Download Lecture 4 Igneous Rocks

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Fig. 6.01b
J. D. Griggs/U.S. Geological Survey
Lecture 4 Igneous Rocks
• Three Main Rock Types
• Formation of Igneous Rocks
Where does magma come from?
The nature of volcanic eruptions
Volcanic products
Intrusive rock bodies
• Igneous rock textures
• Igneous rock classification
• Engineering considerations of igneous rocks
• Three Main Rock Types
• Rocks are divided into three main types depending on their
origin:
• Igneous rocks are cooled from a molten state.
• Sedimentary rocks are deposited in a fluid medium
(usually water).
• Metamorphic rocks are formed from preexisting rocks by
heat and pressure.
• Formation of Igneous Rocks
• magma: igneous rocks form from the cooling of molten (or
partially molten) rock materials called magma, which
consists liquid, dissolved gas, and crystals.
• lava: magma that reaches Earth's surface
• extrusive or volcanic: igneous rocks that form when
molten rock solidifies at the surface
• intrusive or plutonic: igneous rocks that form at depth.
W. W. Norton
• Where does magma come from?
•
The Earth's crust and mantle are composed of solid, not molten, rock. So what
is the source of magma that produces igneous activity? Magma is generated in
the lower crust and upper mantle (at depths of 50 to 200 km also) by (1)
raising the temperature, (2) reducing the pressure, (3) adding water. Thus,
magma forms in distinctive tectonic settings, generally related to plate
boundaries.
•
The temperature increases with depth (on average by 20-30 degrees centigrade
per kilometer in the upper crust known as the geothermal gradient). At
convergent plate boundaries as the oceanic crust descends into the mantle, it is
heated and dehydrates. The fluid reduces the melting temperature to cause
melting.
•
At divergent boundaries as hot mantle rock ascends, it moves to zones of
lower pressure to trigger melting even without additional heat.
Melting of rocks from
the heat of rising
magma and
decompression
W. W. Norton
The Earth’s
geotherm and
melting curve for
mantle rock
(peridotite). A rock
rises up from A to B
will start to melt
(known as
decompression
melting).
W. W. Norton
Melting by addition
of water and
volatiles.
W. W. Norton
The addition of water and volatiles decreases the
melting temperature of rocks. (W.W. Norton)
•
At divergent boundaries, hot mantle rock
ascends from the ashenosphere and moves
to zones of lower pressure as the
overlying lithosphere splits and moves
apart. This reduced pressure triggers
melting even without additional heat.
•
At convergent boundaries, as the oceanic
crust descends into the mantle, it is heated
and dehydrates. The fluid reduces the
melting temperature to cause melting.
Explosive Mt. St. Helens eruption 1980. (Photo by Austin Post of USGS)
• Why some eruptions are explosive and
some are "quiet"?
• The primary factors are viscosity and dissolved gas content.
The viscosity depends on temperature and silica content.
The lower the temperature or the higher the silica content,
the greater the viscosity. Very fluid basaltic (low silica)
magmas allow expanding gases to migrate easily out of
the vent, making the eruptions less violent.
• Viscosity describes resistance to shear during the motion of
a fluid.
• Volcanic products
• Lava flows: pahoehoe flows, and aa flows.
• pahoehoe: smooth, ropy surface, from low viscosity flows
• aa: rough, jagged blocks, from high viscosity flows.
• Lava tubes: lava conduits
(Left) pahoehoe flows.
(Right) aa flows.
• Corrections for the first paragraph of West
p.38: The correct one should be:
“... Pahoehoe forms ropy features on the surface
but aa forms a rough jagged blocky surface.
Pahoehoe shows a relatively smooth surface. This
illustrates that pahoehoe is the more fluid of the
two lavas and it yields thinner individual flows.”
View of an active lava tube as seen through the collapsed roof. (Photo
by Jeffrey B. Judd, USGS).
• Intrusive rock bodies: Igneous intrusions are masses of
rock formed when magma cools beneath the surface (generally called plutons).
They are classified according to their sizes, shapes, and relationships to the
rock they have invaded (known as the country rock). Important intrusive rock
bodies include:
•
batholiths: large exposures (over 100 km^2) of intrusive rock. Batholiths
typically form in the deeper zones of mountain belts and are exposed after
considerable uplift and erosion, e.g. Sierra Nevada batholith.
•
stocks: small plutons with an exposure area of less than 10 km^2.
•
dikes: narrow, tabular intrusive bodies. They are quite common. All dikes are
discordant, cutting across preexisting structure (such as bedding planes). Dikes
are related to the fractures caused by magma intrusions on the country rocks.
•
sills: tabular intrusive bodies formed when magma is injected along the
bedding planes of layered rocks.
•
laccoliths: lens-shaped intrusive bodies that formed when the injection of
magma arches up the overlying strata.
Basic igneous structures. A)
relationship between volcanic and
intrusive igneous activities. B)
intrusive igneous structures. C) A
stock and batholith are exposed
after uplifting and erosion.
Fig. 6.11de
Paul Hoffmann
The uniformity of the granite pluton makes carving
possible. (S. Marshak)
• Igneous rock texture
• The texture of an igneous rock describes the overall size,
shape, and arrangement of of its constituents. Texture
reveals a great deal about the environment in which the
igneous rock formed. The most important factor
contributing to the texture of igneous rocks is the rate at
which magma cools.
• Types of igneous rock texture
•
glassy texture: no distinct grains, conchoidal fracture, produced by rapid
cooling.
•
aphanitic (fine-grained) texture: grains not discernible with the naked eye,
relatively rapid cooling at the surface (Greek: phaneros -- visible).
•
phaneritic (coarse-grained) texture: grains discernible with naked eye, slow
cooling below the surface.
•
porphyritic texture: grains of two distinct sizes, representing two stage
cooling
•
pyroclastic texture: composed of rock fragments ejected by volcanic
eruptions.
•
pegmatitic texture: composed of unusually large crystals (> 1cm). Pegmatites
are formed at the last cooling stage of magmatic liquid; crystals are able to
grow in the fluid (water) rich final melt.
(UL) Aphanitic, (UR) phaneritic, (LL) porphyritic, (LR) glassy
• Igneous rock classification
• Igneous rocks are classified on the basis of texture and mineral
composition. The major kinds of igneous rocks are granite, diorite,
gabbro, rhyolite, andesite, and basalt.
•
A general relationship between color and mineral composition can be
used: Dark color generally indicates ferromagnesians; light color
generally indicates high silica content.
The classification of common igneous rocks is based on texture and
composition. The print size of the rock names is proportional to their
relative abundance at the Earth’s surface.
• Engineering considerations of igneous rocks
• An engineering siting must consider: 1) rock types and distribution; 2)
type of alteration after formation (tectonic fracturing, weathering).
• Unaltered intrusive igneous rocks are generally suitable for most
engineering projects because of the tight interlocking network of
mineral crystals.
• Problems for extrusive rocks: The water-bearing capacity is much
greater than intrusive rocks, making them unsuitable for reservoir or
tunnel construction. Extrusive rocks with pyroclastic materials are
much weaker.