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Kinematics III: Hot-spots and plumes

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Kinematics III: Hot-spots and plumes . Important : This chapter follows mainly on chapter 2 and hilo.hawaii.edu/~kenhon/GEOL205. Hotspot tracks: Global distribution . Volcanic chains and hotspot tracks:. Hotspot tracks: A view on the Pacific . A closer look at the Pacific:.

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slide1

Kinematics III: Hot-spots and plumes

Important: This chapter follows mainly on chapter 2 and hilo.hawaii.edu/~kenhon/GEOL205

slide2

Hotspot tracks: Global distribution

Volcanic chains and hotspot tracks:

slide3

Hotspot tracks: A view on the Pacific

A closer look at the Pacific:

slide4

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.

slide5

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.

slide6

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.
slide7

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.
slide8

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.
slide9

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.
slide10

Hotspot tracks: The plume model

Hawaii

Figure from Ribe, 2004.

The topographic swell:

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.

slide11

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.

slide12

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

slide13

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

slide14

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

Incompatible rich

Figure from Hofmann, 1997

slide15

Hotspot tracks: The plume model

Figure from Hofmann, 1997

  • 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.
slide16

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.

slide17

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 Dynamic Earth by G.F. Davies

Figure from White and McKenzie, 1989

slide18

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 suggests that as volcanic material is 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.
slide19

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.

slide20

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.

slide21

Hotspot tracks: The fixity of hotspots

Parallel hotspot tracks also within the Indian Ocean.

slide22

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.
slide23

Hotspot tracks: Absolute plate motion

Question: In previous lectures we have discussed the relative plate motion. Can we infer absolute plate motions as well?

We have seen that the relative motion between plates and plumes may be inferred from the trend of hotspot tracks and the island ages.

Plumes are almost fixed.

From 1 and 2, it follows that hotspot tracks can be used to infer absolute plate motion.

slide24

Hotspot tracks: A plume next to a mid-ocean?

Difference in age between the volcanoes and the underlying seafloor as a function of distance along the island chain:

  • 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.

Question: how is that possible?

slide25

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.

slide26

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:

slide28

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..
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