is a fine-grained homogeneous sedimentary rock composed of clay
or volcanic ash which has been metamorphosed (foliated) so as
to develop a high degree of fissility or slaty cleavage which
is usually at a high angle to the planes of stratification. This
high degree of fissility makes the better grades of slates an
extremely useful roofing material (although its use has fallen
in favor of synthetic and manufactured substitutes).
choosing roofing materials, one must take into consideration the
longevity of the roofing material, as well as the installation
cost. Slate roofs have been known to last hundreds of years in
comparison to the synthetic roof which typically lasts 10-20 years.
Synthetic and manufactured roofing materials may initially save
money at the time of installation, but the slate roof will last
many lifetimes, making it the more permanent, economical choice.
of the finest slates in the world come from Portugal, Wales in
the United Kingdom, Brazil and The Slate Valley of Vermont and
SEDIMENTARY ROCKS GENERAL
Sedimentary rock is one of the three main rock groups (along with igneous and metamorphic rocks) and is formed in three main ways—by the deposition of the weathered remains of other rocks (known as clastic sedimentary rocks); by the deposition of the results of biogenic activity; and by precipitation from solution. Sedimentary rocks include common types such as chalk, limestone, sandstone, and shale.
rocks are formed from overburden pressure as particles of sediment
are deposited out of air, ice, or water flows carrying the particles
in suspension. As sediment deposition builds up, the overburden
(or lithostatic) pressure squeezes the sediment into layered solids
in a process known as lithification ("rock formation")
and the original connate fluids are expelled.
rocks are composed largely of silica (i.e. quartz), with other
common minerals including feldspars, amphiboles, clay minerals
and sometimes more exotic igneous minerals. Sedimentary rocks
are classified as clastic, that is, they are composed of discrete
clasts of material (rather than being composed of organic material
as is the case for a limestone).
minerals precipitating out of the ocean cover the ocean floor
with layers of calcite which can later form limestone.
rocks are economically important in that they can be used as construction
material. In addition, sedimentary rocks often form porous and
permeable reservoirs in sedimentary basins in which petroleum
and other hydrocarbons can be found.
is believed that the relatively low levels of carbon dioxide in
the Earth's atmosphere, in comparison to that of Venus, is due
to large amounts of carbon being trapped in limestone and dolomite
sedimentary layers. The flux of carbon from eroded sediments to
marine deposits is known as the carbon-cycle.
shape of the particles in sedimentary rocks has an important effect
on the ability of micro-organisms to colonize them. This interaction
is studied in the science of geomicrobiology. One measure of the
shape of these particles is the roundness factor, also known as
the Krumbein number after the geologist W. C. Krumbein.
rock is one of the three main rock groups. See also igneous and
rock is formed from the weathered remains of other rocks.
CLASSIFICATION OF ROCKS