra (Greek for “ash”)
Tephra (Greek, for ash) is the generic term for any airborne pyroclastic material
Classified according to size
Heaviest particles settle out first

21. Volcanic Ash fine-grained fragments < 2 mm in diameter
Volcanic ash forms when gases trapped in magma or lava form bubbles.
When the bubbles explode, they create millions of tiny pieces.

22. Volcanic Ash on Plymouth, Monserrat after 1995 Eruption

23. Lapilli (Latin for “little stones)
Lapilli are small bits of lava that harden before they hit the ground. Pea- to walnut-size pyroclasts (2-64mm) All types of lava produces lapilli. All types of volcanoes produce lapilli

24. Lava Bombs
Lava bombs are large (< 64mm) blobs of ejected while still molten, gaining a streamline shape, harden in the air and cool before they reach the ground.

25. Lava Blocks Angular fragments ejected in solid condition.
larger than 64 mm in diameter
Can be as big as houses

26. Pyroclastic Flows
Very hot (700ºC), Very fast (100 km/h), current of gas and tephra. Capable of reaching 700 km/h
Very dangerous (can destroy everything in its path).
A pyroclastic flow from the eruption of Mount Pinatubo is shown in the figure below.
Play only 1st minute

27. Other weird Tephra: Pele’s Tears
lapilli-size fragments of basaltic lava may cool quickly while airborne, to form glassy teardrop-shaped lapilli

28. Pele’s Hair
During strong winds, these molten fragments can be drawn out into fine filaments
Pele's Hair is the name used for hair-like strands of volcanic glass that sometimes form in areas where lava fountaining, lava cascades and vigorous lava activity occur. They are less than 1/2 millimeter in width, but can be up to two meters in length. They resemble golden-brown human hair in their size, shape and color. They are a mineraloid formed from basaltic lava. Creative Commons photograph by Cm3826.

29. Homework
Volcano WS, Questions #1-7
Research a Volcano Assignment

30. Volcano Research Assignment: Showcase a volcano from the list or one of your choice
Name of volcano, location, date of last eruption
Type of volcano
Type of eruption (explosive/ effusive)
Type of eruptive material
Consequences/damage/death toll
Images (before, during, and after eruption)