Physical properties of silicate melts W4L2

Question 1

physcial behaviour of magma has effects on

Answer 1

Style of Eruption
• Volcano Architecture
• Form of Igneous Intrusions

Question 2

silicate melt not

Answer 2

magma bcs can change above orbelow ground

Question 3

properties depend on

Answer 3

(chemistry), temperature, density, • viscosity

Question 4

properties are governed by

Answer 4

y deth of melt and variations in the chemistry of the melt, chiefly …
• silica content, composition (chemistry)
• volatile content

Question 5

structure if melts

Answer 5

not structureless,
they have strong bonds
some degree of polymerisation between tetrahedras occurs

Question 6

as melt cools the structure changes

Answer 6

polymer chains break down

• stable crystals begin to nucleate

Question 7

Volatiles are

Answer 7

gases and liquids dissolved in magma at high Pressuree

main from volcanic magma : H2O, CO2, SO2,

Question 8

Volatiles released when

Answer 8

are released or exsolved when P decreases

Question 9

Melt temperatures

Answer 9

Basalt 1000---1200°C 
Andesite 950---1200°C 
Dacite 800---1100°C 
Rhyolite 700---900°C T 
decreases as silica increases

Question 10

gravity controls

Answer 10

Gravity controls a magma’s ability to rise in the crust

therefore rising magma deflects reflects density

Question 11

hot liquids bouycane

Answer 11

hot liquids are far more buoyant than cold solids!

Question 12

degassing

Answer 12

on crystallisation, meltslose their volatiles – ‘degas’

i.e. melts will be less dense than their solid equivalents

Question 13

Melt density

Answer 13

is also related to the degree of silica polymerisation
Melt densities decrease with increasing T
hot liquid risee rapidly through crust

Question 14

Typical density rocks

Answer 14

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

Question 15

viscosity def

Answer 15

the consistency of a substance, resistance to flow

Question 16

Stress and

Answer 16

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

Question 17

Fluid Dynamics, Newtonian Fluzids

Answer 17

Liquids like water behave “ideally”; they will flow spontaneously

Question 18

Fluid Dynamics: Bingham Fluids

Answer 18

Silicate Melts do not behave ideally. They will only flow when a force is applied

Question 19

Viscosity yield strength

Answer 19

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

Question 20

viscisity measured in

Answer 20

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)

Question 21

factors that effect viscosity

Answer 21

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

Question 22

basalts formed from

Answer 22

Basalts are derived fromt he ‘dry’mantle

• they have low water contents

Question 23

Granites formed

Answer 23

Granites are formed in continental crust

• they have high water contents

Question 24

Degassing causes vescicles

Answer 24

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

Question 25

Vesicles

Answer 25

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

Question 26

Granite Viscosity

Answer 26

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 …