Transcript Regional vs Contact Metamorphism

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 and can change on the atomic scale.
 Metamorphism most often occurs during one of two
geologic events;
1) mountain building (regional metamorphism)
2) intrusion of molten rock (contact metamorphism)
Reference:
Tarbuck and Lutgens
Pages
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;
1) extreme pressure, and 2) extreme heat.
Regional Metamorphism
Pressure
 Pressure on rocks within mountain
environments is mainly caused by;
1) the weight of the material above, and
2) stress caused 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. 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 the surface, and
2) Intrusion of hot molten rock called magma
which causes the surrounding
rock to heat up as heat from
magma radiates outward.
Regional Metamorphism
WHERE DOES IT OCCUR?
(1) Areas of mountain building; and
(2) Subduction zones.
(1)
• Ocean-continent collision
(convergent plate boundary) is an
example of where subduction and
mountain building occurs.
• A compressional environment
resulting in increasing pressure. As
the mountains are forming,
increasing burial depth results in
increasing temperature.
Regional Metamorphism
(2) This is a subduction zone. The metamorphism is not
occurring within the subduction zone, but within the
mountain system. As you can see, as burial depth
increases so does the GRADE OF METAMORPHISM.
Shale to Slate to Phyllite to Schist to Gneiss
Contact Metamorphism
 This type of metamorphism occurs when rock is in
contact with, or near, a mass of magma (Heat).
 The changes are caused primarily by the high
temperatures of the molten rock, which in effect
“bake” the surrounding rock.
 Examples include Marble and Hornfels.
Contact Metamorphism
is indicated by the “xxxx”
in the diagram.
Contact Metamorphism
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WHERE DOES IT OCCUR?
(1) Beneath lava flows (Hint: buried lava flows)
(2) Adjacent to magma intrusions
(3) Dykes and sills
B is a magma intrusion and R is
a buried lava flow.
Dyke = Vertical
Sill = Horizontal
Contact Metamorphism
Remember:
[Refer to diagram on previous slide (left side).]
• Buried Lava Flow
Contact Metamorphism is located
Below the Lava Flow (Only).
• Magma Intrusion
Contact Metamorphism is located
Above and Below the Magma Intrusion.
Chill Out – “Chilled Margins” – Contact Metamorphism
• When lava “pours” over rock or magma “intrudes” rock, it starts to
crystallize (igneous rock activity). The contact metamorphism occurs
where the hot lava or hot magma “touches” the old, previously
existing rock.
Look at the magma intruding through a
rock unit! This magma starts to cool
slowly so crystals grow to be large.
HOWEVER, the magma cools faster on
the outsides since the cold, previously
existing rock unit causes it to cool
faster. THEREFORE, YOU GET
LARGER CRYSTALS TOWARDS
THE MIDDLE AND SMALLER
CRYSTALS BY THE OUTSIDES. The
chilled effect!
Sample Problem
What feature is represented by “xxxx” in the diagram
below?
Once molten rock
(A) contact metamorphism
(B) faulting
(C) regional metamorphism
(D) unconformity
Sample Problem
How can contact metamorphism distinguish an igneous
intrusion from a buried lava flow?
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,
whereas, a lava flow would only show metamorphism below
the igneous rock unit.
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 (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 heat only.