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Lecture 16

EPSC201 Lecture 16 Notes.doc

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McGill University
Earth & Planetary Sciences
EPSC 201
Anthony Williams- Jones

EPSC 201 Lecture 16 Notes Recap lecture 15 at the beginning... There are two types of weather; physical and chemical. In the desert, there is predominantly physical weathering. Chemical weathering occurs in warm, wet climates. The chemical reactions need water and heat to occur. The breakdown of limestone and marble requires acid. Acid can be found naturally from the breakdown of organic material, such as pine needles. Carbon dioxide is generated from the breakdown of limestone, which easily leaves. The products of the breakdown are either gaseous, or soluble in water, which makes the breakdown very easy. Potassium feldspar when exposed to acid breaks down into clay and quartz, along with potassium ion. If the structure hasn’t broken down after weathering, there will be lots of clay and quartz present, because that what granite breaks down into. Quartz is resistant to weathering, and this is why we see nice sandy beaches of quartz. Water is a polar molecule, and helps dissolve ions through its dipoles. The water molecules point their negative oxygen towards positive ions, such as potassium. The water molecules point their positive hydrogen’s to- wards negative ions, such as chlorine. These polar properties make it a great solvent (the universal sol- vent). When the rock is broken down, the ions are dissolved into the water. As iron is oxidized, it makes sulphuric acid. The rust on cars is actually the mineral goethite. There are vertical and horizontal joints in the rocks. Joints cause physical and chemical weathering, which explains why we observe block like rocks. Pure physi- cal weathering causes sharp joints, while chemical weathering causes the edges to be round. A large number of joints greatly increases the surface area, which causes rapid weathering. The greater the sur- face area, the more rock is exposed to the weathering, the faster it weathers. Biotite and feldspar breakdown and yield their ions to be washed out. Clay is also produced, and can be washed out. The quartz is left over, and does not break down further. Perfect spheres can be created through chemi- cal weathering. Transport and deposition of sediment depends on how broken down the material is. If the sedi- ment is course and large, it will be deposited close to the source. If the sediment is fine grained, it will be transported very far from the source. Ions can travel a long way, even in underground water, because they are soluble. As the river moves further along, its energy is less. So at a far distance, the river is only able to carry the fine particles, but at the beginning of the transport, the river can carry everything. There is also the effects of breakdown, that as the particles travel further, they breakdown even more, causing them to be smaller. So the effects of breakdown, in addition to the energy of the river, cause there to only be fine particles at the end of transport, where as at the beginning of transport, there is a mix of particles. The grains are rounded at the end of transport, but at the beginning they are sharp and angular. How do we go from broken down material to rocks? Cementation. There is a large number of grains present, for instance quartz sand. We can cement sand into sandstone. The weight of water forces the sediment to pack together. The warm groundwater moves in-between the spaces of the sediment, and cools as it moves through the cold sediment. The ions in the solution will precipitate (solidfy out of solution) as the water cools. The ions are only dissolved because the water is warm. For example, SiO2 (quartz) will precipitate out as the groundwater is cooled in- between the sediment. This precipitated silica joined together all the claustic sediments. Many sediments are cemented by silica. Calcium carbonate (CaCO3) is another important cementing agent. It glues together the sediment, the same way silica does. Calcium carbonate precipates out when the ground water is alkalai (basic). As water goes deeper in the earth, it gets warmer. Key point – warm water brought through cold sediment forces silica to precipitate out, and joins together the sediments Gravel beds of rivers can be cemented together. The long axis of the rocks always point down- stream, so the direction of the river can be determined from rocks. These rocks form one bigger rock with fined grained material in between the rocks, called a conglomerate. If the river is steep and flowing fast, it is a high-energy environment, and can move all sized material. Low en- ergy rivers are flat and can only move fine-grained material. The material of the rocks can indicat- ed how much energy was present in the environment. Sandstone, made primary of small grained material, forms in low energy environments. Shale is thin layers of rock formed from mud. Another type of sediments is glacial moraines. These are extremely variable in size, and poorly sorted. Lots of angular fragments. Once the glacier picks up the material, it is just moved along, and not subjected to weathering. Rivers meander more when they are lower in energy. Fast flowing rivers are straight. The posi- tions of meanders change over time, relatively quickly (decades). There are floodplains of each side of the river, which flood every year (spring when the snow melts). O
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