Download Lecture#5: Rocks

Rocks
GEOG 3251 Mountain Geography
summer term B 2010
Rock cycle
Definition & classification
• Rock = a naturally occurring aggregate of
minerals and/or mineraloids.
• Classification:
 mineral and chemical composition;
 the texture of the constituent particles
 processes that formed them.
Types of rocks
 igneous
 sedimentary
 metamorphic
Igneous rocks
Formed from molten magma (latin ignis =
"fire")
2 types:
 intrusive: magma cools and crystallises slowly
within the Earth's crust
 extrusive: magma reaches the surface either as
lava or fragmental ejecta.
Classification based on mode of
occurrence
Mode of occurrence Acid
Intrusive
Granite
Extrusive
Rhyolite
Composition
Intermediate Basic
Diorite
Gabbro
Andesite
Basalt
Ultrabasic
Peridotite
Based on composition
FELSIC ROCKS: texture
Intrusive
Granite
coarser
Extrusive
Muy
extrusive
Rhyolite
Obsidian
finer
Yosemite Valley: El Capitan
Intrusive: slow cooling, large texture
ARROWHEADS
Extrusive: fast cooling, fine texture
Obsidian
Igneous Rock Types
Figure 11.7
Sedimentary rocks
• Two ways of forming:
– from overburden pressure as particles of
sediment are deposited out of air, ice;
– chemical precipitates (water flows carrying the
particles in suspension), eg. carbonate-rich
sediments
What makes the Flatirons so
good for climbing?
Type of rock?
•Sandstone
•conglomerate
-large particles
-good handholds
Limestone
(Ca CO3)
• Piatra
Craiului,
Carpathians
(Romania)
• Karst
landscape
Limestone caves
Metamorphic rocks
• formed by subjecting any rock type
(including previously-formed metamorphic
rock) to different temperature and
pressure conditions than those in which
the original rock was formed
Metamorphic rocks
• Foliated: stress that deforms the rock in
one plane, e.g. slate is a foliated
metamorphic rock, originating from shale.
• Non-foliated: do not have planar patterns
of stress; rocks that were subjected to
uniform pressure from all
SLATE
Because slate easily flakes, it’s ideal for
Floors, roofs, blackboards.
GNEISS
Gneiss is characterized by a banded or foliated pattern.
Granite is a precursor rock to gneiss
Stress vs.strain
• Stress = force per unit area
• Strain = amount of deformation an object
experiences compared to its original size
and shape
– Compression: Longitudinal strain that
shortens an object.
– Tension: Longitudinal strain that lengthens an
object.
Behavior of Materials
• Elastic
– Material deforms under stress but returns to its
original size and shape when the stress is
released.
• Plastic
– Material does not flow until a threshold stress has
been exceeded.
• Brittle
– Material deforms by fracturing. Glass is brittle.
Rocks typically brittle at low temps and pressures.
How do rocks behave when subjected to
stress?
Folding
Faulting