Chapter 18 Volcanic Activity

Magma A mixture of molten rock, suspended mineral grains, and dissolved gases deep beneath Earth’s surface. Forms when temperatures are high enough to melt rock 800°C-1200°C

Factors That Affect Magma Formation Temperature and Pressure As pressure increases the temperature at which a substance melts increases. Presence of water Lowers the melting point temperature

Three Major Types of Magma Basaltic magma Least explosive Andesitic magma Rhyolitic magma Most explosive Magmas differ in the source rock from which they form, viscosity, silica content, gas content and explosiveness

Intrusive Activity Affects Earth’s Crust Magma can force rock apart and enter newly formed fissures (cracks) Magma can cause rocks to break off and sink into the magma chamber Magma can melt rock into which it intrudes

Types of Intrusions Batholiths, stocks, sill, dikes, and laccoliths are plutons that are classified according to their size shape and relationship to surrounding rock. Batholiths are the largest plutons and often form the cores of many of Earth’s major mountain chains.

Volcanoes Lava flows onto Earth’s surface through a vent Multiple lava flows can accumulate to form a volcano A crater is a depression that forms around a vent at the summit of a volcano A caldera is a large crater that forms when a volcano collapses during or after an eruption.

Three Types of Volcanoes Shield volcanoes Cinder cone volcanoes Composite cone volcanoes

Tephra Rock Fragments ejected from a volcano

Where Volcanoes Form Convergent and divergent plate boundaries Hotspots Usually hot areas in the mantle that are stationary for a long period of time Flood basalts when lava flows from fissures to form flat plains or plateaus