CHAPTER 18: VOLCANOES SECTION 1: MAGMA Objectives: Standards:

CHAPTER 18: VOLCANOES SECTION 1: MAGMA is a mixture of molten rock, suspended mineral grains, and dissolved gasses deep below Earth’s surface Magma forms when: Temperatures are high enough to melt rock (between 800 oC and 1200 oC ) Temperatures exist at the lithosphere/asthenosphere border Magma is less dense than the surrounding rock Forced upward

FACTORS THAT AFFECT MAGMA FORMATION TEMPERATURE: Temperatures increases with depth. This is why the asthenosphere is plastic-like. PRESSURE: Pressure increases with depth due to weight of rock above. Pressure explains why rocks in Earth’s lower crust and upper mantle do not melt even though the temps are high enough WATER CONTENT: Water can be found in the spaces of rocks. A wet mineral or rock will melt at a lower temp than a dry rock.

CHARACTERISTICS OF BASALTIC MAGMA: Rocks in upper mantle melt and rises to crust quickly Low viscosity – fluid magma Small amounts of dissolved gases Small amounts of silica Quiet/non-violent eruptions

CHARACTERISTICS OF ANDESITIC MAGMA: Found along continental-oceanic subduction zones Oceanic crust and/or oceanic sediments melt 60 % silica Intermediate viscosity – thicker than basaltic and intermediate gas content Intermediate eruptions

RHYOLITIC/GRANITIC MAGMA CHARACTERISTICS OF RHYOLITIC/GRANITIC MAGMA: Continental crust Rich in silica and water content High viscosity – thickest magma, slows movement Large amounts of trapped gases Very explosive eruptions

VISCOSITY AND HOW IT AFFECTS THE EXPLOSIVENESS OF MAGMA Viscosity: A substance’s internal resistance to flow. EX: water has a lower viscosity than honey and flows more easily. Honey has a higher viscosity than water because it does not flow as easily. Viscosity and explosiveness of magma: The lower the viscosity, the more fluid-like the magma, the least explosive eruption. BASALT Intermediate viscosity, intermediate magma, intermediate explosion. ANDESITIC The higher the viscosity, the less fluid-like the magma – magma gets caught and pressure builds up causing a more explosive eruption. RHYOLITIC/GRANITIC

TABLE 8.1 – PG. 474. Basaltic Upper mantle Low 1-2% About 50% Least Composition Source Material Viscosity Gas content Silica content Explosive-ness Location of Magma Basaltic Upper mantle Low 1-2% About 50% Least Both oceanic and continental crust Andesitic Oceanic crust and oceanic sediments Inter- mediate 3-4% About 60% Intermediate Continental margins associated with subduction zones Rhyolitic/ Granitic Continental crust High 4-6% About 70% Greatest

SILICA CONTENT AND VISCOSITY OF EACH TYPE OF MAGMA Hotter magma or lava = lower viscosity Basaltic: temp. between 1000 oC and 1250 oC – low viscosity Rhyolitic: temp between 700 oC and 900 oC – high viscosity High amounts of silca = higher viscosity Basaltic: low amounts of silica = low viscosity Rhyolitic: high amounts of silica = high viscosity Andesitic: intermediate amounts of silica = intermediate viscosity