Transcript Weathering

Weathering
Objectives
1.
2.
3.
4.
5.
Describe the Rock Cycle.
Briefly contrast weathering and erosion.
Contrast physical, chemical and biological weathering.
List and describe the types of mechanical weathering.
List and describe the types of chemical weathering.
The Rock Cycle
• The three major types of rocks: igneous,
sedimentary and metamorphic.
• Rocks are interrelated by a series of natural
processes.
• Igneous rocks (e.g. granite, basalt) form from
the cooling and crystallization of hot molten lava
and magma. Igneous rocks undergo weathering
and erosion to form sediments. When
Sediments are deposited they form
Sedimentary rocks (e.g. sandstone, coal and
chalk).
• Sedimentary rock becomes buried by additional
sedimentary deposition, and when they are
deep within the Earth, they are subjected to
intense heat and pressure which causes them
to become Metamorphic rocks (e.g. marble
and slate).
Igneous
Rock
Sedimentary
Rock
Metamorphic
Rock
• With further burial and heating, the metamorphic
rocks begin to melt. Partially molten metamorphic
rocks are known as migmatite.
• As melting proceeds with increasing
temperatures and depths of burial, eventually the
rock becomes molten (magma), which can be
erupted onto the Earth's surface as lava, and
cools and crystallizes to form volcanic igneous
rock.
Complications within the rock cycle include:
1. Weathering of sedimentary and metamorphic
rocks (in addition to igneous rocks)
2. Metamorphism of igneous rocks and repeated
metamorphism of metamorphic rocks.
Definition of weathering
• Weathering is the disintegration (break down)
and decomposition (decay) of rocks in situ (in
their place of origin) to form sediment.
• Weathering (unlike erosion) need not involve the
movement (transport) of material.
A) Physical or Mechanical
weathering
• Freeze / Thaw – water expands when it freezes
Talus slope, Lost River, West Virginia
Shale chips, West Virginia
• Exfoliation or unloading rock breaks off into leaves or sheets along joints
which parallel the ground surface;
caused by expansion of rock due to uplift and
erosion; removal of pressure of deep burial.
• Thermal expansion repeated daily heating and cooling of rock;
heat causes expansion; cooling causes
contraction.
different minerals expand and contract at different
rates causing stresses along mineral boundaries.
B) Chemical weathering
Rock reacts with water, gases and solutions
(maybe acidic). Thus chemical processes can
add or remove elements from minerals.
• Dissolution (or Solution, Carbonation) – dissolving of calcium carbonate (limestone) in acidic
rain or ground water.
– Several common minerals dissolve in water
(halite, calcite)
– Limestone and marble contain calcite and are
soluble in acidic water
– Marble tombstones and carvings are particularly
susceptible to chemical weathering by dissolution.
• Photo taken in
an aboveground
cemetery in
New Orleans
• Caves and caverns typically form in
limestone (karst environments)
– speleothems are cave formations
– speleothems are made of calcite
• Stalactites: hang from ceiling
• Stalagmites: on the ground
– Karst topography forms on limestone terrain
and is characterized by:
• caves/caverns, sinkholes (dolines), disappearing streams,
springs.
Sink-hole (doline) formation
• Oxidation
– Oxygen combines with iron-bearing
silicate minerals causing "rusting"
(olivine, pyroxene,
amphibole,biotite).
– Iron oxides are produced (limonite,
hematite, goethite)
– Iron oxides are red, orange, or
brown in color.
– Mafic rocks such as basalt (which
may contain olivine, pyroxene, or
amphibole) weather by oxidation to
an orange color.
– "Georgia Red Clay" derives its color
from the oxidation of iron bearing
minerals.
Weathering Rind, Wilhite
Formation, eastern
Tennessee
• Hydrolysis
– Silicate minerals weather by hydrolysis
to form CLAY.
– Feldspar alters to clay (kaolinite) plus
dissolved materials (ions)
– ‘Feldspars’ are stable at high
temperatures and pressures (but not at
the temperatures and pressures of the
Earth's surface)
– Clays are stable under conditions at the
Earth's surface
– Feldspars and clays are similar in
composition.
– Feldspar readily alters to clay when in
contact with acid and water.
• Iron-bearing silicate
minerals weather to form
clays by hydrolysis (in
addition to iron oxides)
• Spheroidal weathering is
caused by chemical
weathering of jointed
rocks. The jointed rocks
weather to form roughly
spherical shapes.
Spheroidal weathering in jointed
basalt, Culpeper Basin, Virginia
C) Biological weathering
•
Organisms can assist in breaking down
rock into sediment or soil.
1. Roots of trees and other plants
2. Lichens, fungi, and other micro-organisms
3. Animals (including humans)
Roots of trees and
plants
Lichens