Slate 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).
When 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.
Some of the finest slates in the world come from Portugal, Wales in the United Kingdom, Brazil and The Slate Valley of Vermont and New York.
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.
Sedimentary 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.
Sedimentary 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).
Carbonate minerals precipitating out of the ocean cover the ocean floor with layers of calcite which can later form limestone.
Sedimentary 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.
It 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.
The 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.
Sedimentary rock is formed from the weathered remains of other rocks.
CLASSIFICATION OF ROCKS