Physical & Chemical Properties of Magmas

Temperature of Magmas

• Temperature (T) measurements from experiments and nature include:
  • Optical pyrometer
  • Thermocouple probe (used in lava lakes)
• Magmas crystallize over a range of temperatures:
  • General temperature range: $650$ to $1200$˚C
  • Can reach as high as approximately $1600$˚C
  • Mafic magmas are generally hotter than felsic magmas.

Pressure in Magmas

• Lithostatic pressure: pressure created by the overlying rocks, described by the formula: $P = \rho g h$
  • Continental crust density: $\rho = 2.7 g/cm^3$
  • Pressure increases at $265\, bars/km$ or $26.5\, MPa/km$
  • For every $1 kbar$ (approx. 10 bars), the thickness is about $3.8 km$
  • $1 GPa$ (10 kbars) corresponds to approximately $38 km$
  • Oceanic crust density: $\rho = 3.0 g/cm^3$
  • Pressure increases at $295\, bars/km$ or $29.5\, MPa/km$
  • For every $1 kbar$, the thickness is about $3.3 km$
  • $1 GPa$ corresponds to approximately $33 km$.

Viscosity of Magma

Composition and Polymerization

• X-ray studies indicate:
  • Silicon-oxygen networks in silicate glasses lead to chains and polymerization of silicon tetrahedra.
  • Network formers include:
  • Silicon (Si)
  • Aluminium (Al)
  • Oxygen (O)
  • Network modifiers include cations like H₂O, Fe, Mg, Ca, Sr, Ba, Li, Na, K, Rb, which can vary in effectiveness.

Types of Magmas

• Granite (Felsic) Magmas:
  • Composed of quartz and feldspars that link significantly, resulting in high viscosity.
• Basalt (Mafic) Magmas:
  • Contain less silica, more olivine and pyroxene, leading to a more unlinked tetrahedral structure and hence lower viscosity.

Viscosity Control Factors

• Temperature (T) and Pressure (P) affect viscosity:
  • As temperature increases, viscosity decreases (described as: $\text{Viscosity} \propto \frac{1}{\log T}$).
  • Example: Basalt magma 100 times more viscous at $950$˚C compared to $1200$˚C.
  • As pressure increases, viscosity generally decreases (due to Al becoming a network modifier at higher pressure).
  • Crystallization increases viscosity as more suspended crystals form.

Viscosity Measurements

• Measured for various magmas at $1200$˚C:
  • Olivine Basalt: $1 - 5 \times 10^3$ Poise
  • Andesite: $0.25 - 5 \times 10^4$ Poise
  • Rhyolite/Granite: $10^6 - 10^8$ Poise

Density of Magmas

• Density ranges from $2.2 - 3.1$ gm/cm³:
  • Composition is the most significant factor; more mafic magmas are denser.
  • Density decreases with increasing temperature and increases with pressure.

Crystal Settling and Magma Ascent

Settling Dynamics

• The speed of crystal settling compared to crystallization rates:
  • Crystals can settle quickly and may be carried by moving magma, impacting porphyritic texture formation.

Factors Influencing Ascent of Magma

• Factors affecting the ascent rate include:
  • Density difference between magma and country rock.
  • Viscosity of magma; low viscosity leads to higher ascent rates.
  • Width and roughness of the magma conduit, heat loss to surrounding rocks may also influence ascent speed.

Examples of Ascent Rates

• Diapir of granite $10$ km across rising through granitic crust at $500$˚C takes approximately $10$ million years.
• Rapid example: In Hawaii, activity shows ascent of $60$ km in about $2$ months.

Volatiles in Magma

Importance and Composition

• Volatile species include:
  • H₂O, CO₂, H₂S, SO₂, Cl, and F (HCl, HF).
• Water is the most abundant and critical volatile in magmas, influencing many properties of magma.

Solubility of Water

• Experimental determination of maximum H₂O content:
  • Basalt: $3.1 ext{ wt. } \, \%$ at $1000\, bars$ and $8.5\text{ wt. } \, \%$ at $5000\, bars$
  • Andesite: up to $4.5\%$ at $1000\, bars$ and $9.8\%$ at $5000\, bars$
  • Reactivity increases with decreasing lithostatic pressure while maintaining constant H₂O pressure.

Crystallization and Cooling Dynamics

Time Factors

• The rate of crystallization and cooling depends on:
  • Volume, shape, heat conduction, heat of crystallization, and country rock composition (e.g., H₂O content).
• Example: Granitoid batholiths may take approximately $1$ million years to crystallize and cool, while smaller intrusions have shorter timeframes due to less volume.