The Rock Cycle
& Igneous Rocks
Enchanted
Rock
The Rock
Cycle
-Melting & Intrusion
-Solidification of melt
-Mountain Building
-Uplift & Exposure
-Weathering
-Erosion & Transport
-Deposition & Burial
-Metamorphism
-Melting & Intrusion
Fig 4.9
Genetic Classification of Igneous Rocks
• Intrusive: crystallized from slowly cooling
magma intruded within the Earth’s crust;
e.g. granite, gabbro.
Granite
Gabbro
The Rock Cycle
Extrusive
rocks
Intrusive
rocks
Fig 4.9
Rock
Texture
Fig. 5.1
Intrusive:
Course-grained,
interlocking
Genetic Classification of Igneous Rocks
• Extrusive: crystallized from rapidly cooling
magma extruded on the surface of the Earth
as lava, …
Rhyolite
Basalt
The Rock Cycle
Fig 4.9
Rock
Texture
Fig. 5.1
Intrusive:
Course-grained,
interlocking
Extrusive:
Fine-grained, discrete
crystals, often glassy
Genetic Classification of Igneous Rocks
• Extrusive: … erupted as pyroclastic
material, i.e., fragmented pieces of magma
ejected and cooled in the air.
Ash
Pumice
Scoria
Formation of Igneous Rocks
Pyroclasts
Extrusive
Intrusive
Porphyry:
Fig. 5.3
partially
crystalline
Composition and Classification
of Igneous Rocks
• Chemistry: e.g. % SiO2
• Mineralogy: e.g.
– Felsic - high silica (continental crust)
– Intermediate - intermediate silica
– Mafic - low silica (oceanic crust)
– Ultramafic - very low silica (mantle)
Felsic Igneous Rocks:
Igneous rocks rich in minerals high in silica
and low in iron and magnesium. They
come from thick, slow moving magma.
They include:
Granite
Rhyolite
Mafic Igneous Rocks:
Igneous rocks rich in minerals low in silica
and high in iron and magnesium. They
come from magma that is thinner, and
faster moving (more fluid). They include:
Gabbro
Basalt
Common Minerals
Intermediate Igneous Rocks:
Igneous rocks intermediate in composition
between felsic and mafic igneous rocks.
They include:
Granodiorite (Dacite)
Diorite (Andesite)
Ultramafic Igneous Rocks:
Igneous rocks with very low silica content,
consisting dominantly of mafic minerals.
The most common ultramafic rock is:
Peridotite (no extrusive equivalent)
Fig. 5.4
Granite
Quartz
Orthoclase
Granite
Biotite
Plagioclase
Compositional
Classification
Granite
Granodiorite
Granite
Compositional
Classification
Quartz
Amphibole
Plagioclase
Granite
Granodiorite
Compositional
Classification
Diorite
Granite
Plagioclase
Amphibole
Granite
Granodiorite
Compositional
Classification
Diorite
Granite
Gabbro
Plagioclase
Pyroxene
Granite
Granodiorite
Compositional
Classification
Diorite
Granite
Gabbro
Pyroxene
Olivine
Peridotite
Table. 5.2
Felsic
Intermediate
Mafic
Granite
Granodiorite
Diorite
Gabbro
Rhyolite
Dacite
Andesite
Basalt
Viscosity
Melting Temperature
How do magmas form?
When rocks melt (or partially melt).
When do rocks melt?
When the temperature exceeds the melting point
of the rock or some minerals within the rock.
Factors that Affect Melting of
Minerals (and Rocks)
• Composition: Felsic minerals melt at lower
temperatures than mafic minerals
• Pressure: Increased pressures raises melting
points
• Water Content: Increased water content
lowers melting points
Magma pools in
magma chamber
The Formation of
Magma Chambers
Magma rises
Buoyant melt migrates through rock
pores and fractures.
Less dense magma
Partial melting
Melt is less dense than solid. Low
density minerals tend to melt first.
Some minerals melt before others.
Results in mixture of melt and solid.
Igneous Intrusions…
• Plutons are masses of rock that forms
when magma cools inside Earth’s
interior.
• Plutons reach Earth’s surface after uplift
and/or weathering exposes them.
• Dikes
– A sheet of igneous rock that cuts through
previously formed rock vertically or at a
steep angle.
– Can be hundreds of kilometers long, and
can range from 1 cm to many meters thick.
• Sills
– A sheet of igneous rock that lies parallel to
the layers it goes through. Forms when
magma is forced between, not across rock
layers.
– Can be hundreds of metters thick, and
many km long.
• Laccoliths
– When magma doesn’t flow in sheets, but
bulges upward, creating a dome.
• Volcanic Neck
– The plug of hardened magma left after the
volcanic material around an inactive volcano
erodes away.
• Batholiths
– Largest of all plutons – the ‘core’ of many of
Earth’s mountain ranges.
– Usually made of either granite or granodiorite,
and can be very huge.
– A ‘Stock’ is a small exposed section of a small
batholith.
- Covers 15,400 mi2 !!
- Composition
- Granite
- Monzonite
- Granodiorite
- Diorite
- Even Gneiss
- All in one “magma chamber”