 Explain how the composition of magma affects volcanic eruptions and lava flow.  Describe the five major types of pyroclastic material.  Identify the three main types of volcanic cones.  Describe how a caldera forms.  List three events that may signal a volcanic eruptions.

 2 general types of lava › Mafic: magma or rock rich in Mg and Fe; commonly dark in color  Commonly makes up oceanic crust  May also make up continental crust › Felsic: magma or rock that is rich in light- colored silicate materials  Makes up continental crust

 Viscosity – the resistance of a liquid to flow; determined by magma’s composition  This can affect the force with which a volcano may erupt  Mafic magmas are runny with low viscosity and typically cause QUIET eruptions (typically oceanic volcanoes – Hawaii)  Felsic magmas are very sticky and therefore highly viscous.  They can trap large amounts of dissolved gases, which leads to more EXPLOSIVE eruptions.

 Pahoehoe – mafic flows that cool & form a crust, continues to flow causing wrinkles in crust; “ropy” lava flows  Aa – forms if crust deforms rapidly or grows too thick to wrinkle, and surface breaks into jagged chunks. › Same composition but different textures due to gas content, rate and slope of flow  Blocky – Higher silica and is more viscous, forming large blocks of volcanic rock when cooled

 Felsic lavas of continental volcanoes such as Mount St. Helens tend to be heavier and stickier (more viscous)  Contain large amounts of trapped gases, such as carbon dioxide and water vapor  During an eruption, gases escape and molten and solid particles are thrown into the air › Pyroclastic material: fragments of rock that form during a volcanic eruption

 Particles are classified according to size  Volcanic ash : particles < 2mm in diameter  Volcanic dust : particles < 0.25 mm in diameter  Lapilli : “little stones” < 64 mm in diameter  Generally fall near the vent  Volcanic bombs : clumps of red-hot lava that spin and cool in the air; round or spindle  Volcanic blocks : solid rock blasted from the vent; may be as large as a small house

 Table 1, page 328  Lava &/or pyroclastic material ejected during both quiet &/or explosive eruptions build up around the vent  Forms 3 Types of Volcanic Cones › Shield volcanoes: Broad at base with gentle sloping sides; quiet eruptions; mafic lava › Cinder cones: Very steep slope; explosive eruptions with cones of pyroclastic material › Composite volcanoes: alt. layers of hardened lava flows and pyroclastic material; a.k.a stratovolcanoes ; lg. volcanic mtns.

 Large circular or basin-shaped depressions in the ground › Form when the magma chamber below a volcanic cone collapses › Figure 4, page 329 › Eruptions discharge large amounts of magma can cause a magma chamber to collapse › Krakatau: volcanic island in Indonesia exploded in 1883 forming a caldera with diameter of 6km › May fill with water to form lakes

 Earthquake activity may signal a volcanic eruption before it occurs › Due to increased pressure, increased temps, or fracturing of surrounding rock › Also increased strength and frequency of quakes  Patterns in Activity › Sides of volcano may bulge as magma moves › Instruments measure tilt of ground on slopes › Knowledge of previous eruptions › Daily measurements vs. past measurements