chapter 6 volcanoes and igneous rocks. volcanoes and igneous rocks
TRANSCRIPT
Chapter 6
Volcanoes and Igneous Rocks
VOLCANOES AND IGNEOUS ROCKS
Objectives• Identify several different categories of volcanic
eruptions.• Identify the volcanic hazards.• Describe how temperature, pressure, and water
content affect a rock’s melting point.• Identify three properties that distinguish one
lava from another.• Distinguish between and identify volcanic and
plutonic rocks.• Describe the most common plutonic formations.
Volcanoes and volcanic hazards
• Volcano– A vent through
which lava, solid rock debris, volcanic ash, and gasses erupt from Earth’s crust to its surface
– Can be explosive or nonexplosive
• Lava– Molten rock that
reaches Earth’s surface
• Magma– Molten rock, which
may include fragments of rock, volcanic glass and ash, or gas
Volcanoes and volcanic hazards
Eruptions, landforms and materials
• Eruption types– Shield volcano
• A broad, flat volcano with gently sloping sides, built of successive lava flows
• Produce flood basalts or basalt plateaus
Eruptions, landforms and materials
• Eruption types– Strombolian
• More explosive than Hawaiian• Create loose volcanic rock called spatter cones or
cinder cones
Eruptions, landforms and materials
• Eruption types– Vulcanian
• More explosive than Strombolian and, as a result, can generate billowing clouds of ash up to 10 km
• Produce pyroclastic flows
– Hot volcanic fragments (tephra) that, buoyed by heat and volcanic gases, flow very rapidly
Eruptions, landforms and materials
• Eruption types– Plinian
• Named after Pliny the Elder, who died during eruption of Mount Vesuvius
• Most violent eruptions, generating ash columns the can exceed 20 kilometers
• Produce steep sided volcanoes, called stratovolcanoes
– Composed of solidified lava flows interlayered with pyroclastic material.
– Steep sides that curve upward
Eruptions, landforms and materials
Eruptions, landforms and materials
Eruptions, landforms and materials
• Viscosity– The degree to which a
substance resists flow, • Less viscous liquid is
runny• More viscous liquid is thick
• Volcanic materials– Pyroclasts– Tephra– Ash– Agglomerates– Tuff
Eruptions, landforms and materials
• Other volcanic features– Craters– Resurgent dome– Thermal spring– Geysers– Fumaroles
Volcanic hazards• Primary effects
– Pyroclastic flows– Volcanic gases
• Secondary effects– Related to, but
not a direct result of, volcanic activity
• Fires• Flooding• Mudslides• Debris avalanche
Volcanic hazards
Volcanic hazards
Volcanic hazards• Tertiary and
beneficial effects– Change a landscape– Affect climate on
regional and global scale
– Renew mineral content and replenish fertility
– Geothermal energy– Provide mineral
deposits
Predicting Eruptions• Establish a volcano’s
history– Active– Dormant
• Monitor changes and anomalies– Earthquakes– Changes in shape or
elevation– Volcanic gases– Changes in ground
temperature– Composition of water
Predicting Eruptions
How, Why and Where Rocks Melt
• Heat and pressure inside Earth– Continental crust: temperature rises 30°C/km,
then about 6.7°C/km– Ocean crust: temperature rises twice as rapid
How, Why and Where Rocks Melt
How, Why and Where Rocks Melt
• Effect of temperature and pressure on melting
How, Why and Where Rocks Melt
• Heat and pressure inside Earth– Fractional melt
• A mixture of molten and solid rock
– Fractionation• Separation of melted
materials from the remaining solid material during the course of melting
How, Why and Where Rocks Melt
How, Why and Where Rocks Melt
• Magma– Molten rock below
surface
• Lava– Magma when it
reaches the surface– Differ in
composition, temperature and viscosity
How, Why and Where Rocks Melt
• Lava– Composition
• 45-75% of magma by weight is silica• Water vapor and carbon dioxide
– Temperature• Lavas vary in temperature between 750°C and 1200°C
• Magmas with high H2O contents melt at lower temperatures
– Viscosity• Lavas vary in their ability to flow• Influenced by silica content and temperature
How, Why and Where Rocks Melt
How, Why and Where Rocks Melt
• The tectonic setting– Lava characteristics influenced by location
• Oceanic, divergent margins– Lava is thin with a steep geothermal gradient
• Subduction zones– Typically have high water content and melt at lower
temperatures
• Hot- spots– Lava tends to be hot and basaltic– Build giant shield volcanoes
• Continental divergent margins are all different– High silica lava
How, Why and Where Rocks Melt
How, Why and Where Rocks Melt
How, Why and Where Rocks Melt
Cooling and Crystallization
• Crystallization– The process
whereby mineral grains form and grow in a cooling magma (or lava)
– Classified as:• Volcanic• Plutonic
Rate of Cooling• Rapid cooling:
Volcanic rocks and textures– Volcanic rock
• An igneous rock formed from lava
• Glassy• Aphanitic• Porphyritic• Pumice• Vesicular basalt
Rate of Cooling
Rate of Cooling• Slow cooling:
Plutonic rocks and textures– Plutonic rock
• An igneous rock formed underground from magma
– Phaneritic-a coarse grained texture
• Can have exceptionally large grains
Chemical composition
• Igneous rocks subdivided into three categories based on silica content– Felsic– Intermediate– Mafic
Fractional Crystallization
• Separation of crystals from liquids during crystallization– Bowens reaction series– Predictable melting and cooling of minerals
Fractional Crystallization
Plutons and Plutonism• Plutons
– Any body of intrusive igneous rock, regardless of size or shape
• Batholith– A large, irregularly
shaped pluton that cuts across the layering of the rock into which it intrudes
Plutons and Plutonism• Dikes
– Forms when magma squeezes into a cross cutting fracture and solidifies
• Sills– Magma that
intrudes between two layers and is parallel to them
Mount Saint Helens
Critical Thinking
• What factors might prevent magma from reaching Earth’s surface?
• What reasons can you think of for living near a volcano? Do you think the advantages outweigh the disadvantages?
• If you were to heat up a glass beaker full of crushed rock, the beaker would melt before you could finish studying the rock melting process. How do you think geologists study rock melting?