Gradation and Weathering Tectonic processes are constantly acting on the surface of the earth in a long term and massive way to uplift, depress and compress the earth’s surface. Gradational forces are responsible for the finer detailed shaping of the earth’s surface that we see on a day-to-day basis. The hills, valley, mountains, shorelines, etc. that result form these gradational processes are called landforms and the study of landforms is called geomorphology. Gradational process are driven by solar energy, producing wind, rain, snow, ice, and waves, which act to break up the surface of the earth. A second energy source - gravity - then acts with the agents of gradation to move the broken up material from higher areas to lower areas of the earth’s surface. In motion the agents of gradation have kinetic energy that can cause further breaking up of the earth’s surface. The overall tendency of gradational forces is to lower the higher parts of the earth’s surface and to fill in the lower parts to reduce and then eliminate differences in relief. If tectonic processes were to stop, then gradational forces would ultimately reduce the earth’s surface to level plains. This action has been called peneplanation. Weathering is the first step in gradation. It is all processes that cause rocks to decompose and disintegrate into what is called regolith, which can then be acted upon by flowing water, wind and so on. There are two broad categories of processes that act to weather rock: mechanical and chemical. Both are a function of climate. Mechanical causes rocks to disintegrate. Chemical causes rock to decompose. Mechanical Weathering Processes Freezing of water (apart from glaciers) •also called frost wedging •water’s volume expands by 10% when it freezes •exerts tremendous pressure •especially important at a very small scale but also moves large boulders •effective in locations where temperatures change frequently to either side of 00, eg., in mountains day to night http://www.geo.wvu.edu/~wilson/Geol1/lec17/lec171.htm Frost wedging has fragmented this outcrop of jointed rock occupied by an Adelie penguin; Antarctic Peninsula, Antarctica. Mechanical Weathering Processes Changes in Temperature •occurs where rock surfaces are exposed to diurnal (daily) heating and cooling •rock minerals, because they have different combinations of elements, have different thermal expansion and contraction rates •this causes internal stresses and eventually the rock crumbles •slow process •responsible for most of the world’s sandy deserts Mechanical Weathering Processes Water • moving water • Rain • waves • Rivers • runoff www.kfshrc.edu.sa/directory_services/ html/red_sands.html Sun or salt spilt rock http://geogweb.berkeley.edu/GeoImages/Wells/ geomorph/rock1.html Mechanical Weathering Processes The Wind •This is self explanatory Mechanical Weathering Processes Salt Crystal Growth •occurs in dry climate areas, usually in sandstone •during drought periods, water is drawn to the surface by capillary action and it carries with it dissolved mineral salt from the rock •at the surface the water evaporates but the salt precipitates •salt crystals slowly grow, and force the rock grains apart •in the arid south-west of the US huge salt caves formed at the base of large cliffs, later inhabited by the Pueblo Indians large sandstone cliff rain falls water seeps through porous rock hard impervious layer impedes downward movement waterdrawn to surface by capillary action as water evaporates, salt crystals force grains of sandstone apart wind blows them away cave gets bigger impermeable layer Salt Crystal Growth volcano.und.nodak.edu/.../ north_america/Ban26.jpg Mechanical Weathering Processes Action of Plants and Animals •plant roots help to widen cracks in the rock •especially important is the action of tiny root hairs of small plants, eg., lichens, mosses •animals act to aerate the soil down to the bedrock, exposing it to other mechanical and chemical processes •plant mechanical action is works with plant chemical action Mechanical Weathering Processes Release of Pressure •also called unloading •when surface layers are removed newly exposed rock often can expand due to the release of overlying weight and the rock cracks •happens often in quarries •can create exfoliation domes where large sheets of rock crack and peel away, like an onion Exfoliation Domes As erosion removes rock material from the surface of the land, the underlying rocks are under less pressure. As the pressure is lowered on them, the rocks expand upwards, creating fractures. Slabs of rock then 'peel off' along the fractures, creating domical hills. Famous 'Half Dome Mountain' is an example of an 'exfoliation dome.' http://geology.about.com/library/bl/images/blexfoliation.htm?terms=exfoliation Chemical Weathering Processes •minerals in rock come in contact with CO2, H2O, or O2 to form new minerals that are either larger in volume or softer and more water soluble •there are three basic types of chemical weathering processes: solution, oxidation, and hydrolysis. Chemical Weathering Processes Solution •dissolved carbon dioxide from atmosphere and organic acids from decaying plant and animal matter are added to rain water to form a weak carbonic acid •the acid reacts on basic rocks such as limestone •certain minerals like calcite are dissolved and carried away in solution CaCO2 + H2CO3 Ca(HCO3) 2 calcite calcium bicarbonate carbonic acid •calcium bicarbonate is water soluble and easily carried away •eventually the limestone is completely removed leaving caverns, sink holes and even underground rivers •where limestone is widespread in a region, a distinctive type of typography called Karst is produced http://images.google.com/images?num=20&hl=en&q =Karst+topography Karst Topography www.geologyone.com/esa/geopro/ karst/illustra.jpg A Sink Hole! Go to the site below to see two animations that illustrate Karst Topography and the creation of a Sink Hole. http://www.watersheds.org/blue/earth/hotearth.htm Chemical Weathering Processes Oxidation •results from the reaction caused by iron bearing minerals and oxygen dissolved in water •the iron is changed into iron hydroxide, what is commonly called rusting •there is a lot of iron in crustal rocks and therefore this is a common form of weathering •rock discolours to yellow-brown or reddish-brown 4FeO iron oxide + 3O2 2Fe2O3 oxygen iron hydroxide Chemical Weathering Processes Hydrolysis •similar to solution (which is mostly underground), this process occurs more on the surface and affects mainly the silicate group of rock forming minerals - one of the most common rock forming minerals •water in the atmosphere forms weak carbonic acid in the presence of carbon dioxide •this acid (in rain) then reacts with silicates to form clay like minerals, which are washed away •the rock is weakened and falls apart or is rounded •This is most common on buildings and grave stones. The End!