E&ES 380: VOLCANOLOGY

Date: 20 October 2004

• Handout ‘Physical Properties of magmas and related issues of melting and crystallization (10/16/04)
• Textbook: Chapter 2, sections 2.3 (Basically Basalt, p. 33-36); 2.5 (Within plate volcanoes. ‘hotspots’ and mantle plumes, p. 39-50). Chapter 6, p. 137-141
• On-line Introduction to ‘Large Igneous Provinces: http://www.largeigneousprovinces.org
• On-line reading on ‘The Great Plume Debate:
  • Saunders: An alternative to the Alternative http://www.geolsoc.org.uk/template.cfm?name=Saunders
  • D. Anderson: Ptolemy, Piltdown, phlogiston, polywater and... plumes? http://www.geolsoc.org.uk/template.cfm?name=Anderson
  • G Foulger: Plumes, Plates and Popper http://www.geolsoc.org.uk/template.cfm?name=NakedEmperor
  • Saunders, G. Fitton and L. Coogan http://www.geolsoc.org.uk/template.cfm?name=StrikesBack

• On the Laki Giga Eruption (the Krafta Fires, ‘Skaftareldahraun’; textbook p. 140-141): http://volcano.und.nodak.edu/vwdocs/volc_images/europe_west_asia/laki.html
• On the Columbia River Flood Basalts (textbook p. 137-141) http://volcano.und.nodak.edu/vwdocs/volc_images/north_america/crb.html
• On the Great Plume Debate: http://www.mantleplumes.org/

• In the geological record, we know of much larger lava flows than have historically erupted (flood basalts: >1000 km³ per flow; flowing over >500-600 km distances), commonly occurring within very large lava-flows piles (volumes of millions of km³ lava), sometimes called ‘traps’.
• Areas with such large lava out-pourings are called Large Igneous Provinces (LIPs), representing episodes during which large volumes of mafic magmas were generated and emplaced by processes unrelated to “normal” sea-floor spreading and sub-duction (not clearly linked to plate boundaries).
• LIPs occur in many different tectonic settings including 1. continental flood basalts, 2. volcanic rifted margins, 3. oceanic plateaus, 4. ocean basin flood basalts, 5. submarine ridges, and 6. seamount chains. Note that LIPs are commonly, but not always, associated with regional-scale uplift and continental break-up. The millions of km³ of lava in LIPs were erupted in geologically short periods of time (few millions of years)
• Since Morgan’s 1971 paper (http://www.mantleplumes.org/Morgan1971.html) the production of such very large volumes of magma has been explained by decompression melting in mantle plumes, “localized (conduit ~100 km, plume head ~1000 km), roughly axisymmetric upwelling of buoyant (hotter than surrounding mantle) rock (melt), originating from a boundary layer deep within the Earth”. Or: “"A plume is an upwelling of hotter stuff from depth that carries a distinctive chemical and isotopic signature." Note that mantle plumes have been argued to occur on other planets. The boundary layer is commonly said to be the D” layer at the boundary between core and mantle (at ~2900km; compare to the depth of the upper-lower mantle boundary at ~650-670 km depth). The plumes originate as a small thermal irregularity.
• The existence of mantle plumes (resulting in places where there was alleged to be a very high heat flow at the surface of the Earth, ‘hotspots’) was said to be expressed in such features as the Hawaii-Emperor seamount and island chain (p. 41 textbook, fig. 2.21): volcanoes formed when a plates moves over a stationary, deep source of magma.
• Such linear chains of volcanoes etc. where then seen as linked to a LIP: when the plume head reaches the surface (diameter ~1000 km), LIPs are erupted. Afterwards, the ‘stem’ (conduit) with much smaller diameter sends in the lavas for the underseas ridge, chains of volcanoes, etc. The starting LIP for the Hawaiian-Emperor Seamount chain was thought to have been subducted (Kuriles).
• The magma erupted in LIPs was seen as being derived from great depths (core-mantle boundary).
• The number of plumes and hot spots proposed to exist has varied considerably, from the original 20 proposed by Morgan, to at least 49-50, but recently the number of ‘true, deep’ plumes has been said to be only 7 (Courtillot et al., 2003, EPSL 205, 295-308; doi:10.1016/S0012-821X(02)01048-8; if you want to look up this paper go to http://www.doi.org and paste the doi number in the appropriate box). It was argued that there are different types of plumes, only some of which are derived from the base of the mantle, others from the boundary interval between upper and lower mantle, still others from within the crust.
• Recently (in the last 3-4 years) the ‘Plume Paradigm’ has been strongly attacked, with many scientists now arguing that plumes do not exist (see reading on-line). The debate is being waged vigorously, with all evidence for the existence of plumes (geochemical and geophysical) being doubted (see also http://www.mantleplumes.org/FUA.html).
• The scientists denying the existing of plumes argue that LIPs formed at relatively shallow levels (<670 km; upper mantle), and that melting and outpouring is caused by within-plate effects of plate tectonic processes, such as stress, and that the differences in composition of various magmas (OIB, MORBS) are not caused be derivation from different depths, but by differences in upper mantle composition (‘fertility). ‘Hot spots’ are not ‘hot’; the low-velocity zones in the mantle are not caused by differing temperatures, but by differing composition (see figure, caption on next pages)
• Compare the left and right side of the figure provided in the hand out (Plume Model versus Plate Model).

A schematic cross-section of the Earth showing the plume model (to the left, modified from Courtillot et al., 2003, with additions from other sources) and the plate model (to the right). The left side illustrates three proposed kinds of hotspots/plumes. In the deep mantle, narrow tubes (inferred) and giant up-wellings coexist. Melting anomalies are localized by narrow upwelling plumes, which bring material from great depth to the volcanoes. In the various plume models the deep mantle provides the material and the deep mantle or core provides the heat for hotspots; large isolated but accessible reservoirs, rather than dispersed components, and sampling differences account for geochemical variability. Deep slab penetration, true polar wander, core heat and mantle avalanches are important. Red regions are assumed to be hot and buoyant; blue regions are cold and dense. Only a few hotspots are claimed to be the result of deep narrow plumes extending to the core-mantle boundary&endash;different authors have different candidates. The schematic is based on fluid dynamic experiments that ignore pressure effects and, of necessity, have low viscosity relative to conductivity. The right side indicates the important attributes of the plate model; variable depths of recycling, migrating ridges and trenches, concentration of volcanism in tensile regions of the plates, inhomogeneous and active upper mantle, isolated and sluggish lower mantle, and pressure-broadened ancient features in the deep mantle. Low-density regions in both the shallow and deep mantle cause uplift and extension of the lithosphere. Melting anomalies are localized by stress conditions and fabric of the plate and fertility of the mantle. Large-scale features are consistent with the viscosity-conductivity-thermal expansion relations of the mantle. In the plate model the upper mantle (down to about 1000 km, the Repetti Discontinuity) contains recycled and delaminated material of various ages and dimensions. These materials equilibrate at various times and depths. Migrating ridges, including incipient ridges and other plate boundaries, sample the dispersed components in this heterogeneous mantle. The upper 1000 km (Bullen’s Regions B & C) is the active and accessible layer. The deep mantle (Regions D and D”), although interesting and important, is sluggish and inaccessible. The geochemical components of MORB, OIB etc. are in the upper mantle and are mainly recycled surface materials. Red and blue regions are respectively low and high seismic velocity regions, not necessarily hot and cold, although some of the red regions at the top and base of the mantle are due to the presence of a melt.