Laboratory Manual for Introductory Geology 4e

2.5.5 Hot Spots and Hot-Spot Tracks
Paleoclimate anomalies show that plates have moved, magnetic anomaly stripes
record the rate of seafloor spreading, and satellites measure the rates and directions
of plate motion today. But how can we determine whether a plate has always moved
at its current rate? Or whether it has changed direction over time? The answers
come from the study of hot-spot volcanic island chains.
FIGURE 2.13 shows how hot-spot island chains form. Each volcano in the chain
forms at a hot spot: an area of unusual volcanic activity not associated with processes
at plate boundaries. The cause of this activity is still controversial, but many geologists
propose that hot spots form above a narrowly focused source of heat called a mantle
plume: a column of very hot rock that rises by slow plastic flow from deep in the mantle.
FIGURE 2.13 Origin of hot-spot island chains and seamounts.
A volcano forms on a moving
plate above a mantle plume.
Active hot-spot
volcano #1
Plate
motion
The first volcano moves off
the plume and dies.
A second volcano
forms above the hotspot.
Mantle
plume
Extinct
volcano #1
Active hot-spot
volcano #2
Asthenosphere
Crust
Seamount
(remnant of
volcano #1)
Extinct
volcano #2
Active hot-spot
volcano #3
Time
Lithospheric
mantle
Plate movement carries
each successive volcano off
the hot spot.
Asthenosphere
Lithosphere
When the plume reaches the base of the lithosphere, it melts the lithosphere
rock and produces magma that rises to the surface, erupts, and builds a volcano.
The plume is thought to be relatively motionless. If the plate above it moves, the
volcano is carried away from its magma source and becomes extinct. A new volcano
then forms above the hot spot until it, too, is carried away from the hot spot.
Over millions of years, a chain of volcanic islands forms, the youngest at the hot
spot, the oldest farthest from it. As the volcanoes cool, they become denser, subside,
and are eroded by streams and ocean waves. Eventually, old volcanoes sink below
the ocean surface, forming seamounts. The chain of islands and seamounts traces
plate motion above the hot spot, just as footprints track the movement of animals.
The Hawaiian Islands, for example, are the youngest volcanoes in the Hawaiian–
Emperor seamount chain. Most of the older volcanoes in the chain are seamounts
46 CHAPTER 2 THE WAY THE EARTH WORKS: EXAMINING PLATE TECTONICS