1. Types, Structures, and Eruptions
Volcanoes!
Types, Structures, and Eruptions

3. What is a Volcano? A volcano is…
A rupture in the surface of a planet that…
Allows molten rock, ash, and/or gases to escape from lower levels onto the surface
It can be large or small
It can be violent or quiet

4. The Anatomy of a Volcano
All volcanoes have the following:
A magma chamber
A main vent
A crater
Volcanoes may also have:
Secondary cones, vents, and craters
Lava domes
Pyroclastics (ash, rocks, bombs)

5. The Anatomy of a Volcano

6. Types of Magma
Magma is classified based on its composition, temperature, and viscosity
Composition includes the amount of mafic minerals and silica and the amount of dissolved gasses
Viscosity refers to its resistance to flow
Three basic types (although intermediate magmas are common)

7. Types of Magma Basaltic Magmas High amounts of mafic minerals
Low silica content (45-55%)
Low viscosity (flows like water)
High Temperature ( C)
Produces pahoephoe and aa lavas (based on temperature)
Gases escape easily

8. Types of Magma (Pahoehoe)

9. Types of Magma (Aa Flow)

10. Types of Magma (Aa Flow)

11. Types of Magma Andesitic Intermediate Lava
Moderate amount of mafic minerals
Moderate silica content (55-65%)
Moderate gas content
Moderate viscosity
Moderate temperature ( C)

12. Types of Magma Rhyolitic Poor in mafic minerals
Rich in silica (more than 65%)
High gas content
High amount of ash
High viscosity
Low temperature ( C)

13. Types of Volcanoes The major types of volcanoes include:
Shield (Hawaiian-type)
Cinder Cones
Stratovolcanoes (Composite volcanoes)
Other (less common) volcanoes include:
Fissures
Lava Plateau
Caldera

14. Shield Volcanoes Shield volcanoes are dominated by their bases
They have low slopes and are built entirely from basaltic lava flows
Most associated with divergence or hot spots
Ex: Hawaiian islands

15. Shield Volcanoes Eruption style Very gentle
Typically produces little ash or pyroclastics
Frequently results in lava fountains or lava lakes
Lava tubes are also produced

17. Shield Eruptions

18. Cinder Cones
Cinder Cones are the smallest type of volcano and the shortest lived
Rarely taller than 1000 ft
Have very steep sides
Lava erupts from the base of the volcano, not the crater!
Have large, deep craters and steep sides
Formed by eruptions of brittle, rocky lava and pyroclastic material

19. Cinder Cone Eruptions Typically moderately explosive eruption
Basaltic lava with a higher gas content
Leads to the production of cinders – cooled small pieces of lava blown into the air
Obsidian (volcanic glass) and scoria are common

20. Stratovolcanoes (Composite)
“Typical” volcanoes
Tall, with steep sides
Built by alternating layers of andesitic and rhyolitic lava and pyroclastics
Common around convergent boundaries
Most major volcanoes are of this type

21. Stratovolcanic Eruptions
Most violent volcanoes!
Eruptions produce high quantities of ash, pyroclastics, gas, and lava (relatively little lava)
Can also result in nuees ardentes (glowing clouds) – huge pyroclastic clouds that rush down mountain at sonic speeds

22. Fissures and Lava Plateaus
A fissure is a crack in the surface of the Earth that opens along a divergent boundary
A lava plateau is related to a large scale field of fissures that open due to an intense magma plume (rare!)
Can be related to mass extinctions

23. Fissures and Lava Plateaus
Fissure eruptions and lava plateaus
Fluid basaltic lava extruded from crustal fractures called fissures
e.g., Columbia Plateau

25. Almost everything about this volcanic province is impressive
Almost everything about this volcanic province is impressive. The Columbia River Flood Basalt Province forms a plateau of 164,000 square kilometers between the Cascade Range and the Rocky Mountains. In all, more than 300 individual large (average volume 580 cubic km!) lava flows cover parts of the states of Idaho, Washington, and Oregon. At some locations, the lava is more than 3,500 m thick. The total volume of the volcanic province is 175,000 cubic km. Eruptions filled the Pasco Basin in the east and then sent flows westward into the Columbia River Gorge. About 85% of the province is made of the Grande Ronde Basalt with a volume of 149,000 cubic km (enough lava to bury all of the continental United States under 12 m of lava!) that erupted over a period of less than one million years. Flows eventually reached the Pacific Ocean, about 300 to 600 km from their fissure vents. The Pomona flow traveled from west-central Idaho to the Pacific (600 km), making it the longest known lava flow on Earth (the major- and trace-element compositions of the flow do not change over its entire length).

26. Fissures and Lava Plateaus

27. Calderas Calderas are the largest volcanic craters on Earth
Can be tens of miles in diameter (just the crater!)
Form when the roof of a large magma chamber collapses during an eruption of any other type of volcano
Results in a massive eruption, destroying the volcano and leaving a circular depression
Examples – Yellowstone, Krakatoa, Santorini, Crater Lake

28. Calderas

29. Caldera

30. Calderas

31. Eruptions Eruptions produce the following: Lava (molten rock)
Pyroclastics
Ash (smaller than 2 mm)
Lapilli (between 2 mm and 64 mm)
Blocks and Bombs (larger than 64 mm)
Gases

32. Eruptions

33. Types of Eruptions The severity of an eruption depends on:
Viscosity of magma
Higher viscosity – more violent eruption
Gas content
Greater gas content – more violent
Silica content
Higher silica content – more violent
In other words – basaltic magmas will be less violent than rhyolitic magmas

34. Volcanoes and Other Igneous Activity Tutorial

35. Volcanoes and Plate Tectonics

36. Intrusive Igneous Activity
Plutons: structures that result from cooling and hardening of magma beneath the Earth’s surface.
Pluton derived from Pluto, Roman god of the underworld

39. Igneous Rocks Type of rock that forms when:
magma cools and hardens beneath the surface of the Earth (intrusive) or
magma cools and hardens on the Earth’s surface from volcanic eruptions (extrusive or volcanic)

40. Plutonic Structures Two types of plutons:
Concordant – parallel to rock beds
Discordant – cutting across rock units

41. Concordant Plutons Laccolith - mushroom shaped
Sills – small flat intrusions

44. Sill and Laccolith Formation
Sills and Laccoliths are plutons that form when magma intrudes between rock layers close to the surface.

45. Sill and Laccolith Formation
Form between layers of sedimentary rock.
Form at shallow depths. Why??

46. Sill and Laccolith Formation: Why are Shapes Different?
Magma that forms laccoliths has higher viscosity than magma that forms sills.

47. Discordant Plutons
Dike – small feature that cuts across rock units

48. Relationship between concordant and discordant plutons
Concordant pluton
Discordant pluton

49. Batholiths and Stocks Batholith Stock
The largest bodies of intrusive igneous rock.
A batholith has a surface exposure of over 100 square kilometers!!
Stock
Similar to batholiths, but under 100 kilometers in size.

50. Batholith Formation Begin as magma deep beneath the surface.
Plutons slowly rise through the crust.
Batholiths are formed when individual plutons join together.

51. Batholiths Batholiths form the core of many mountain ranges!
The Idaho batholith forms part of the northern Rocky Mountains and is more than 40,000 square kilometers!