Earth Science 10.3 Intrusive Igneous Activity
Intrusive
Igneous
Activity
Classifying Plutons:
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Volcanic rock that cools and hardens
covers much of Earth’s surface.
Most magma, however, cools and
hardens deep within the Earth.
This magma forms the roots of
mountain ranges and a variety of
landscape features.
Classifying Plutons:
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Recall that magma rises through the
crust toward the surface.
As it rises, it may rise through
fractures in the rock or force it’s
way between layers of rock.
The magma may form thin sheets a
few centimeters thick or collect in
vast pools that can be kilometers
wide.
All these formations have various
names; dikes, sills, laccoliths and
batholiths.
Classifying Plutons:
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Structures resulting from the cooling
and hardening of magma beneath
Earth’s surface are called plutons.
The word pluton is derived from
Pluto, the Roman god of the
underworld.
Plutons form in continental crust
wherever magma slowly crystallizes
and forms intrusive igneous rock.
Intrusive igneous rock = rock that
forms as magma cools deep within the
Earth.
Extrusive igneous rock = forms once
magma emerges as lava and cools
above earth’s surface.
Classifying Plutons:
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Over millions of years, uplift and
erosion can expose plutons to the
world above the surface.
There are several types of
plutons:
 Sills
 Laccoliths
 Dikes
 Batholiths
Sills and Laccoliths:
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Sills and laccoliths are plutons that
form when magma intrudes
between rock layers close to the
surface.
Sills and laccoliths differ in shape
and often differ in composition.
A sill is a pluton that forms when
magma flows between parallel
layers of sedimentary rock.
Horizontal sills are the most
common to be found.
Sills and Laccoliths:
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Sills form only at shallow depths,
where the pressure from the
weight of the overlying rock is low.
For a sill to form, the magma must
lift the overhead rock to a height
equal to the thickness of the sill.
While this takes energy; forcing
the magma between rock layers
requires less energy than forcing
the magma all the way up to the
surface.
Exposed Sill
Sills and Laccoliths:
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A laccolith is a lens-shaped pluton
that has pushed the overlying rock
layers upward.
Like sills, laccoliths form when
magma intrudes between
sedimentary rock layers close to
the surface.
The magma that forms laccoliths
has a higher viscosity (thickness)
than the magma that forms sills.
For this reason, the magma that
collects bulges upward instead of
spreading out in a thin layer.
Exposed laccoliths
Dikes:
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Some plutons form when magma
from a large magma chamber
moves into fractures in the
surrounding rocks.
A dike is a pluton that forms
when magma moves into
fractures that cut across rock
layers.
Dikes are sheet-like structures
that can range in thickness from
less than a centimeter to more
than a kilometer.
Most dikes are a few meters
thick and extend for no more
than a few kilometers.
Two examples of dikes
Batholiths:
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Batholiths are very large bodies
of intrusive igneous rock.
A batholith is a body of
intrusive igneous rock that has a
surface exposure of more than
100 square kilometers.
Much larger than a pluton, a
batholith can be hundreds of
kilometers long and tens of
kilometers across.
Batholiths:
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Gravity studies and seismic
evidence indicate that
batholiths are very thick,
sometimes extending tens of
kilometers downward into the
crust.
A body of igneous rock similar
to a batholith but having an area
less than 100 kilometers is
called a stock.
How are Batholiths formed?
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Batholiths form from many
individual plutons that begin as
blobs of magma deep beneath
the surface.
The plutons slowly rise through
the crust in the form of hot
magma.
They clump together forming a
huge irregular mass of
underground magma, molten
rock.
How are Batholiths formed?
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This magma never works it’s way
to the surface. Instead, it cools
and hardens underground
forming granitic rock.
Over millions of years, uplift and
erosion gradually expose the
batholith at the surface.
Batholiths form the core of the
Earth’s great mountain ranges;
from the Northern Rockies to
the Sierra Nevada Mountains in
southern California.
How are Batholiths formed?
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Examples of batholiths
that have been exposed
by erosion
Batholiths can be huge,
extending hundreds
of miles
Half Dome Yosemite
Sierra Nevada batholith
Computer Lab Assignment:
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Use the internet to research and write 3
paragraphs about one of the following
geological formations. Describe how it was
formed. DO NOT COPY CUT OR PASTE: each
paragraph must be a minimum of 4 sentences.
 The Giants Causeway in Northern Ireland
 The Northern Canadian Rocky Mountains
 Devil’s Tower, Wyoming
 Mount Shasta, California
 Crater Lake, Oregon