ChapterRocks
5 : Rocks
fromliquids
molten
from molten
liquids
Including adaptions from Dupre and
Copeland (2004)
Andesite - granite at the
surface
Hand sample-Santiago de Chile
Microscope
slide
Genetic Classification of
Igneous Rocks
• Intrusive: crystallized from
slowly cooling magma intruded
within the Earth’s crust; e.g.
granite, gabbro
Genetic Classification of
Igneous Rocks
• Extrusive: crystallized from
rapidly cooling magma extruded on
the surface of the Earth as lava or
erupted as pyroclastic material.
Extrusive Igneous Rocks
Include:
• rocks formed from the cooling of
lavas
• rocks formed by the cooling of
pyroclastic material, i.e. fragmented
pieces of magma and material erupted
into the air
Last Days of Pompeii-Karl Briullov-Copyright © 1999. George Mitrevski.
Auburn University
Mt St Helens
crater 08-780, 60 mph,
800 deg Fpyroclastic
flow- USGS
Composition and Classification
of Igneous Rocks
• Chemistry: e.g. % SiO2
• Mineralogy: e.g.
–
–
–
–
Felsic (Feldspar and Silica)
Intermediate
Mafic (Magnesium and Ferric)
Ultramafic
Why Figure 5.4 I so important
• The color and mineral distribution
indicate an increasing density and
melting temperature. Darker igneous
rocks generally weigh more and are
formed at higher temperatures and
pressures. This reflects the densitystratification of the whole Earth!
Felsic Igneous Rocks:
-Igneous rocks rich in minerals
high in silica. They include:
Granite
Rhyolite
Intermediate Igneous
Rocks:
-Igneous rocks in between in
composition between felsic and mafic
igneous rocks. They include:
Granodiorite
Dacite
Diorite
Andesite
Mafic Igneous rocks
-very low silica content, and consist
primarily of mafic minerals. The most
common ultramafic rock is:
Peridotite
What controls the melting
temperatures of minerals?
*External pressure
*and Water content
*Internal composition
(including internal
water content)
Factors Affecting Melting
of Minerals (and Rocks)
• Pressure: Increased Pressures raises
melting points
• Water Content (internal and external
to the mineral): Increased Water
Content lowers melting points
• Composition: Felsic minerals melt at
lower temperatures than mafic
minerals
How do magmas form?
When rocks melt (or partially melt).
Why do rocks melt?
When the temperature exceeds the
melting point of the rock or some
minerals within the rock.
Partial Melting
--Occurs when some of the
minerals forming a rock melt at
lower temperatures than other
minerals within the same rock
If different minerals melt at
different pressures that means
that different minerals become
solid at different temperatures
too.
What does this imply about
the internal composition of a
magma body as it cools???
Fractional
Crystallization!
Magmatic
Differentiation ?
What is
If, during fractional crystallization, the remaining
magma were to erupt it would be
(a) more felsic or
(b) more mafic than the original magma????
What do we know about the
shape of magmatic bodies?
Are they sheets? Blobs?
Spheres?
“Magmatic Pipes -101”
• Dykes are near-vertical
• Sills are horizontal and squeeze in
between other layers of rock
• Plutons are deep (km) bodies of
solidified magma. An example of a
plutonic rock is granite or gabbro.
www.jemmoore.com/flying/ shiprock.html
Summary of Volcanoes
• Subduction volcanoes in continental
crustal and oceanic crust at
convergent plate margins
• Hot spot volcanoes (anywhere)
• decompression volcanoes at divergent
plate margins
END of CHAPTER 5
IGNEOUS ROCKS-rocks from
liquid melts