18.2 Eruptions
The composition of magma is what decides if the volcanic eruption is going to be explosive or quiet. Lava
can be thin and runny or thick and lumpy. Temperature, pressure and the presence of water affect
the formation of magma.
TEMPERATURE: Rocks melt at temperatures between 800oC (1,472oF) and 1200oC (2,192oF). These
temps can be found in the crust and upper mantle.
PRESSURE: Pressure increases with depth because of the weight of the overlying rocks. As pressure
increases so does the temp at which the rocks melt.
COMPOSITION of magma: As the amount of dissolved gases in magma increase, so does its explosivity.
The gases in magma that give it its explosive power are water vapor, carbon dioxide, sulfur dioxide and
hydrogen sulfide. Water vapor is the most common and determines where magma forms.
VISCOSITY: The resistance to flow is viscosity. Temp, and silica content affect the flow of magma. The
thicker and cooler the material the higher the viscosity, like chilled honey. Thick magma has a high silica
content and is sticky. Therefore, it is able to trap gases, which will produce explosive eruptions. The
thinner and warmer the material, the lower the viscosity, like warm syrup. Thin magma has a lower silica
content and is not able to trap gases, therefore it will flow easily and produce quiet, non-explosive
eruptions.
TYPES of MAGMA: There are three types of magma: Basaltic, andesitic, and rhyolitic
Basaltic: When rock in the upper mantle melts, it is basaltic magma. It has a low silica content, less than
50%, therefore a low viscosity. (thin and runny) Because the gases are able to escape, the eruptions are
quiet.
Andesitic: This type of magma is found along oceanic, continental subduction zones. The source
material for this magma is either from the oceanic crust or the continental crust. Its silica content is
average. 50-60%. The viscosity of andesitic magma is intermediate and therefore produces intermediate
explosions.
Rhyolitic: This type of magma has a silica content higher than 60%. It mixes with continental crust and
has the same composition of granite. It has a high viscosity which means the magma moves slowly.
Because of its high viscosity it is able to trap gases and therefore produces very explosive eruptions.
Explosive eruptions: If the magma cannot flow freely from the vent, it explodes, throwing lava and rock
into the air. The erupted material is called tephra. Tephra is classified by size. The smallest, less than
2mm in diameter is called ash. The largest are called blocks(bombs). Some blocks can be the size of
cars. Large explosive eruptions can disperse tephra over much of the planet. Ash can rise 40km (25mi)
into the atmosphere and can even change the weather. The 1991 eruption of Mt. Pinatubo in the
Philippines sent up a plume of ash 40km high. Tiny sulfuric acid droplets and particles remained in the
stratosphere for about two years, blocking the suns rays and lowering global temps.
Pyroclastic flows: Some tephra cause tremendous damage and kill thousands of people. Some eruptions
can send clouds of ash and tephra down a mountain slope at speeds of nearly 200km/h (124mi/h). The
rapidly moving clouds of tephra mixed with suffocating gases are called pyroclastic flows. These flows
can have internal temps of more than 700oC (1,292oF).