M012 - Geo PDF

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Landmass Denudation As your book explains, denudation refers to “any process that wears away or rearranges landforms…including weathering, mass movement, erosion, transportation, and deposition” (394). Those processes are brought on by the combination of gravity and moving water, waves, wind and ice. I will discuss these processes as we go on, but one key thing to remember is that weathering and erosion are not the same thing. Weathering involves the break down of materials (like disintegrating rocks) while erosion is the simply the transport of weathered materials from one location to another. Oh, one other thing! Your book and I constantly refer to “rocks” in this chapter. Just keep in mind, that “rock” is a relative term – it means little pebbles, as well as entire mountain sides; thus, rock is a term for the crusty parts of Earth. So, as the earth denudes, it starts to take shape. This is all part of Earth’s constant need for equilibrium and stability, as mountains are formed through tectonic uplift – an endogenic processes and referred to as an initial landscape, exogenic processes act to tear down these landforms and bring the landscape back into equilibrium – which then creates sequential landscapes. This balance is explained with the dynamic equilibrium model: the idea that landscape formation is a balance between uplift (endogenic) and reduction (exogenic) processes. Landforms will reach a tipping point – a geomorphic threshold in which it can no longer sustain the amount of denudation, and it will be forced to adjust – for example, a landslide, or when a river is forced to change course. On page 395, you can read about the sequence of events that make up this model. Slopes play a role in this model because depending on the grade (how steep or gentle it is) land will either stay in the same place, or it will drastically shift. So the gentler the slope, the less movement, the steeper the slope, the more movement you will have.

Weathering Processes Okay, moving on to Weathering. Weathering is actually defined by how it sounds – weather. It deals with how weather (wind and rain) affect landforms. Weathering weakens rocks (massive and small) and make them more susceptible to the pull of gravity. Weathering is the process of breakdown the rock surface of Earth by either disintegrating them into mineral particles (in a physical/mechanical process of force) or dissolving them by water (this would be a chemical process). This is how a lot of out soil gets created. A typical hillside has bedrock, and after millions of years of weathering, the top layer becomes weathered and loosened which is then called regolith. This regolith is then transported and deposited down the hill, and as it continues to break-

up and become weathered, it mixes with decaying vegetation and forms soil. Sometimes, you can see the un weathered bedrock exposed on a hill slope. Many of the hikes around the Bay Area have cliffs that expose the interior of hill slopes, and you can see the regolith and bedrock. There are several things that influence weathering processes: Rock composition is really important: is it hard or soft, broken or unbroken, soluble or insoluble? If there are cracks, known as joints, it will be more easily affected by weathering. Climate: wetter, warmer climates speed up chemical weathering, while colder climates speed up physical weathering (we’ll discuss this more in a bit) Slope orientation: is it facing the sun? Receiving more rain? Think about the what the mountains look like when you are standing in the middle of the San Jose Valley, you will notice that the Santa Cruz Mountains (not facing the sun) look green and lush, while the hills to the east (facing the sun) are much more dry and sparsely vegetated. Subsurface water is also important, how high is the water table? If it is near the surface, then the ground will likely experience chemical weathering from water. Vegetation: It can protect rock and soil, by stabilizing them with their roots and intercepting rain, to slow it down. But roots can also destabilize rocks by forcing their way through crevices and separating them (this is physical weathering); roots can also produce acids that start to decay rocks (chemical weathering). Time: longer exposure to the elements of nature (wind, frost, ice, rain, gravity), the more it will become weathered. Physical Weathering So the list above already mentioned several weathering processes, but here we will discuss it in a bit more detail. Physical weathering is also known as mechanical weathering and it refers to the disintegration of rock material without changing it’s chemical makeup. So basically breaking up rocks by force. The most likely ways are through frost action, salt-crystal growth, and exfoliation. Frost action or freeze-thaw is when water seeps into the crevices or joints of rocks, and freezes – as we know when water freezes it’s volume expands – this forces the joints apart ever so slightly, then the ice thaws, allowing more moisture to enter the now larger joint. That water will eventually freeze, and again, force the rock joints even further apart – the process is called frost wedging. Over time, this can break the rock completely. Obviously, this only

occurs in areas that seasonally have temperatures both above and below freezing. Salt-Crystal Growth (or salt weathering) is common in hot, dry climates, where evaporation is high and draws moisture to the surface of the rock, which then evaporates, left behind are the previously dissolved, crystalized minerals (salts). Overtime they accumulate and grow in the joints and crevices, eventually forcing the rocks apart. Again, remember -this can happen in rock strata (lateral sheets of rock) not just small little rocks. Exfoliation is the last form of physical weathering that we will discuss. It is when a rock peels or slips off in sheets instead of breaking into grains. Sort of like when you get a facial – through exfoliation, you are removing dead layers of skin – same thing with rocks. This process emerges when pressure is released from the removal of overlying rock. This happens when lots of weathering is happening on top of a huge rock formation – like a pluton, and the pluton reaches that geomorphic tipping point and lets go of that entire loosened layer at once – like Half Dome in Yosemite. Now on to Chemical Weathering Keep in mind that Physical and Chemical Weathering do not happen in isolation of each other. They are usually occurring at the same time, and working together. Now, Chemical weathering refers to the chemical breakdown of rock material, which always involves water. As you know it’s easier to dissolve minerals in hotter water, well that happens at this larger scale too. The higher the temperature, and the more water there is, the more accelerated the weathering will be. Think about hand washing dirty dishes. Say you have a plate with some baked-on food – which will remove the dirt faster, a gentle stream of cool water? Or a whole bunch of fast moving hot water? The answer is the more, hot water. – Same goes with chemical weathering on rock. You have probably seen weathering on tombstones – where you can barely read the etching because of acid rain dissolving the stone. We’ll talk about 3 main forms of chemical weathering: Hydration & Hydrolysis, Oxidation, and Dissolution of Carbonates. Hydration refers to the combination of water and a mineral, but it doesn’t actually form a new chemical compound, but it does change the chemical structure of a mineral. This can result in the expansion of the mineral, which can then cause physical weathering by wedging the rock apart. Hydrolysis is when water actually chemically changes a mineral into a new chemical compound - a different mineral. Keep in mind that rocks contain many different minerals, and not all minerals will undergo hydrolysis, so for example silica is formed from the hydrolysis of feldspar in granite, this silica is then resistant to any further chemical breakdown, so as the rocks are still being broken down around it (granular disintegration), it (the silica) will

erode and get transported elsewhere downstream, eventually becoming sand on a beach somewhere. Thus minerals with a lot of silica are more resistant to chemical weathering. The same thing happens with clay – it is a product of feldspar too, and then it can’t be chemically weathered anymore, so it usually becomes part of soil or sedimentary rock. The next kind of chemical weathering is oxidation. This occurs when certain metallic elements combine with oxygen to form oxides. You’ve all seen this in the form of rust. Rust is actually iron oxide it’s produced by the combination of iron and oxygen. Oxidation slowly breaks down minerals making them more and more susceptible to weathering. Finally, there is carbonation. This occurs when water vapor dissolves carbon dioxide, which later creates acid rain which can then dissolve many minerals such as limestone, and marble (a form of limestone). This is the case with what happens to a lot of tombstones.

Karst Topography Okay, moving on. Let’s talk about Karst Topography. These are landscapes dominated by chemically weathered limestone. The landscapes themselves are a massive formation of weathering, and we call them Karst because the name references an area in Slovenia where these processes were first studied. Here are the prerequisites for a limestone landscape to form into karst topography: The limestone must contain 80% or more calcium carbonate (which will end up getting dissolved) A complex pattern of joins in the limestone so water can form routes to subsurface channels An aerated zone between the ground surface and the water table Vegetation is needed to supply organic acids to enhance the dissolution process. Here are some of the characteristic features of karst topography: Sinkholes: these are circular depressions in the ground from dissolved limestone. You can have a solution sinkhole which slowly sinks between major joints. These can be several hundred feet deep and thousands of feet in diameter. Eventually these might turn into a collapse sinkhole if there happens to be over an underground cavern beneath it, in which case, the solution sinkhole completely collapses into the cavern.

Karst Valleys: These are elongated depressions, up to several kilometers long, they may contain sinkholes, bogs (sort of like a swamp in a karst depression), and disappearing streams. Tropical Karst: Karst topography in tropical climates are characterized by landforms called cockpits and cones. Cockpits are deep-sided pits that kind of look like egg cartons, many of the pits have sinkholes which collapse into underground caverns. Cones are limestone formations that have resisted weathering, unlike the rest of the landscape, so they stick out like towers. These are also called Tower Karst. Caves and Caverns Caves are natural underground areas large enough for humans to enter. They form in limestone because it is so easily dissolved. Large caves formed by chemical weathering are called caverns. These usually form just below the water table, where lowering of the water table helps to form and expose them even more. As water seeps down into the caverns and caves, dissolving limestone, several new formations are created: Speleothems: formed from minerals deposited inside the caves occurring in a variety of shapes. See figure 14.21 in your book for detailed images. Dripstones: Speleothems formed from water containing dissolved minerals slowly dripping from the cave ceiling. Calcium carbonate precipitates out of the evaporating solution, which accumulates in layers. Stalactites: Dripstones growing on the ceiling of a cave or cavern. Stalagmites: build up on the ocean floor just below dripping stalactites. The way I remember the difference between the two is this: “hold on tight” to a stalactite or you will fall to the floor… Column: when a stalactite and stalagmite meet. Flowstones: billowy sheets of calcium carbonate on cave floors and walls. The study and exploration of caves and caverns is called speleology and it’s an area that has hardly been studied. There is very little that we know about these ecosystems and the life in them. So if you’re interested, there is still tons to explore and learn, so go spelunking!

Mass Movement Finally, we are going to talk about mass movement or mass wasting which refers to the downslope movement of materials made up of soil, sediment, or rock propelled by the force of gravity (410) – so basically, stuff that falls down hills. The driving force in mass wasting is gravity, and the resisting

force is the strength of the slope material – how cohesive it is and its internal friction. The steeper the slope the more susceptible it will be to mass wasting. Classes: Rockfall: A volume of rock that falls through the air and hits a surface. Debris Avalanche: A mass of falling and tumbling rock, debris, and soil. Landslides; A sudden rapid movement of a cohesive mass of regolith or bedrock that is not saturated with moisture is a landslide. There are two forms of landslide: translational slides and rotational slides. Flows: So the technical term for a mudslide is actually a mudflow. Technically, if the moisture content of the moving material is high, which is the case with mud, then you use the suffix -flow rather than –slide. So really, mud can’t actually slide – it can only flow. So from now on, you can correct your friends and family with that fun fact! The general public still refers to mudflows as mudslides – I’ve even heard weather forecasters on the weather channel refer to them as mudslides, but that’s not the scientific term… silly weather channel. Creep: A persistent, gradual mass movement of surface soil is soil creep....