Physical Geology 101 Weathering and Soil p 166 182 Disintegration of Rock Rocks exposed to the elements break down into smaller particles called which eventually breaks down further to form This process is called weathering Definition of weathering Definition of erosion Definition of mass wasting Weathering takes place where the 4 earth systems lithosphere atmosphere hydrosphere and biosphere can interact with each other i e at the Earth s surface How deep can weathering penetrate into the Earth Descriptions of the two kinds of weathering rock is physically broken down but the minerals don t change rock is broken down by chemical or biochemical alteration of minerals or by dissolving minerals and removing them in solution Mechanical Weathering Any weathering that involves the physical breakdown of rock is called mechanical weathering There are five main types 1 when water freezes and expands forcing rock to break apart How much does water expand when it freezes What is the process called where fractures in rock are forced apart by the water inside them freezing this is just another way of saying frost wedging The chunks of rock that then fall off and collect on a slope are called 2 rocks under pressure deep inside the Earth s crust get brought slowly to the surface over geologic time e g by the effect of erosion or mountain building At the surface the rocks expand and break apart The process whereby rocks get uplifted is called The rock breaks into thin slabs or sheets during uplift The fractures between these slabs of rock are called Physical Geology 101 The process that produced these thin slabs or sheets is called The large bodies of weathered rocks that are created at the Earth s surface as a result are called Example 3 temperature extremes and sudden temperature changes may cause a rock to peel off in layers like an onion This is a type of exfoliation called In what type of environment is this likely to happen So are rocks good or poor conductors of heat GOOD or POOR 4 the forces of growing crystals can break bits off rocks This is common where water containing dissolved minerals evaporates What mineral commonly grows when water evaporates 5 plant roots penetrate cracks and force rocks apart whereas burrowing animals help break down rocks and soil into smaller particles Chemical Weathering 1 One type of chemical weathering is when ions are removed from minerals in the rocks causing them to dissolve completely In this case we say that the ions have been carried away in solution and the process itself is called What two types of minerals frequently get dissolved and removed in solution 1 2 So what three types of rocks made up of these minerals can potentially dissolve 1 2 3 What types of ground collapse features in limestone are caused by dissolution 2 Rainwater is actually a weak acid Raindrops falling through the air dissolve CO2 resulting in the formation of an acid compound called H2CO3 This acid facilitates rainwater s ability to chemically attack rock minerals Also water that seeps into the ground absorbs additional CO2 from decaying organic matter which causes it to become even more acidic Rainwater interacts with man made atmospheric pollutants such as and compounds resulting in an even stronger acid called which speeds up the weathering process Physical Geology 101 The reason rain can cause weathering is because dissolved H ions displace larger cations in minerals in rocks such as K Na and Mg2 This is a type of chemical weathering involving reactions with water which we call reactions This is exactly how feldspar is able to break down into clay and is why marble gravestones lose the inscriptions on them over time 3 A third type of chemical weathering is when oxygen is added to a metal to form an oxide This is called Such chemical reactions are particularly common in areas affected by i e waste water from mining operations Example iron pyrite oxidizes to form acid This is commonly introduced into streams in regions near old mines Factors Influencing Weathering How long does it take for a layer of topsoil to develop from the weathering of underlying rocks The main factors that control how susceptible a rock is to mechanical or chemical weathering are some minerals are more susceptible to weathering than others Example is very resistant to weathering is not very resistant to weathering In general minerals get more resistant to weathering as we move down Bowen s Reaction Series i e olivine and Ca plagioclase are least resistant if rocks contain lots of fractures they are more susceptible to weathering such as through frost wedging The greater the number of joints the more surface area that s available for chemical weathering Jointed rocks are weathered by having the corners rounded off to form smooth rounded boulders This process is called weathering What type of climate is best for chemical weathering Where is this most likely to occur What type of climate is best for mechanical weathering Where is this most likely to occur Physical Geology 101 Soil Soils can originate through one of two ways they can either be formed in place or brought in from somewhere else Soil deposited during river floods are transported e g along the Nile River each year Transported soils may also be deposited by the wind For example the soils in the Palouse region were blown in by winds about 10 000 years ago We call this type of wind blown soil We recognize five different horizons in a soil profile that reflect the progressive downward migration of weathering action The horizons are from the top down The O horizon is mostly decaying organic matter called The A horizon is a mix of organic matter and minerals like clay The O and A horizons together are called The E horizon is not always present it contains very little organic matter or oxidized minerals so it tends to be light colored It is common in acidic soils The B horizon is predominantly brown or red consisting of oxidized clay minerals The C horizon is mostly original rock that has started to oxidize and weather Do we always see each of these soil horizons in every soil profile YES or NO We also distinguish three main families of soil types rich in clays and are common in areas of moderate rainfall and temperate climates such as the eastern central and NW United States These soils are the best for agriculture are common in areas having dry climates like the SW United States They are rich in CaCO3 and other soluble minerals like gypsum