What is a plume? By Julian Winter Alexandra Witze (2003)

Introduction Aims Primarily to confuse you; see conclusion Briefly look at modeling Understand their characteristics & evolution Conclusion; confused?

Birth of Plumes -Morgan 1971 Intra plate volcanism – failure of plate tectonics Fixed upward rise of hot, buoyant mantle Comes from the deep mantle Downward flow dispersed uniformly Column shape inferred from swell

Radiating Dykes Centrally located source Outward injection of magma Plume provides point source Found on Venus – no tectonics here But not all intraplate volcanism has such dykes! Richard E. Ernst 2004

Plume Modelling- Whitehead & Luther 1975 Liquids heated from below - TBL TBL less dense & less viscous Head and tail structure & entrainment Griffiths & Campbell 1990

Plumes Refined: Flood basalts - Campbell & Griffiths 1990 A plumes head can:- –Produce 500m-1000m uplift but later subsidence –Spread > km –Melt – Flood basalts –Evolution to plume tail melts But wait, flood basalts not found at all hotspots e.g. Hawaii!!!

Geochemistry Typically have enrichment in incompatible elements Higher concentrations of He 3 (high RA) Typically HIMU, EM-1 & 2 & FOZO Isotopic ratio distinguished from DM

Angelo Peccerillo

Subduction derived plumes 1. Suducted slabs drive convection Slabs reheated in lower mantle; bouyant Accounts for geochemistry

Subduction derived plumes 2. Slab sinks to a 1600km deep TBL Slab creates high spot  Plume Interface of TBL irregular Depth of plume can therefore be irregular thus plume can be both deep & shallow?

Subduction derived plumes 2. Kellogg et al 1999

Supersrwells e.g. Polynesia Anomalously shallow seafloor several 1000km in extent Unusually dense concentrations of hotspots SUPERPLUMES Doming of Superplume  Plumes

Superswells Davaille (1999)

Plumes: a summary Plumes from CMB Shallower origins & subduction driven Superswells & Superplumes ‘Top down’ models & shallow plumes –Function of plate related stresses –Shallow mantle upwelling

Conclusion Jules’ criteria for plume recognition: LIP but no worries if not. Monotic age progression; if it suits you. A DEEP source but a shallow source will do. A superswell e.g. Polynesia & Africa- they’re just showing off. Low seismic velocity? not if you don’t want. Dare I say ‘radial dykes’?

So a plume is…….? Morgan 1971 Griffiths & Campbell 1990 Kellogg 1999 Ivanov 2004 Daville (1999) Anderson, Foulger 2004

Questions?

Condie 2003

The plate tectonic paradigm Intra plate volcanism – failure of plate tectonics Hotspots –Linear tracks –age progression –Fixed position Why a hot mantle? Unrealistic!