Dissolution
occurs when rocks and/or minerals are dissolved by water. The dissolved
material is transported away leaving a space in the rock. One
consequence of this process is the formation of caves in limestone
areas.
rain + carbon dioxide (from air) à carbonic acid (reacts with rocks)H2O + CO2 à H2CO3
The details of marble statues and tombstones will be gradually worn away by dissolution. Image on right courtesy of Dr. Annabelle Foos. Carbon dioxide (CO2) from the air is dissolved in rainwater to create a weak acid, carbonic acid
(H2CO3), that preferentially dissolves certain rocks and minerals, e.g.
limestone, marble. All rain is mildly acidic (average pH ~5.6, compared
with neutral fluids [pH 7] and highly acidic [pH 1] substances).
Caves form when dissolution occurs
along a series of fractures in limestone to create a larger opening.
Water passing through the rock enlarges the cave and associated
re-precipitation can form a variety of features. The dissolved limestone
is transported through the cave and may be precipitated to form new
features such as stalagtites that grow downward from the cave ceiling
and stalagmites that grow up from the floor. If they meet they form a
compound cave formation such as a column.
Crystal Ball Cave, Utah. Features hanging down from the ceiling are stalactites. Pillar-like features on the cave floor are stalagmites. Both stalagmites and stalactites are formed from the precipitation of limestone within the cave. Image courtesy of Dr. Ira Sasowsky.The
Virtual Cave website contains images and descriptions of a variety of
cave formations. You can review pictures of Lechuguilla Cave in Carlsbad
Caverns National Park and effectively take your own virtual tour of the
cave.
Not all the products of dissolution are below ground. Sinkholes
form at the surface from the collapse of the roof of an underlying
cavern or by dissolution of rock along a series of fracture surfaces.
Image on right shows a sinkhole, (Watlings Blue Hole) in the Bahamas. Road at bottom left of image for scale. Image courtesy of Dr. Ira Sasowsky.
Hydrolysis
Carbonic acid ionizes (breaks down) into two ions, hydrogen (H+) and bicarbonate (HCO3-1).
H2CO3 à H+ + HCO3The
free hydrogen ions may alter mineral composition by replacing other
ions in a mineral’s atomic structure; this reaction is termed hydrolysis.
Hydrolysis occurs when minerals react with water to form other
products. Feldspar, the most common mineral in rocks on the earth's
surface, reacts with water to form a secondary mineral such as
kaolinite (a type of clay) and additional ions that are dissolved in
water. The weaker clay is readily worn away by physical weathering.
Oxidation Oxygen,
the second most common element in the air we breathe, reacts with iron
in minerals to form iron oxide minerals, e.g. hematite (rust). As many
minerals contain iron, it is not unusual to see red-colored rocks like
the example from Arches National Park shown here.
Delicate Arch, Arches National Park, Utah.
Physical weathering breaks rocks down into smaller pieces thus increasing the surface area over which chemical weathering can occur (see diagram). All forms of chemical weathering tend to promote the decomposition of minerals in rocks to a less resistant form. Working in concert, physical and chemical weathering can reduce a once resistant rock to nothing (dissolved limestone) or to easily eroded weaker materials (clays).
الأحد فبراير 25, 2018 3:13 pm من طرف carate
