1. Hot Spots
- Building a Tropical Paradise - Hot Spots in the Pacific
- Review for Exam 1
Reading:
4th Ed. Ch 3, Sec 27
5th Ed. Ch 3, Sec 28
Exam 1: Thurs. Sept. 21, 11am - 12:15 pm, Bowman 133
Graphic: Steam explosion off the coast of Hawaii caused by lava reaching the
sea. Volcanic debris produced by the explosion washes ashore and forms black
sand beaches. J.D. Griggs, photographer. Courtesy of USGS.
2. Hawaii - Living with a Hot Spot
Scientists routinely monitor Hawaiian volcanoes
Hazards: Lava flows and earthquakes
Graphics: (top left) Oct. 1987, (top right) June 1989, both J.D. Griggs
photographer,
courtesy of NOAA, (bottom) CNN.com 14 Sept 2004.
3. What is a Hot Spot?
Hot spot = surface expression of a magma plume that rises through the mantle
After erupting, this magma cools to form volcanic mountains
As lithospheric plates move above hot spots, volcanic island chains can form
Graphic: Eruption of Pu'u 'O'o. J.D. Griggs, photographer. Courtesy of USGS.
4. Hot Spots - A View from Earth's Interior
Graphic: Garrison, 4th Ed. Fig. 3.13, pg 71, see 5th Ed. Fig. 3.32, pg 81.
5. Locations of Hot Spots
Hot spots are associated with processes deep inside the Earth - they are not
"tied" to lithospheric plates
Hot spots can occur anywhere on the planet - at plate boundaries or plate
interiors
Examples: Iceland, Hawaii, Yellowstone
Graphic: Map courtesy of USGS.
6. Two Hypotheses Regarding the Formation of Hot Spots
Temperature variations in the outer core:
- Very hot areas of the outer core may melt material in the lower mantle
Melting of previously subducted material:
- Material that has broken off of subducted slabs may melt in the lower mantle
Graphic: Garrison 4th Ed., Fig. 3.1, pg 59.
7. Hot Spot Volcanoes: Inflation-Deflation Cycles
Hot spot volcanoes form due to eruptions over many years
- Magma swells up toward the surface
- The magma reservoir inflates, cracking the ground and triggering many small
earthquakes
- An eruption releases magma and the magma chamber deflates
Graphic: Drawings courtesy of USGS. See http://hvo.wr.usgs.gov/howwork/subsidence/inflate_deflate.html
8. Explosive Eruptions
Some hot spot volcanoes erupt explosively
- Water in the crater contacts molten rock and flashes to steam
- Under stress, the walls of the crater collapse, blocking the crater
- Pressure builds up and the volcano erupts explosively
Graphics: (left) Courtesy of USGS, (right) 1924 eruption of Kilauea, courtesy of
USGS.
9. Hot Spot Volcanoes and Earthquakes
Small, shallow earthquakes occur as magma moves through the upper part of the
volcano's interior (these can provide advance warning of a major eruption)
Larger, deep earthquakes occur due to stresses exerted by magma rising deep in
the chamber
Graphic: Earthquakes on and around the Big Island of Hawaii, 1929-1994. Courtesy
of USGS.
10. Formation of a Volcanic Island Chain
Heat weakens the plate above the hot spot and magma migrates upward to form a
volcano
As the plate moves, the magma supply is cut off and the volcano cools and sinks
isostatically
A new volcano begins to form above the hot spot
Graphic: Garrison, Fig. 3.29, pg 83. (animation)
11. Hot Spot Volcanoes and Time
Volcanoes farther from the hot spot are older
(Ages shown in yellow are in millions of years)
Graphic: Ages of volcanoes in the Hawaiian-Emperor Chain, Garrison, 4th Ed. Fig.
3.30, pg 83, 5th Ed. Fig. 3.33 pg 81.
12. Measuring Plate Motion
Plate direction: Determined from the orientation of volcanoes within the chain
Plate speed: Determined from ages of volcanoes and distance between volcanoes
Graphic: Garrison, Fig. 3.29, pg 83.
13. Changes in Plate Direction Over Time
One hot spot created both the Hawaiian Islands and Emperor Seamounts
The Hawaiian-Emperor "bend" in the chain is due to an abrupt change in plate
direction 40 million years ago
Hot spot volcanoes help reconstruct plate motion over long periods of time
Graphic: Garrison, Fig. 3.30, pg 83.
14. Lo'ihi - Hawaii's Youngest Submarine Volcano
- Lo'ihi rises 3000 m above the sea floor (summit = 969 m below sea level)
- Generates frequent earthquake swarms
- Expected to break the surface and become an island in 30,000-100,000 years
Graphics: (top) Map courtesy of USGS, (bottom) Earthquakes on and around the Big
Island of Hawaii, 1929-1994. Courtesy of USGS.
15. Exam 1
Thursday Sept 21
Worth up to 26% of your grade; your lowest midterm score is dropped
Be on time! Bring two pencils and an eraser!
Format: (1) Multiple choice (~50-60 questions) except for late arrivals (2)
Students arriving late will take a short-answer and essay exam on the same
material (3) May include interpretation of maps and diagrams
Content: Lectures 1-7 (inclusive) Introduction to Oceanography through Hot Spots