Physical & Chemical Properties of Molten

Rock

Physical & Chemical Properties of Molten Rock

The most abundant chemicals in magma are silicon and oxygen. •Silica- Chemical Formula: Si + O2=SiO2

1 Silicon Atom + 2 Oxygen Atoms= 1 Silica Molecule

Physical & Chemical Properties of Molten Rock

Silica molecules stick together and form long chains. They get tangled together and the molecules don’t slide past each other easily.

Physical & Chemical Properties of Molten Rock

Viscosity: the ability of a substance to resist flow (Water vs. Molasses)

Viscosity Determines if a volcano will have: pyroclastic flow or lava flow.

Physical & Chemical Properties of Molten Rock

Magma and lavas viscosity is controlled by: temperature, composition and gas content.  The higher the silica content of magma, the more viscous.

Lava that is low in silica is less viscous.

When the minerals in rocks melt they tend to form gases:Mostly H2O (water vapor) & some CO2 (carbon dioxide)Minor amounts of Sulfur, Chlorine, and Fluorine gases

Physical & Chemical Properties of Molten Rock

These gases do have an effect on viscosity…their more important role is that they provide the driving force for volcanic eruptions. The more viscous it is the more likely it is for gas pressure to build.  The combination of lots of dissolved gases and high viscosity created conditions for the most explosive and deadly volcanoes.

Physical & Chemical Properties of Molten Rock

Low silica lava flows such as Kilauea, Hawaii happen weekly or daily and the eruptions are far less explosive. These types of lavas are called basaltic lavas. 

Intermediate to high silica magmas produce violent eruptions such as Mt St. Helens, Krakatoa, and Pinatubo. These eruptions happen every several hundred to thousands of years.Theses types of lavas are called andesitic and granitic lavas.

Form of Volcano

Type of Magma

Shape of Volcano Materials in Volcano

Shield BasalticLow SiO2

gentle slope lava

Cinder Cone

AndesisticMedium SiO2

steep sided Tephra- ash, cinders, rocks, & bombs

Composite AndesisticHigh SiO2

Intermediate slope -large mountain

tephra & lava

Eruptions and Forms of Volcanoes