Warm Up 11/18
The broad, slightly dome-shaped volcanoes of Hawaii
are ____.
a. cinder cone volcanoes
c. pyroclastic volcanoes
b. composite cone volcanoes d. shield volcanoes
The most violent volcanic eruptions are associated
with what type of volcano?
a. composite cones
c. fissure eruptions
b. shield volcanoes
d. cinder cones
A magma’s viscosity is directly related to its ____.
a. age
c. silica content
b. depth
d. color
Answers: 1) d. 2) a. 3) c.
Intrusive Igneous Activity
Chapter 10, Section 2
Plutons
Plutons – structures that result from the
cooling and hardening of magma at depth
They can only be studied after uplift and
erosion have exposed them to the surface
Intrusive igneous bodies, or plutons, are
generally classified according to their
shape, size, and relationship to the
surrounding rock layers
Sills and Laccoliths
Sills and laccoliths are plutons that form when
magma is intruded close to the surface
Sill – forms when magma is injected along
sedimentary bedding surfaces, parallel to the
bedding planes
For a sill to form, the sedimentary rock must be
lifted to a height equal to the thickness of the sill
Laccoliths – forms when magma intrudes into
sedimentary bedding surfaces, collects as a
lens-shaped mass that pushes the overlying
strata upwards
Sills
Laccoliths
Concept Check
How are sills and laccoliths alike?
They are both plutons formed by magma
intrusions close to the surface
How are sills and laccoliths different?
They differ in shape and usually
composition
Dikes and Batholiths
Dikes – plutons that form when magma is
injected into fractures, cutting across
preexisting rock layers
Many dikes form when magma from a
large magma chamber invades fractures in
the surrounding rocks
Batholiths – the largest intrusive bodies,
must have a surface exposure greater
than 100 square kilometers
Batholiths may form the core of mountain
ranges
Dikes
Batholiths
Basic
Igneous
Structures
Origin of Magma
Earth’s crust and mantle are composed
primarily of solid, not molten, rock, and the
liquid iron in the outer core is too dense to
travel all the way to the surface
Geologists conclude that magma originates
when essentially solid rock, located in the
crust and upper mantle, partially melts
The most obvious way to generate magma
from solid rock is to raise the temperature
above the level at which the rock begins to
melt
The Role of Heat
The rate of change of temperature as you move
down into the upper crust is ~20-30º per
kilometer
Geothermal Gradient – the change in
temperature with depth
At a depth of ~100 km, temperature ranges from
1400ºC and 1600ºC, almost the melting point of
rock
Subduction zones add friction (another way to
produce heat) and water (which lowers the
melting temperature of rock) to give the added
heat needed to melt the crustal rocks
Hotter mantle rocks can rise and intrude into
crustal rocks, melting them
Geothermal Gradient
Concept Check
What is a geothermal gradient?
The change in temperature with depth
Magma Formation at Subduction
Zones
The Role of Pressure
Pressure also increases with depth
Melting occurs at higher temperatures as you go
deeper into Earth, because an increase in the
confining pressure causes an increase in the
rock’s melting temperature
Reducing that confining pressure lowers a rock’s
melting temperature
Decompression Melting – triggered when the
confining pressure drops enough to melt the
rock
Decompression Melting
Assignment
Read Chapter 10
Do Chapter 10 Assessment
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# 1 – 35 p. 303 – 304
# 1 – 7 p. 305