Physical properties of silicate melts W4L2 Flashcards
physcial behaviour of magma has effects on
Style of Eruption
• Volcano Architecture
• Form of Igneous Intrusions
silicate melt not
magma bcs can change above orbelow ground
properties depend on
(chemistry), temperature, density, • viscosity
properties are governed by
y deth of melt and variations in the chemistry of the melt, chiefly …
• silica content, composition (chemistry)
• volatile content
structure if melts
not structureless,
they have strong bonds
some degree of polymerisation between tetrahedras occurs
as melt cools the structure changes
polymer chains break down
• stable crystals begin to nucleate
Volatiles are
gases and liquids dissolved in magma at high Pressuree
main from volcanic magma : H2O, CO2, SO2,
Volatiles released when
are released or exsolved when P decreases
Melt temperatures
Basalt 1000---1200°C Andesite 950---1200°C Dacite 800---1100°C Rhyolite 700---900°C T decreases as silica increases
gravity controls
Gravity controls a magma’s ability to rise in the crust
therefore rising magma deflects reflects density
hot liquids bouycane
hot liquids are far more buoyant than cold solids!
degassing
on crystallisation, meltslose their volatiles – ‘degas’
i.e. melts will be less dense than their solid equivalents
Melt density
is also related to the degree of silica polymerisation
Melt densities decrease with increasing T
hot liquid risee rapidly through crust
Typical density rocks
Eclogite 3.3---3.5 Peridotite 3.1---3.4 Basalt 2.8---3.1 Diorite 2.7--3.0 Granite 2.5---2.8 ∂ decreases as silica increases
viscosity def
the consistency of a substance, resistance to flow
Stress and
Stress: - a measurement of the internal forces within a body and their resistance to deformation;
Strain: a measure of the extent to which a body is deformed when stress is applied
Fluid Dynamics, Newtonian Fluzids
Liquids like water behave “ideally”; they will flow spontaneously
Fluid Dynamics: Bingham Fluids
Silicate Melts do not behave ideally. They will only flow when a force is applied
Viscosity yield strength
Yield strength: In silicate melts, flow will not begin until a critical Yield Strength has been exceeded
Yield strength increases with an increase in silica content
viscisity measured in
Pa s (Pascal seconds)
N s m—2 (this is the SI unit — too small for geological applications)
Poise (dyne seconds per cm2— a dyne is the unit of measurement of tangential force)
factors that effect viscosity
melt viscositie changes with melt composition and temperature
other factors: volatile content, crystal content, bubble content, P , T
can actually see flow lines in crystals
Pressure is a control on water solubility in magmas
As P increases (depth) the potential of magmas to dissolve water increases
basalts formed from
Basalts are derived fromt he ‘dry’mantle
• they have low water contents
Granites formed
Granites are formed in continental crust
• they have high water contents
Degassing causes vescicles
Degassing: As magmas rise and pressure falls, volatiles exsolve as gas from the melt, and the magma undergoes degassing
The gas may not completely escape into the atmosphere and is entrained in lava as bubbles – these are known as VESICLES
Vesicles
ubbles
nuvelate when melt decompressed and volatiles exsolve
crystallisation begins → residual melt becomes relatively enriched in volatiles
The effects of bubbles on melts are variable and depend upon volatile content and composition of the melt
affects degre of vesiculation and bubble size and frequency
basalts —bubbles have minor effects or reduce viscosity
rhyolites — bubbles increase viscosity
Granite Viscosity
Granites are continental crustal melts; they are ‘wet’…
when magma rises = Plithostatic decreases
water exsolves, and crystallisation procedes
this results in a net increase of melt viscosity
Granitic magmas (rhyolites) are highly viscous, only flow short distances and erupt explosively …
