Geology 101 - Lecture 9
Volcanic Rocks (continued)
I. WHERE DO ERUPTIONS OCCUR?
Great
Visual Review of Plate Tectonics
(see especially the section on plate motions)
Click here for plate tectonic
distribution of volcanoes
- Divergent Boundaries
- Convergent Boundaries
- Intraplate Settings - hotspots
Get both INTRUSIVE and EXTRUSIVE rocks in these localities
- Mid-ocean Ridge Eruptions - basalt, pillow lavas
Recent eruption on the Juan de Fuca Ridge!
- Subduction-zone volcanoes (click here
for an example)
- Intraplate eruptions - Hotspot volcanoes - i.e. Hawaii
- These are typically mellow eruptions of lava
- The Columbia River basalt in eastern
Washington is hypothesized to have erupted while the region
passed over a hotspot (the same one that now sits under Yellowstone)
A Guide to
Volcanoes of the World and other useful volcano information
(A good review of lecture material)
Volcano World: A fun volcano site to visit
Active Eruption!!!: Montserrat Volcano erupting now
II. TYPES OF VOLCANOES
1. Shield volcanoes - Hawaii
(diagram of shield volcano)
-
Docile lava outpouring. Only minor pyroclastic material
-
Lava forms broad dome with central crater
-
Slope is 2-10 degrees, like flattened shield
-
Very long lived, very large, massive amounts of lava (pahoehoe and aa)
-
Example: Kilauea
2. Cinder cones (diagram of cinder cone)
- Erupt pyroclastic material
- Steep slopes (30 to 40 degrees)
- Not very long lived.
- Typically small, less than 1000 feet tall
- Often parasitic on larger volcanoes
- Examples: Paricutin in Mexico, Sunset crater in Arizone
3. Composite volcanoes (diagram of composite
volcanoe)
- Erupt lava and pyroclastic material
- Intermediate slopes because lava acts like protective coating on pyroclastic layers
- Built up over long periods of time
- Most picturesque, but most violent
- Examples: Mt. Vesuvius, Mt. Shasta, Mt. Fuji
III. WHAT HAPPENS AFTERWARDS?
- In huge pyroclastic eruptions, lots of magma removed from magma chamber.
- Get collapse feature called a CALDERA
Can be few km wide to 50 km or more
IV. OTHER KINDS OF ERUPTIONS AND RELATED EVENTS
A. Phreatic eruptions
- Hot magma encounters groundwater.
- Superheated steam causes explosions
- Example: Krakatoa
B. Fissure eruptions
- Basalt lava eruping out of linear crack
- Produces fire fountains
C. Lahars - mudflows of wet volcanic debris
- Wall of crater lake breaks
- Pyroclastic debris hits river
- Lava melts glacial ice
- Example: Nevado del Ruiz in Colombia in 1985. Killed 25,000 people
D. Hot springs and geysers
Circulating groundwater heated by buried magma
