- Sedimentary Rock - Jeri Schott
The natural progression of rock formation is constant. Fragments of previous rock bodies come together following physical erosion and chemical weathering. The fragments of various sizes eventually cement through a process of lithification to compose new rocks.
B.W. Murck and B.J. Skinner (Visualizing Geology, 2008) claim the new rocks, sedimentary rocks, provide information as to past climates and life forms that inhabited the earth. The constant process of plate tectonics, results in sedimentary rock development in various valleys and ocean basins. Eighty percent of surface earth consists of sedimentary rock lying atop igneous and metamorphic rock layers.
Physical Erosion and Chemical Weathering
Physical erosion and chemical weathering produce sediments which later form sedimentary rock. Though similar end products result, the progressions differ greatly. Erosion impacts the rock on a physical level, changing the shape through constant exposure to wind and water. The elements work on the rock, slowly disintegrating it into smaller particles.
Chemical weathering alters the actual mineral composition of solid rock. Acidic water solutions partially or entirely dissolve receptive minerals, recreating the rock. The disbanded sediment travels and precipitates, eventually bonding with other sediment to form graded or cross bedding. Erosion and weathering should not be mistaken for one another despite comparable final results.
Sedimentary Rock Categories
Sedimentary rock types are classified in one of three categories. The clastic category forms from fragmented sediment. Portions of eroded or weathered rock bind, producing clastic subcategories of conglomerate, sandstone, siltstone, or shale. Grain size distinguishes rocks in each clastic subcategory.
A second category of sediment and sedimentary rock is termed chemical. As a result of dissolution, sediment of pre-existing rock(s) somewhat congeals and travels in a body of water. Usually evaporation settles the deposit which then completely solidifies. The product is a type of chemical sedimentary rock, commonly limestone.
The last category of sedimentary rock is known as biogenic or organic. The biogenic method, while chemical, is categorized separately because leftovers of living organisms compose the sediment. Shells and plant fragments unite in biogenic sedimentary rocks. Biogenic rocks form from evaporative and chemical processes which glue fossils into one mass. Limestone and coal are examples of biogenic sedimentary rocks.
Characteristics of Sedimentary Rock Types
Worthy characteristics distinguish the types of sedimentary rocks. Clastic sedimentary rock is grainy. Though some rocks in this category contain such fine grains as to appear smooth, a microscopic view reveals the miniscule particles. Clastic sediment combines after compaction, or pore loss. Pressure forces the fragments together and they combine to create a solid, seemingly cemented form. Many road surfaces mimic the outcome, where pebbles and stones are held together by dried tar.
Chemical sedimentary rocks are often banded in appearance. Often, this category of rocks is visibly crystalline and at times contains quartz. Because sea and lake water contribute to the making of chemical rock, certain types actually have a salty taste or texture. Chemical rocks are angular and light in color.
The fossil composition of organic or biogenic sedimentary rocks may be obvious but in several examples the relics are too tiny for detection by the naked eye. Biogenic rocks containing plant remnants are dark colored. Biogenic samples may emerge clear and soft to the touch, chalky, sooty, or falling to pieces. Biogenic and chemical sedimentary rocks both produce limestone but the biogenic version contains particles of once-living things.
Plate Tectonics and Common Sedimentary Rock
The most common type of sedimentary rock is the clastic form. Birthed almost directly from plate tectonics, clastic sediment is common where physical changes to earth abound. Earth plates converge and diverge around the world and in doing so, slowly alter the existing landscape. Eroded fragments eventually find a new existence as sedimentary rock.
Because plate tectonics is a physical process, chemical sedimentary rocks rarely result. Despite the comparatively small amount of chemical and biogenic outputs, all sedimentary rocks stand as revitalized specimens, representing the earth’s constant destruction and rebirth cycle.
Freeman, Tom. (2007). Geoscience Laboratory Manual. ( 4th Edition). Chapter 5. New Jersey: John Wiley & Sons. Retrieved August 10, 2009.
Murck, B.W. & Skinner, B.J. (2008). Visualizing Geology. New Jersey: John Wiley & Sons
Jeri Schott - Hi, Thanks for checking in with me. I am attending college but my ultimate dream is to write and earn money for doing so! I love ...
