RECORDS OF GEOLOGIC PROCESSES

ROCKS:
RECORDS
OF
GEOLOGIC
PROCESSES
( 地地地地地地地 )


ROCK , by definition,

is composed of an aggregate of
minerals
( 地地 )
( 地地 )

( 地地 )


• Appearance of a rock is related to:
1. Mineralogy = types and relative proportions of constituent
minerals
2. Texture = size, shape and orientation of mineral grains

• These features relate to the origin and history of the rock.
When many rocks are understood, the geologic history (and
tectonic setting) of a region can be inferred.



Where do we see rocks?

• Not on the
surface of course!
• They are at
depth in most
places covered by
soil and regolith
( 地地地 ).


Three general rock types:
1. Igneous - formed by solidification of molten
rock (>700°C)
2. Sedimentary - formed as burial products of
sediments, on land or sea
3.Metamorphic - formed by transformation in
the solid state of pre-existing rock due to
changing pressure and temperature.


( 地地地 )

( 地地地 )

( 地地地 )


Sedimentary rocks
comprise a
majority of the
surface of the

Earth, but are a
minor volume of
the total volume of
the crust.


IGNEOUS ROCKS ( 地地地 )
Definition: A rock formed by the solidification of magma
(molten rock)

Igneous environments


IGNEOUS ROCKS ( 地地地 )
• Can be intrusive (crystallizing below the surface of
the Earth) or extrusive (crystallizing at the surface
of the Earth)
• Textures in igneous rocks relates
to the composition of the magma,
rate of cooling of the magma and
any flow features of the magma
• Extrusive ( 地地 ) rocks - rapid
cooling, therefore fine-grained or
glassy; some surface flows
• Intrusive ( 地地 ) rocks - slower
cooling, therefore coarser grained;
flow features possible



IGNEOUS ROCKS ( 地地地 )


SEDIMENTARY ROCKS ( 地地地 )
Definition: A rock formed by the accumulation and
cementation of mineral grains by wind, water or ice
transportation to the site of deposition, or by chemical
precipitation at the site.

Typical sedimentary
depositional environments


• Sedimentary
rocks - generally
characterized by
bedding i.e.
parallel layers of
sediments
• Textures of
sedimentary
rocks relate to
the manner of
sediment
transport and
deposition e.g.
high vs. low
velocity
transport

• This is an
interface
between the
atmosphere,
hydrosphere and
lithosphere.


SEDIMENTARY ROCKS ( 地地地 )
• Types:
1. Clastic - physically deposited sedimentary particles
(clasts - broken rocks, remains or products of animals or
plants) transported and laid down by running water,
wind or ice. Generally silicate minerals.
2. Chemical and biochemical sediments
-chemical precipitation of dissolved
components of rocks at the
depositional site e.g. halite, calcite
3. Mixtures of both of the above
• Lithification of sedimentary rocks –
hardening into a solid rock due to
compaction and cementation




METAMORPHIC ROCKS ( 地地地 )
Definition: A rock whose original mineralogy, texture, or
composition has been changed by the effects of pressure,
temperature, or the gain or loss of chemical components.

• Occurs between temperatures of roughly 250°C (end of
sedimentary lithification processes) and melting of the rock
(>700°C)
Contact and regional metamorphism
at plate-collisional mountain ranges

Burial metamorphism in
deep sedimentary rocks

Cataclastic metamorphism
along faults

Contact and regional
metamorphism at
subduction zones

Hydrothermal metamorphism at
mid-oceanic ridges


METAMORPHIC ROCKS( 地地地 )
• General classification of type of metamorphism relates to
location of heat source
1. Regional metamorphism - heat
source in not apparent, but
generally higher; associated with
mountain building episodes
extending over large regions
2. Contact metamorphism - heat
source is apparent i.e. associated

with intrusion of a magma that
"cooks" the surrounding rocks
• Textures relate to the pressure,
temperature and deformational
history of metamorphic rock




Metamorphic environments


THE ROCK CYCLE ( 地地地地 )
• Inter-relationships among the three rock types

• The rock cycle is directly related to plate tectonics ( 地地
地地 ) - in turn, the rocks that we currently see can infer
tectonic conditions in the past.


• This is an
interaction
among the
atmosphere,
hydrosphere,
lithosphere and
asthenosphere
(the mantle
layer) and is
unique to Earth

Recommended Web-sites:
Chinese web-site:
岩石圈
English web-sites:
Discover How Rocks Are Formed
Rocks For Kids
Your Gemologist
Rockdoctors
Guide to Minerals, Igneous, Metamorphic, and Sedimen
tary Rocks