Download Chapter 4: Origin and Evolution of Igneous Rocks

Chapter 4:
Origin and
Evolution of
Igneous Rocks
Caption. (Figure X.X)
Fig. 4.20
OBJECTIVES
•
Describe how igneous rocks relate to the two other rock
groups (sedimentary and metamorphic).
•
Describe how magma forms and the factors that influence
magma’s ascent toward the surface and its cooling history.
•
Explain how magmas produce a variety of igneous rocks
with textures that vary according to the environment of
their formation.
•
Compare and contrast the different types of igneous rock
and explain the basis of their classification.
OBJECTIVES
•
Explain how the chemical composition and physical
characteristics of magma evolve as magma cools.
•
Identify the processes that cause volcanic eruptions and
the various types of volcanoes and volcanic rocks that
eruptions produce.
•
Explain the role that plate tectonics plays in the formation
and composition of magma and igneous rocks.
Igneous Rocks and the Rock Cycle
• Igneous rocks are one of the three main rock types (igneous,
sedimentary, metamorphic).
• Igneous rocks form through the solidification of molten or
partially molten rock (lava on the surface or magma
underground).
• Magma and Lava: mixture of liquid rock, solid crystals,
gases
• Igneous rocks constitute one stage of the rock cycle.
Igneous Rocks and the Rock Cycle
Igneous rocks form when other rocks melt, or partially melt, and then
solidify. Igneous rocks become sedimentary rocks through weathering,
erosion, deposition, and lithification. Igneous rocks become metamorphic
rocks through heating and/or compression.
Fig. 4.1
Igneous Rocks and the Rock Cycle
•
Geologists study igneous rocks to better understand
• Chemistry of igneous rocks
• Cooling history
• Composition of source rocks (and of Earth’s interior)
• Transport of magma toward the surface
• Volcanic eruptions
• Plate tectonics
Magma Formation and Transport
Three Main Steps of Formation
1) Partial melting of source rock
•
Increase in temperature
•
Decrease in pressure
•
Addition of volatiles
2) Transport of magma
•
Magma less dense than
surrounding rock
•
Rises through fractures
•
May “digest” wall rock
•
May cause collapse of surrounding rock
3) Crystallization of magma
•
Generally: decrease in temperature (cooling)
•
Sometimes: decrease in pressure (decompression)
•
Sometimes: increase in pressure
•
Sometimes: removal of volatiles
Fig. 4.7
Textures of Igneous Rocks
• Texture: size, shape, arrangement of crystals
Figs. 4.4,
4.14
Coarse-grained
Fine-grained
Glassy
Porphyritic (phenocrysts
in groundmass)
Textures of Igneous Rocks
• Two main types of igneous rocks:
• Intrusive (aka Plutonic): turn solid underground
• Extrusive (aka Volcanic): turn solid on the surface
• Texture is a result of melting, transport, and crystallization
history
Texture
History
Igneous Rock Type/
Environment
Coarse-grained
Generally slow crystallization
Intrusive (plutonic)
Fine-grained
Generally fast crystallization
Extrusive (volcanic)
Glassy
Turn solid almost instantaneously
Extrusive (volcanic)
Porphyritic
Mixture of crystals that form slowly
and those that form quickly
Intrusive (plutonic) or
extrusive (volcanic)
Textures of Igneous Rocks
Fig. 4.5
Igneous Rocks in the Field
•
Geologists study igneous rock formations in the field to
better understand
• Relative ages of formations
• Partial melting and the source of igneous rocks
• Magma transport
• Crystallization history
• Volcanic processes
Igneous Rocks in the Field
Fig. 4.10
Classifying Igneous Rocks
• Classified based on
• Composition
• Relative abundance of various minerals
• Relative amounts of oxides
• Texture (coarse, fine, glassy, porphyritic)
• Field relationships
• Four main groups based on mineralogy/chemistry (SiO2, Fe,
Mg) Group
SiO2 content
Fe, Mg content
Color
Felsic
High
Low
Light-colored
Intermediate
Intermediate
Intermediate
Medium or
speckled
Mafic
Low
High
Dark grey to
black
Ultramafic
Very Low
Very High
Dark Green to
Black
Classifying Igneous Rocks
Fig.
4.13
Evolution of Igneous Rocks
• Magmas change over time.
• Chemistry of the magma changes as crystals form.
• Mineralogy changes as crystals react with magma.
• Crystals settle out or separate from the magma
(fractionation).
• Magma mixes with other magmas.
• Magma incorporates wall rock (assimilation).
• Volatile content changes.
• Temperature changes.
• Viscosity changes.
• Bowen’s Reaction
Series
• Different minerals
are stable at
different pressure
and temperature
conditions.
• Crystals react with
the remaining
magma to form
new minerals.
Fig. 4.15
A magma may
fractionate through
the settling of crystals.
Magmas of different
compositions may mix
together.
Figs. 4.16,
4.18, 4.19
A magma can become more
viscous as minerals with more
complex structures become
stable.
Volcanic Eruptions
• Eruption styles and volcanic structures are related to
lava chemistry.
• Mafic lavas
• More fluid
• Less explosive eruptions
• Low shield volcanoes and small cinder cones
• Ropy lava, blocky lava, fissure vents, lava tubes,
lava fountains, pillow lavas
• Intermediate and felsic lavas
• More viscous
• More explosive eruptions
• Large, steep composite volcanoes
(stratovolcanoes)
• Blocks, bombs, lapilli, breccia,
tuffs, pumice, pyroclastic flows, lahars
Fig. 4.25
Volcanic Eruptions
Explosive eruptions are
associated with convergent
boundaries.
Effusive eruptions of basalt are associated with
seafloor spreading centers and oceanic hot spots.
Figs. 4.33, 4.24
Igneous Rocks and Plate Tectonics
• Most volcanoes form along plate boundaries.
• Tectonic environment affects magma composition and
volcano type.
Feature
Boundary Type
Lava Composition
Volcano Types
Mid-Ocean Ridge
Divergent (oceanocean)
Mafic
Low linear shield
Volcanic Arc
Convergent
(subduction zone)
Intermediate
Composite
Rift Volcanoes
Divergent
(continental rift)
Felsic to mafic
Composite
Ocean Island
Intraplate (not
along a boundary)
Mafic
Shield
Igneous Rocks and Plate Tectonics
Volcanism and Plate Boundaries: Igneous rocks are associated with
divergent and convergent boundaries.
SUMMARY
• Igneous rocks are one of the three main groups of rocks.
• Igneous rocks can evolve from or into metamorphic and
sedimentary rocks.
• Magma forms from the melting of existing rocks. Magma rises as
a result of its low density compared to surrounding rocks.
• Igneous rocks are classified based on composition, from ultramafic (silica poor and iron rich) to felsic (silica rich and iron poor).
• Igneous rocks are also classified based on texture, from glassy
(no crystals) to fine-grained (small crystals) to coarse-grained
(large crystals) and porphyritic (large crystals within a fine-grained
matrix). The texture of an igneous rock reflects its cooling history.
• Magma evolves from mafic to silicic as minerals crystallize within
it.
SUMMARY
• Magma changes in temperature, viscosity, volatile content, and
composition as it is transported toward the surface, mixes with
other magmas, assimilates surrounding rock, and fractionates.
• Eruption styles and volcanic structures are related to lava
chemistry.
• Mafic lavas generally result in gentle eruptions and low-profile
volcanic structures. Intermediate and felsic lavas generally result
in violent eruptions and steep volcanoes.
• Many igneous rock formations formed along divergent and
convergent plate boundaries, though some form within plates.
• The composition of an igneous rock reflects its tectonic
environment.
• Geologists study igneous rocks to better understand the
composition of Earth’s interior, igneous processes, and plate
tectonics.