Three key points about hotspots: 1) Hotspots are volcanic regions thought to be fed by anomalously hot mantle material rising independently of tectonic plate boundaries. Well-known examples include Hawaii, Iceland, and Yellowstone. 2) There are two hypotheses for the origin of hotspots - one involving mantle plumes rising from the core-mantle boundary, the other involving passive rising of melt from shallow depths. 3) Hotspot tracks provide evidence for the plate tectonics model, as volcanic chains like the Hawaiian Islands can be explained by plates migrating over fixed mantle plumes.

1. Hotspot
In geology, the places known as hotspots or hot
spots are volcanic regions thought to be fed by
underlying mantle that is anomalously hot compared with the
surrounding mantle.
Their position on the Earth's surface is independent of tectonic
plate boundaries.
There are two hypotheses that attempt to explain their origins.
One suggests that hotspots are due to mantle plumes that rise as
thermal diapirs from the core–mantle boundary.
The other hypothesis is that lithospheric extension permits the
passive rising of melt from shallow depths.
This hypothesis considers the term "hotspot" to be a misnomer,
asserting that the mantle source beneath them is, in fact, not
anomalously hot at all. Well known examples
include Hawaii, Iceland and Yellowstone hotspots.

3. • The vast majority of earthquakes and
volcanic eruptions occur near plate
boundaries, but there are some exceptions.
For example, the Hawaiian Islands, which
are entirely of volcanic origin, have formed
in the middle of the Pacific Ocean more
than 3,200 km from the nearest plate
boundary.
• How do the Hawaiian Islands and other
volcanoes that form in the interior of plates
fit into the plate-tectonics picture?

4. • In 1963, J. Tuzo Wilson, the Canadian geophysicist who discovered
transform faults, came up with an ingenious idea that became known as
the "hotspot" theory.
• Wilson noted that in certain locations around the world, such as Hawaii,
volcanism has been active for very long periods of time.
• This could only happen, he reasoned, if relatively small, long-lasting, and
exceptionally hot regions -- called hotspots -- existed below the plates that
would provide localized sources of high heat energy (thermal plumes) to
sustain volcanism.
• Specifically, Wilson hypothesized that the distinctive linear shape of the
Hawaiian Island-Emperor Seamounts chain resulted from the Pacific Plate
moving over a deep, stationary hotspot in the mantle, located beneath the
present-day position of the Island of Hawaii.
• Heat from this hotspot produced a persistent source of magma by partly
melting the overriding Pacific Plate

5. • The magma, which is lighter than the surrounding
solid rock, then rises through the mantle and crust to
erupt onto the seafloor, forming an active seamount.
• Over time, countless eruptions cause the seamount
to grow until it finally emerges above sea level to
form an island volcano. Wilson suggested that
continuing plate movement eventually carries the
island beyond the hotspot, cutting it off from the
magma source, and volcanism ceases.
• As one island volcano becomes extinct, another
develops over the hotspot, and the cycle is repeated.
• This process of volcano growth and death, over many
millions of years, has left a long trail of volcanic
islands and seamounts across the Pacific Ocean
floor.

6. • According to Wilson's hotspot theory, the volcanoes
of the Hawaiian chain should get progressively older
and become more eroded the farther they travel
beyond the hotspot.
• The oldest volcanic rocks on Kauai, the northwestern
most inhabited Hawaiian island, are about 5.5 million
years old and are deeply eroded.
• By comparison, on the "Big Island" of Hawaii –
southeastern most in the chain and presumably still
positioned over the hotspot -- the oldest exposed
rocks are less than 0.7 million years old and new
volcanic rock is continually being formed.

7. • Although Hawaii is perhaps the best known hotspot,
others are thought to exist beneath the oceans and
continents. More than a hundred hotspots beneath
the Earth's crust have been active during the past 10
million years.
• Most of these are located under plate interiors (for
example, the African Plate), but some occur near
diverging plate boundaries. Some are concentrated
near the mid-oceanic ridge system, such as beneath
Iceland, the Azores, and the Galapagos Islands.
• A few hotspots are thought to exist below the North
American Plate. Perhaps the best known is the
hotspot presumed to exist under the continental crust
in the region of Yellowstone National Park in
northwestern Wyoming.

8. • In 1963, J. Tuzo Wilson came up with the
idea that volcanic chains like the Hawaiian
Islands arise when plates migrate over hot
spots deep within Earth.
• Ten years later, W. Jason Morgan took
Wilson’s theory a step further when he
proposed that hot material rises up from the
core-mantle boundary in chimney-like
plumes to feed hot spots on the surface. .

9. • Most hotspot volcanoes are basaltic (e.g., Hawaii, Tahiti). As a result,
they are less explosive than subduction zone volcanoes, in which water
is trapped under the overriding plate.
• Where hotspots occur in continental regions, basaltic magma rises
through the continental crust, which melts to form rhyolites.
These rhyolites can form violent eruptions.
• For example, the Yellowstone Caldera was formed by some of the
most powerful volcanic explosions in geologic history. However, when
the rhyolite is completely erupted, it may be followed by eruptions of
basaltic magma rising through the same lithospheric fissures (cracks in
the lithosphere).
• An example of this activity is the Ilgachuz Range in British Columbia,
which was created by an early complex series
of trachyte and rhyolite eruptions, and late extrusion of a sequence of
basaltic lava flows.
• The hotspot hypothesis is now closely linked to the mantle
plume hypothesis.

23. Hotspot tracks: Global distribution
Volcanic chains and hotspot tracks:

24. Hotspot tracks: A view on the Pacific
A closer look at the Pacific:

25. Hotspot tracks: Hawaii
Linear increase of ages with
distance along the Hawaii-
Emperor chain.
Gradual decrease in
elevation with increasing
distance from the active
volcano.

26. Hotspot tracks: Hawaii
The oldest seamounts are
found at the northwest end,
poised to plunge beneath the
Aleutian volcanic arc, carried
downward with the oceanic
lithosphere as it is consumed.

27. Hotspot tracks: Hawaii
• Note the abrupt bend about
44 millions years before the
present, which indicates a
major reorganization of plate
motion at that time.
• While some think it was the
collision of India with the
Eurasian subcontinent, others
suggest it was the beginning
of spreading on the Antarctic
Ridge south of Australia.

28. Hotspot tracks: Hawaii
• Another remarkable
observation is that the eruption
rate for Hawaiian volcanoes has
remained quite constant over
most of the 65 million years of
preserved activity.
• This suggests that as volcanic
material being erupted, new
material is being supplied more
or less continuously from below.

29. Hotspot tracks: Hawaii
• For the 10 million years
following the bend, very little
lava erupted. This is a bit of a
bad situation for the previous
inhabitants of the islands, since
there is very little other dry land
for thousands of kilometers.
Indeed, almost all of the
previous life must have been
exterminated, so that the current
flora and fauna must have
arrived more recently.

30. Hotspot tracks: The plume model
Morgan’s plume model (Morgan, 1971):
• Volcanic islands are produced by
plumes rising through the mantle.
• The plumes come from the lower
mantle - and are therefore fixed.
• Plume flow derives the plates.

31. Hotspot tracks: The plume model
The topographic swell:
Hawaii
Figure from Ribe, 2004.
The sea floor surrounding the
Hawaii chain of islands is
anomalously shallow, relative
to normal sea floor of the
same age, over an area
about 1,200 km wide and
3,000 km long.

32. Hotspot tracks: The plume model
The topographic swell:
Bathymetry of the North Atlantic.
Iceland (shown in the center)
protrudes from the ocean basin
sitting on a large swell.

33. Hotspot tracks: The plume model
Seismic tomography:
Seismic images suggest that
some mantle plumes originate at
the lower mantle.
Figure from Montelli et al., 2004

34. Hotspot tracks: The plume model
Distinct geochemical
signature:
• The content of incompatible
elements is by 1 to 2 orders of
magnitude higher in Ocean
Island basalt (OIB, e.g. Hawaii,
EM-1 and HIMU) than it is in
Mid-Oceanic Ridge Basalt
(MORB).
• This implies different
reservoirs for OIB and MORB.
Figure from Hofmann, 1997

35. Hotspot tracks: The plume model
Distinct geochemical
signature:
• In general, Nd/Nd correlates
negatively with Sr/Sr.
• MORB data are at the upper-
left corner.
• The OIB are enriched in
incompatible elements with
respect to the MORB.
Incompatible rich
Incompatiblerich
Figure from Hofmann, 1997

36. Hotspot tracks: The plume model
Distinct geochemical
signature:
• The position of the OIB
between MORB and
continental crust suggests that
OIB source may be the result
of back mixing of continental
material into the mantle.
• How different chemical
reservoirs may still exist if the
mantle is undergoing global
mixing is yet an open question.
Figure from Hofmann, 1997

37. Hotspot tracks: The plume model
Association with flood basalt:
Morgan, in 1981, pointed out that a number of hotspot tracks originate in
flood basalt* provinces. He explained that flood basalt was produced
from a plume head arriving at the base of the lithosphere.
* Flood basalt are the largest known volcanic eruptions in the geologic record, and
typically comprise basalt of the order of 1 km thick over an area up to 2000 km across.

38. Hotspot tracks: The plume model
• The association of the
Deccan trap in India with the
Reunion hotspot track.
• The flood basalt eruption is
due to the arrival of the plume
head, and the hotspot track is
formed by the plume tail.
Figure from White and McKenzie, 1989
Figure from Dynamic Earth by G.F. Davies

39. Hotspot tracks: The plume model
Summary of the arguments supporting the notion of a rigid plate moving
atop of a deeply rooted mantle plume:
• The straightness of the hotspot tracks and the linear increase of
volcanic ages along the track.
• Topographic expression.
• The nearly constant eruption rate for Hawaiian volcanoes during the
past 65 million years suggest that as volcanic material being erupted,
new material is being supplied more or less continuously from below.
• Distinct chemistry for the OIB suggests deeper origin for the magma
source.
• Seismic tomography.

40. Hotspot tracks: The fixity of hotspots
Paleo-magnetic data
strongly suggests that all
of the lava solidified at
19.5 degrees north
latitude, precisely the
latitude of the hotspot
today. At least with respect
to latitude it would seem
that the Hawaiian hotspot
has been nearly fixed for at
least the past 65 million
years.

41. Hotspot tracks: The fixity of hotspots
That portions of island chains of
similar age are parallel to each
other suggests that the hotspots
themselves remain mostly fixed
with respect to each other,
otherwise the chains might be
expect to trend in different
directions as the plumes
generating them moved
independently.

42. Hotspot tracks: The fixity of hotspots
Parallel hotspot tracks within the
Indian Ocean.

43. Hotspot tracks: The fixity of hotspots
Summary of the geophysical arguments supporting the notion of fixity of
hotspots:
• Paleo-magnetic data indicate that the hotspot latitude has remained
fixed during the past 65 million years.
• Portions of island chains of similar age are parallel to each other
suggests that the hotspots themselves remain mostly fixed with respect
to each other.

44. Hotspot tracks: Absolute plate motion
1. We have seen that the relative motion between plates and plumes
may be inferred from the trend of hotspot tracks and the island ages.
2. Plumes are almost fixed.
3. From 1 and 2, it follows that hotspot tracks can be used to infer
absolute plate motion.

45. Hotspot tracks: A plume next to a mid-ocean?
• At present the age of the sea
floor beneath the Big Island is
roughly 95 millions years old.
• From the bend north along the
Emperor chain the age
difference steadily decreases
until it is less than 10 million
years for the oldest known
volcanoes in the chain.
• If the trend is continued back
to about 80 million years, it
would appear that the hotspot
was building volcanoes on
ocean floor of the same age.
Difference in age between the volcanoes and the underlying seafloor
as a function of distance along the island chain:

46. Hotspot tracks: A plume next to a mid-ocean?
Iceland is a modern example to a plume co-located with a mid-oceanic
ridge.
Iceland is the only place on Earth where an active mid-oceanic ridge is
exposed on land.

47. Hotspot tracks: Yellowstone
There is no reason why plumes be exclusively under oceanic
lithosphere and indeed several plumes are found in continental areas
too. The Yellowstone is one such example:

48. Hotspot tracks: Darfor-Levant volcanic array (Garfunkel, 1992)
Hotspot track in Israel:

49. Hotspot on Mars: Mt. Olympus
• Mars has no plate tectonics, so
hotspot volcanism results in building
huge volcanoes that dominate the
surface of the planet.
• The moving plates on the Earth
prevent any single volcano from sitting
over the hotspot long enough to build
such huge edifices.
• Earth's crust is also far too thin to
support a volcano as massive as
Olympus Mt..

50. Reference
• Plates vs. Plumes:A Geological
Controversy
Gillian R. Foulger