9.2 Magma and Erupted Materials Purpose: After completing this lesson we learn what materials erupt from volcanoes.

Types of Magma Magma is molten rock. The main “ingredient” in rocks is a mineral named silica (SiO2); besides silica, there are many other minerals. Magma also contains significant amounts of fluids dissolved in the molten rock: water vapors and carbon dioxide are most common Silica, however, is the one that determines the type of magma in every igneous activity. Silica determines the viscosity (resistance to flow) of magma as well.

There are three types of magma, depending on the content of silica: 1. Basaltic 2. Andesitic 3. Rhyolitic

Basaltic Magma Some characteristics of basaltic magma are: it has about 50% silica has a small amount of gases dissolved flows easily (least viscous) the eruption is rarely explosive it has the highest melting point (>1300 C degrees)

Basaltic Eruption Lava flows easily, due to reduced viscosity

Andesitic Magma contains about 60% silica Has a moderate amount of gasses Sometimes it generates explosive eruptions The melting temperature is around 1000 C degrees

Andesitic Eruption This is an explosive eruption

Rhyolithic Magma Contains most silica ( over 70%) Contains a significant amount of dissolved gasses It has a high viscosity (flows very slowly) The eruptions are usually explosive and create tremendous damage. The melting point is below 900C degrees.

Rhyolithic Eruption This type is highly explosive

Location of Each Type of Magma Each type pf magma form in certain areas: Basaltic magma is found at rifts and hot spots Andesitic magma is found at subduction boundaries Rhyolitic magma is found at continental hot spots.

Magma versus Lava Magma is the molten material that rises from the deep. It contains a significant amount of volatile compounds; water vapors and CO2 are most common. As the magma reaches the surface of the earth, it loses very quickly the volatiles, because of the difference in pressure. (Shake a can of soda and open it quickly; the fizz is nothing else but the CO2 which is released from the liquid) Lava is magma which has lost most of its volatile components. Lava has temperatures between 700 and 1200 C degrees.

Other Volcanic Products Lava is the main product that is released from a volcano. Pyroclastic materials (“pyro” means hot and “clast” means fragment in Greek). These are hot, solid materials of different sizes. Sometimes the lava cools down on the pipe, creating a plug; the magma is climbing up the pipe at great pressures, and breaks apart this plug, creating fragments of different sizes. The most common pyroclastic materials are: -volcanic ashes (the particles have less than 2 mm in diameter) -lapilli- pebbles with diameters between 2-64 mm - bombs or blocks, are fragments larger than 64 mm

Types of Lava Flow Pahoehoe is lava with smooth, ropelike surfaces.

Pillow Lava Lava that cools under water has a rounded, pillow like form.

Aa Aa is lava that cools quickly with a rough, jagged surface.