Download Metamorphism - Mr. Snelgrove

Metamorphism
 Metamorphism literally means to “change form”. These
changes take thousands to millions of years to complete.
During this period of change, minerals within the rocks
“flow in solid state” (i.e. NO MELTING) and can change
on the atomic scale.
 Metamorphism most often occurs during one of two
geologic events, which include: 1) mountain building
(regional metamorphism); and 2) the intrusion of
molten (contact metamorphism).
Reference:
Tarbuck and Lutgens
Pages 198 - 207
Regional Metamorphism
 Occurs where tectonic plates collide; mainly at ocean–
continent collision boundaries.
 This type of environment produces extensive mountain
building.
 The greatest volume of metamorphic rocks are formed
in this way.
 This type of metamorphism is caused by two main
conditions, which include: 1) extreme pressure; and 2)
extreme heat (i.e. temperature). Of course, chemically
active fluids are also essential.
Regional Metamorphism
Pressure:
 Pressure on rocks within mountain
environments is mainly caused by: 1)
the weight of the material above; and 2)
stress caused by compressional forces as
tectonic plates push against one another.
 This pressure causes rocks to deform and flow as if they
were plastic. The minerals within the rock recrystallize
and reorganize to form metamorphic textures (e.g.
foliation). Folding is a common feature found in rocks
that have undergone regional metamorphism.
Regional Metamorphism
Heat
 Heat responsible for regional metamorphism within
mountain environments is produced as a result of:
1) Geothermal gradient, which states that temperature
increases 30 degrees Celsius for each kilometer below
Earth’s surface.
2) Intrusion of hot molten rock called magma,
which causes the surrounding
rock to heat up as heat from the
magma radiates outward.
Contact Metamorphism
 This type of metamorphism occurs when rock is in
contact with, or near, a mass of molten (i.e. heat
source). The molten could be lava or magma.
 The changes are caused primarily by the high
temperatures of the molten rock, which in effect
“bakes” the surrounding rock.
 Examples include marble and hornfels.
Contact Metamorphism
is indicated by the “xxxx”
in the diagram.
Sample Problem
Contrast the conditions necessary for rock to change by
regional metamorphism compared to that of contact
metamorphism.
Answer:
Regional metamorphism occurs in mountainous environments
where rocks are exposed to high heat from igneous intrusions and
high pressure caused by compressional forces.
Contact metamorphism can occur in any environment where a heat
source (i.e. molten rock) causes the surrounding rock to change.
Thus, regional metamorphism is caused by high heat and high
pressure, whereas contact metamorphism is caused by high heat
only.
Sample Problem
How can contact metamorphism distinguish an igneous
intrusion from a buried lava flow? See the textbook on page
198.
Answer:
Contact metamorphism would reveal if an igneous rock unit
was an intrusion or a lava flow by looking at where the
metamorphism is located. An intrusion would have
metamorphism above and below the igneous rock unit (i.e.
X’s on top and bottom), whereas a lava flow would only
show metamorphism below the igneous rock unit (i.e. X’s
on bottom only).