The Tails of Two Plumes
Monday, 9. February 2009, 14:54:31
Hans Christian Andersen once wrote a fairy tale about a little feather that grew into five hens (It's Quite True!). Mantle plumes may sound as another fairy tale - but is it?
The concept of mantle plumes was advanced in the 1970s to explain intra-plate or hot spot volcanism erupting far from any plate boundary. The concept is still being heavily discussed. Do mantle plumes exist? Where and how do they form? Are all, none, or some hotspot volcanoes associated with mantle plumes? What do they look like, anyway?
First a very brief description of the concept. Mantle plumes are plume-like upwelling currents of hot material from the core-mantle boundary (the D“ layer) - or maybe higher up from the junction of the upper and lower mantle - that finally erupts as hotspot volcanoes or flood basalts.
And what do they look like? One idea is a cavity plume with a large spherical head and a thin trailing tail. Another idea is a diapir plume with a thick tail.
The conditions required for the formation of these two types of plumes are different, and it may seem unlikely that both could coexist in a chemically more or less homogeneous mantle. But on the other hand maybe not so if they are formed in the layer just above the core-mantle boundary, also known as the D” layer (pronounced "dee double prime"). In recent years a better understanding of this layer is accumulating. The core-mantle boundary region is chemically heterogeneous, particularly where mantle plumes are thought to originate. The thickness of the layer also varies a lot from place to place - probably from 0 and 300 km thick.
According to a study just published in the GSA journal Geology (with FREE access!) under the title Tails of two plume types in one mantle the variable thickness of this layer at the base of the mantle can lead to the coexistence of the two mentioned distinct plume types.
The form of mantle plumes is governed by viscosity. A locally thick chemical layer leads to small viscosity variation instabilities and hence to diapir plumes. The diapir plume is characterised by a cylindrical stem with a diameter twice the thickness of the thermal boundary layer capped by a head only slightly larger in radius than the stem. A locally thin chemical layer allows for large viscosity variations across the active portion of the lower mantle thermal boundary layer and, hence, for cavity plume formation. The thin tail reflects the thickness of the lowest viscosity active part of the thermal boundary layer (i.e. the velocity boundary layer), which feeds the upwelling plume.
The authors expect weak hotspots (such as the Azores) to be associated with diapir plumes and strong hotspots (such as the Hawaii) to be associated with cavity plumes.
Reference:
Lenardic and Jellinek:
Tails of two plume types in one mantle
Geology 2009;37;127-130
doi:10.1130/G25229A.1
• http://geology.gsapubs.org/cgi/content/abstract/37/2/127?rss=1
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The concept of mantle plumes was advanced in the 1970s to explain intra-plate or hot spot volcanism erupting far from any plate boundary. The concept is still being heavily discussed. Do mantle plumes exist? Where and how do they form? Are all, none, or some hotspot volcanoes associated with mantle plumes? What do they look like, anyway?
First a very brief description of the concept. Mantle plumes are plume-like upwelling currents of hot material from the core-mantle boundary (the D“ layer) - or maybe higher up from the junction of the upper and lower mantle - that finally erupts as hotspot volcanoes or flood basalts. And what do they look like? One idea is a cavity plume with a large spherical head and a thin trailing tail. Another idea is a diapir plume with a thick tail.
The conditions required for the formation of these two types of plumes are different, and it may seem unlikely that both could coexist in a chemically more or less homogeneous mantle. But on the other hand maybe not so if they are formed in the layer just above the core-mantle boundary, also known as the D” layer (pronounced "dee double prime"). In recent years a better understanding of this layer is accumulating. The core-mantle boundary region is chemically heterogeneous, particularly where mantle plumes are thought to originate. The thickness of the layer also varies a lot from place to place - probably from 0 and 300 km thick.According to a study just published in the GSA journal Geology (with FREE access!) under the title Tails of two plume types in one mantle the variable thickness of this layer at the base of the mantle can lead to the coexistence of the two mentioned distinct plume types.
The form of mantle plumes is governed by viscosity. A locally thick chemical layer leads to small viscosity variation instabilities and hence to diapir plumes. The diapir plume is characterised by a cylindrical stem with a diameter twice the thickness of the thermal boundary layer capped by a head only slightly larger in radius than the stem. A locally thin chemical layer allows for large viscosity variations across the active portion of the lower mantle thermal boundary layer and, hence, for cavity plume formation. The thin tail reflects the thickness of the lowest viscosity active part of the thermal boundary layer (i.e. the velocity boundary layer), which feeds the upwelling plume.
The authors expect weak hotspots (such as the Azores) to be associated with diapir plumes and strong hotspots (such as the Hawaii) to be associated with cavity plumes.
Reference:
Lenardic and Jellinek:
Tails of two plume types in one mantle
Geology 2009;37;127-130
doi:10.1130/G25229A.1
• http://geology.gsapubs.org/cgi/content/abstract/37/2/127?rss=1
Subscribe to olelog by Email
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