1. Earth’s Creators and Destroyers
Volcanoes
Earth’s Creators and Destroyers

2. Structure of the Earth The earth is composed of layers:
Inner Core – solid
Outer Core – liquid
Mantle – solid
Upper Mantle – plastic
Crust – solid

3. Diagram of Earth’s Layers

4. Tectonic Plates
The lithosphere is the Earth’s hard, outermost shell that is divided into a mosaic of 16 major slabs, or tectonic plates.
These plates float on the upper mantle.
As the plates move about they spread apart, collide, or slide past each other

7. Why Plates Move: Convection Cells

9. Earth’s Tectonic Plates

11. Where Volcanoes Occur
Volcanoes occur most frequently at plate boundaries.
Some volcanoes, like those that form the Hawaiian Islands, occur in the interior of plates at areas called hot spots.
The greatest number of volcanoes occur on the ocean floor along spreading ridges.
Over 80% of those on land occur at edges of continents, or subduction zones, where one plate dives, or subducts, under another plate.

15. Why Volcanoes Occur
Temperatures in the mantle are hot enough to melt rock into magma.
Less dense than the solid rock around it, magma rises and some of it collects in magma chambers.
As the magma rises, pressure decreases allowing trapped gasses to expand and propel the magma through openings in the Earth’s surface causing an eruption.
Erupted magma is called lava.

16. How Volcanoes Erupt
Eruptions are described as explosive or effusive (loosely flowing).
How explosive an eruption is depends on the magma’s chemical composition and gas content, which affect the magma’s stickiness, or viscosity.
If magma is fluid, gases can escape rapidly and lava flows; if magma is viscous the gases can not escape and pressure builds inside the magma until the gases escape, sometimes violently.

17. Magma vs. Lava Magma is molten rock beneath the surface.
Lava is erupted magma. There are 2 types of lava:
A a (ah ah) is largely solidified rock that gets pushed forward.
Pahoe hoe (pah hoy hoy) is flowing “liquid” with a ropy, billowy surface.

18. Lava – Pahoe hoe

19. Lava - Aa

21. Types of Lava Flows
Lava flows are superheated streams of molten rock that flow at 1 – 50 mph.
Pyroclastic flows are avalanches of hot ash, rock fragments, and gases that flow at speeds greater than 100 mph.
Landslides are avalanches of rock, snow and ice on slopes of volcanoes (loosened and tumbling due to seismic activity).
Lahars (mud flows) are a mixture of volcanic ash and water (like wet concrete)

22. Volcano Terms A vent is an opening through which eruptions take place.
A crater is a basin like depression over a vent, at the summit of a volcano
A caldera is a depression larger than the original crater (>1km. Diameter) that forms when the summit is blown off, or when the volcano collapses into the empty magma chamber.
Example: Crater Lake atop Mt. St. Helens.

24. Types of Volcanoes
Repeated volcanic eruptions build volcanic mountains of three basic types, or shapes, depending on the composition of the materials deposited by the eruption.
Shield volcanoes
Stratovolcanoes
Cinder cones

25. Shield Volcanoes
Shield volcanoes are broad gently sloping volcanic mountains slowly formed by layer over layer of solidified lava.
Shield volcanoes are formed by effusive eruptions of fluid lava.
These can become very large as the low viscosity lava spreads widely and thickly.
Examples: Kilauea, Hawaii and Mt. Etna, Italy

26. Kilauea, HI

27. Kilauea

28. Mt. Etna, Italy

29. Mt. Etna

30. Mt. Etna

31. Stratovolcanoes (Composite)
Stratovolcanoes are formed from both explosive and effusive eruptions.
Layers of tephra alternating with layers of viscous lava flows create steep-sided, often symetrical cones that can be very large.
Formed over long spans of time as periods of 100,000+ yrs. separate periods of a few years of intense activity.
Examples: Aconcagua, Andes (22,825’) and Mt. St. Helens

32. Aconcagua, Andes

33. Aconcagua

34. Mt. St. Helens

35. Mt. St. Helens

36. Cinder Cone Volcanoes
Cinder cones are the smallest volcanoes (< 500’), formed by explosive eruptions of lava. Blown violently into the air, the erupting lava breaks apart into fragments called cinders that fall and accumulate around the vent.
Cinder cones are temporary geologic features as they are easily eroded. They have short life spans as gas causing violent eruptions is quickly depleted.
Example: Paricutin, Mexico

37. Paricutin, Mexico

38. 1950
1944
1966
1946
1948

39. Evolution of Paricutin

40. Monitoring and Predicting Eruptions
Volcanic activity is monitored using several observations:
Land deformation
Ash clouds
Tremors- measured by seismic data
Volcanic Tremors (VT)
Rockfall (RF)

41. Active,or Recently Active, Volcanoes
Name
Type
Last Erupted
Bezymianmy
Stratovolcano
1993
Erebus
1980
Kilauea
Shield
1995
La Palma
1954
Mt. Etna
Mt. St. Helens
1986
Nevada del Ruiz
1991
Ol Doinyo Lengai
Paricutin
Cinder cone
1952
Pinatubo
1992