GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Note Series Weathering Lecture Goals A Mineral Stability B Types of weathering Physical Chemical Biological C Chemical weathering reactions Reference Press et al 2004 Chapter7 Grotzinger et al 2007 Chapter 16 p 371 381 A Mineral Stability Are you glad to be finally done with igneous rocks All those textures all those compositions all those minerals Well the good news is that you are now done with igneous rocks The bad news is that you still need to know Bowen s Reaction Series In a very short period of time 2 lectures we will start talking about the next group of rocks the sedimentary rocks Before we get to them though we have to discuss the origin of sediment Sediment is a diverse group of materials that are initially unconsolidated fragmented and that can be converted to rock sedimentary rock under the right conditions One of the ways that sediment is produced is through the break up weathering of other rocks Since we left off with igneous rocks we might as well use them as an example of how the weathering process works Igneous rocks are composed of minerals that form from molten rock Minerals that form at high temperature and or high pressure do so because they are stable under those conditions Olivine is very stable at 1800 C but at temperatures significantly less than that like that at the surface of the Earth olivine is unstable Add water in the form of rain fall and the mineral becomes very reactive Olivine rich rocks such as dunite peridotite or basalt porphyry do not survive long at the surface of the Earth Bowen s Reaction Series can also be considered a stability series Those minerals that form first from a melt e g olivine pyroxene Ca plagioclase are at the low stability end of the series while those that form last e g quartz muscovite are at the high stability end of the series Quartz is the most stable of the common minerals which explains why it is concentrated along Alabama s beaches but as will be discussed shortly even quartz can be GY 111 Lecture Notes D Haywick 2008 09 2 weathered under the right conditions For the purposes of mineral stability we will add four other minerals mineraloids to our modified Bowen s Reaction Series Kaolinite a clay mineral is more stable than muscovite Limonite hematite and bauxite are all more stable than quartz B Weathering There are three major types of weathering although most textbooks only distinguish two The first type is physical weathering and is defined as the mechanical breakup of rock The second type of weathering is called chemical weathering This is the most important process in soil formation see the next lecture and involves chemical changes during the breakup of rock The last of the weathering types not always distinguished in texts is biological weathering This involves the actions of plants and animals and is really just a combination of physical and chemical weathering The main thing to remember about these types of weathering is that they all reduce rock into sediment Physical weathering does this with little loss in volume Chemical weathering may result in a significant loss in volume GY 111 Lecture Notes D Haywick 2008 09 3 Physical weathering occurs everywhere but is especially prevalent in areas of the Earth that are either very hot e g deserts or very cold e g mountains tundra In hot areas alternations between hot and cold conditions causes rock to expand and contract It is felt by many geologists that this causes rocks to sheet off in a process called exfoliation see the first of the two photos on this page The second photo shows the end result of this process spherical weathering results in rounded granite boulders atop mountains see second photo on this page By the way both photos are from Doug s recent trip to Germany Another type of physical weathering is called unloading Granite forms well below the surface of the Earth in areas of fairly high pressure When exposed at the Earth s surface the rocks no longer feel the confining pressure and may tend to shatter because of the reduced pressure load Unloading is really a problem in new mine shafts Some granites other rocks too but granite is about the worst will exploded in what is called a rock burst This is just one of the hazards of being a miner In cold climates water is the major agent behind physical weathering Liquid water expands when it freezes so any water within cracks fractures and joints see an upcoming rock deformation lecture exerts tremendous force when it freezes Rocks can be literally split apart as the temperature drops Mountains are particularly good areas to see the results of this frost heaving The piles of rock that occur along the base of mountains called scree or talus was mostly derived from frost heave GY 111 Lecture Notes D Haywick 2008 09 4 Physical weathering produces smaller bits of rock but it doesn t actually change the composition of the rock You would be able to recognize bits of granite or basalt or rhyolite The most important thing it does is increase the relative surface area of the rock The surface area is the amount of contact area in an rock that is exposed to water Water is the principle agent behind chemical weathering so the more surface area the more contact area for chemical weathering Or to put it more succinctly the higher the surface area the faster chemical weathering occurs C Chemical weathering reactions There are three major reactions responsible for chemical weathering 1 solution or dissolution 2 oxidation 3 hydrolysis Solution occurs when a mineral dissolves The next result is that you get ions in solution and nothing is left behind example minerals halite calcite Oxidation occurs when a mineral reacts with oxygen in the atmosphere or in water example mineral pyrite Hydrolysis occurs when a mineral reacts with water example minerals orthoclase pyrite olivine GY 111 Lecture Notes D Haywick 2008 09 5 We ll call it quits here for today In the next lecture we will discuss weathering a bit more as it applies to soil development This will also include a discussion about the weathering of quartz Under the right conditions quartz undergoes chemical weathering specifically solution Important terms concepts from today s lecture Google any terms that you are not familiar with Sediment Sedimentary rock Unconsolidated fragmented Weathering Physical chemical biological weathering Exfoliation Spherical weathering Frost heaving Unloading rock bursts Talus