Melt Migration Melt moves upward why Melt Migration

Melt Migration • Melt moves upward – why?

Melt Migration • Melt moves upward – why? – Buoyancy

Melt Migration • Melt moves upward – why? – Buoyancy • Why does most melt never reach the surface?

Melt Migration • Melt moves upward – why? – Buoyancy • Why does most melt never reach the surface? – Rock density generally lowers nearer the surface

Melt Migration • Melt moves upward – why? – Buoyancy • Why does most melt never reach the surface? – Rock density generally lowers nearer the surface – Eventually low density melt reaches equal density rocks – no buoyancy at this point.

Melt Migration • Melt moves upward – why? – Buoyancy • Why does most melt never reach the surface? – Rock density generally lowers nearer the surface – Eventually low density melt reaches equal density rocks – no buoyancy at this point. • Diapir = large, buoyant region of melt in asthenosphere – Rises into more cold/viscous mantle, then brittle crust – Dikes, sills, laccoliths, batholiths

Physical Properties of Magmas

I. Role of Solids, Liquids and Gas in Melts • Magma = liquid + crystals + volatiles +Xenos – No single Tmelt or Tcrystallization – All affect viscosity, explosivity

I. Role of Solids, Liquids and Gas in Melts • Magma = liquid + crystals + volatiles +Xenos – No single Tmelt or Tcrystallization – All affect viscosity, explosivity • Focus on liquid - Every melt is polymerized to some degree. More polymerized = more viscous

I. Role of Solids, Liquids and Gas in Melts • Volatiles – dissolved H 2 O lowers viscosity – breaks bonds linking tetrahedron

I. Role of Solids, Liquids and Gas in Melts • Volatiles – dissolved H 2 O lowers viscosity – breaks bonds linking tetrahedron • Volatiles out of solution (bubbles) or xtals – increases viscosity according to Einstein-Roscoe (Shaw, 1972; Marsh, 1981)

I. Role of Solids, Liquids and Gas in Melts • Volatiles – dissolved H 2 O lowers viscosity – breaks bonds linking tetrahedron • Volatiles out of solution (bubbles) or xtals – increases viscosity according to Einstein-Roscoe (Shaw, 1972; Marsh, 1981) hr = h 0 (1 -f. R)-2. 5 where hr = relative viscosity, h 0 = initial viscosity, R is constant (empirically ~1. 6 for magmas), and f = fraction of bubbles or xtals. If magma has 40% crystals or bubbles, how much more viscous? >1 order of magnitude

II. Effects of Volatiles • H 2 O, CO 2, SO 2, CO, H 2 S. . . – only H 2 O effect on melting has been studied in detail – Reduces solidus and viscosity of resulting melt

II. Effects of Volatiles • H 2 O, CO 2, SO 2, CO, H 2 S. . . – only H 2 O effect on melting has been studied in detail – Reduces solidus and viscosity of resulting melt • Lumped into one percent value (by weight) – Basalts and rhyolites have 5 -7% volatiles at depth – < 0. 5% (basalts) at surface – 0. 1 -7. 0% (rhyolite) at surface (if fast rise, too viscous for gas to escape – Most is H 2 O

II. Effects of Volatiles • Decreasing P = decreasing solubility – bubbles migrate to top of conduit/chamber – Faster rise if viscosity is low – Super slow for silicic – need 100’s 1000’s of years

III. Eruption Temperatures • Large Range – – Komatiite (Abitibi greenstone belt, Ontario) Komatiites (1400 – 1600 C) Basalts (1200 C) Rhyolites (~800 C) Carbonatites (<500 C) Carbonatite spatter cone, Oldoinyo Lengai (Tanzania)

III. Eruption Temperatures • Large Range – – Komatiite (Abitibi greenstone belt, Ontario) Komatiites (1400 – 1600 C) Basalts (1200 C) Rhyolites (~800 C) Carbonatites (<500 C) • Relates to composition – Mg/Fe-rich magma is hot – Si-rich magma is cooler Carbonatite spatter cone, Oldoinyo Lengai (Tanzania)

III. Eruption Temperatures • Large Range – – Komatiite (Abitibi greenstone belt, Ontario) Komatiites (1400 – 1600 C) Basalts (1200 C) Rhyolites (~800 C) Carbonatites (<500 C) • Relates to composition – Mg/Fe-rich magma is hot – Si-rich magma is cooler • Heat also affects viscosity by reducing polymerization Carbonatite spatter cone, Oldoinyo Lengai (Tanzania)

IV. Magma Behavior - Types of Stress • Compression • Tension • Shear