GEOLOGY 12
CHAPTER 7 METAMORPHIC ROCKS
MAIN TOPIC QUESTIONS & ANSWERS
Read the following notes carefully to find the answers to the main topic questions.
1.
2.
3.
Metamorphic rocks form when increases in heat and/or pressure change the physical
and chemical conditions of an existing rock. Metamorphic temperatures are greater
than those required for diagenesis (i.e. changes during lithification - compaction,
cementation, recrystallization, dolomitization) and less than those required to melt a
given rock. FACTORS WHICH CAUSE METAMORPHISM INCLUDE:
a.
Sources for heat include the normal increase of temperature with depth in the
earth's crust (or GEOTHERMAL GRADIENT) and, more locally, PLUTONIC
ACTIVITY.
b.
Sources for pressure include the normal CONFINING PRESSURE at depth and,
more locally, regional DIRECTED STRESS associated with tectonic activity.
c.
Chemical conditions may also cause metamorphic changes if fluids pass through
the rocks due to HYDROTHERMAL ACTIVITY during PLUTONIC ACTIVITY
(intrusive) or TECTONIC ACTIVITY (mountain building) activity.
Rocks are changed when they are subject to physical or chemical conditions which
differ from those under which they were formed. THERE ARE THREE MAIN TYPES
OF METAMORPHISM.
a.
CONTACT METAMORPHISM is localized over a smaller area and is generally at
shallow depth. Contact metamorphism creates a zone of metamorphic rock
around a pluton in country rock known as a contact aureole
b.
FAULT ZONE METAMORPHISM also occurs over a localized area and is
generally associated with directed stress along faults. Fault zone metamorphism
is characterized by fault breccia which was formed from angular fragments of
rock broken up in a fault zone
c.
REGIONAL METAMORPHISM, as the term implies, occurs over an extensive
area and is associated with major igneous intrusions cooling slowly or extreme
pressure and heat associated with deep burial of rocks and/or sediments.
Regional Metamorphism may also involve very widespread migration of hot
gases throughout a region. The condensation of gases and precipitation of
crystals is called hydrothermal metamorphism.
The effects of metamorphism include changes in mineralogy and textures of the initial
rocks.
a.
As temperatures increase, minerals stable at low temperatures break down to
form minerals stable at higher temperatures. (see #5 below)
b.
With increasing confining pressure, denser minerals may form in metamorphic
rocks. (see #4 below)
4.
5.
c.
With increases in directed stress, compositional or textural banding or layering
may develop in metamorphic rocks. (see #4 below)
d.
Chemical changes are more difficult to generalize but also play an important role
during metamorphism.
Metamorphic rocks are classified on the basis of texture as either foliated or nonfoliated.
a.
FOLIATED ROCKS may form when directed stress is involved in metamorphism.
Examples of foliated metamorphic rocks include slate, phyllite, schist and gneiss.
b.
NON-FOLIATED ROCKS form under a variety of conditions. Examples of nonfoliated metamorphic rocks include anthracite (from coal), marble (from
limestone), meta-conglomerate (AKA stretched pebble conglomerate or pudding
stone was originally the sedimentary rock conglomerate), and quartzite (from
sandstone).
Many geologic conditions exist in which metamorphic rocks may form.
a.
The conditions of temperature and pressure in which metamorphism has
occurred can be described in terms of metamorphic facies and metamorphic
grades. The presence of specific minerals or index minerals, such as garnet,
defines a particular metamorphic facies and METAMORPHIC GRADE.
b.
Geologist can map the distribution of the index minerals to identify areas of
similar metamorphic grade. The map lines used to indicate these areas are called
isograds. (iso means “of the same”).
c.
The presence of hornfels indicate a low grade metamorphism, such as that
associated with CONTACT METAMORPHISM. However, the partially melted look of
migmatite indicates the high grade metamorphism of REGIONAL METAMORPHISM.
Geology 12
Chapter 7 Main Topic Questions
Page 2