Chapter 2. Wearing Down Landforms: Rivers and Ice. Physical Weathering

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1 Chapter 2 Wearing Down Landforms: Rivers and Ice Physical Weathering Weathering vs. Erosion Weathering is the breakdown of rock and minerals. Erosion is a two fold process that starts with 1) breakdown of land (weathering) and also includes the movement (transportation) of these weathered materials 2) Deposition of the eroded material occurs when it is dropped in a new location. Denudation is a term that refers to the wearing down or smoothing off of land features. The processes of weathering and erosion are denudational. They are also gradational because they grade the earth s surface. Physical Weathering vs. Chemical Weathering Physical weathering is the breakdown of rock and minerals by mechanical stress Chemical weathering breaks down rock with chemical reactions often including water. Types of Physical Weathering Frost fracture the expansion of freezing water that causes rocks to crack. Heat expansion rocks can expand and subsequently fracture. Plant growth expansion due to root growth as shown in the picture below. Burrowing animals- tunneling animals can increase the size of existing cracks Exfoliation as internal pressure is released from certain rocks, it can cause layers to split and fall off. Environmental affects on Physical Weathering Fast temperature, changes like those that occur in the desert, increases the amount of physical weathering due to heat expansion. Conversely, in regions like the tropics where there is little temperature change, the amount of physical wreathing due to heat expansion is minimal. Abundant precipitation combined with alternating freezing/thawing temperatures increases the amount of frost fracture. Conversely, the absence of those climatic conditions reduces the amount of frost fracture. Running water increases physical erosion as friction occurs between water and rock. Ocean waves cause hydraulic pressure and abrasion on the shore leading to physical weathering.

2 Chemical Weathering Types of Chemical Weathering Chemical weathering is the breakdown of rocks and minerals by chemical reactions and usually involves the action of rainwater. There are three different types of chemical weathering described below. The formation of solutions as rainwater absorbs CO2, SO2, and other chemicals from the atmosphere along with organic acids from the soil, which then reacts with rock and minerals causing some to dissolve and move away. Hydrolysis, like the first process, involves the minerals in solution. In this case, carbonic acid reacts with silicates in some rocks leaving a soft clay from which potassium, sodium and magnesium are subsequently leached. Oxidation is the reaction of metallic minerals to oxygen (mainly in water). This results in the formation of oxides, which tend to be softer than the original mineral. For example, rust on iron. How Environmental conditions influence Chemical Weathering Rivers Heavy rain, running water, and abundance of water increases the amount of dissolving that occurs. Conversely less abundance of water leads to less dissolving. High temperatures will increase the rate of chemical reactions such as oxidation. Ocean water contains salt which can increase the rate of many reactions like oxidation. Drainage Basin - is the area of land drained by a river and its tributaries. Life Cycle of a River Rivers change over time and seem to go through three stages: Youth Maturity/Late Maturity Old Age Youth (figure 2.7 p. 28) Are usually found in highland or mountain regions. They tend to have a steep slope (high gradient) Usually have a small volume of water They have a rapid flow of water There is usually very rapid erosion especially vertically A narrow V shaped valley is characteristic Water falls and rapids are common

3 Maturity (figure 2.7 p. 28) Most high relief is eroded Gentler slope Many well developed tributaries Broad flat river valley Well developed flood plain More lateral erosion than vertical Meandering results Late Maturity (figure 2.7 p. 28) Remember the stages are not distinct. These changes occur over long periods of time. Old Age (figure 2.7 p. 28) Almost no slope Very little relief Elaborate meandering Oxbow lakes develop Often swampy areas around river Very muddy due to slow speed Most susceptible to flooding because of large flood plain. Evaluating Evidence to Determine the Age of Rivers There are six common pieces of evidence you can look for to determine the stage of a river. Slope of the river (steeper = younger) Relief of the banks (steeper = younger) Width of the valley (wider = older) Meandering (more = older) Size of flood plain (wider = older) Rapids or water falls (more = younger) Two Directions of River Erosion Vertical erosion makes rivers deeper as is the case in young rivers Lateral erosion makes rivers wider leading to the meandering of mature rivers.

4 River Deltas Delta is a low lying area at mouth of river formed by deposition of silt deposition occurs because river slows as it enters ocean or lake. There are three types of deltas arcuate digitate estuarine. Arcuate Delta: named from Latin word for curved in the shape of a bow. Fan shaped ex. Nile Delta ( p. 32) Digitate Delta: From Latin for finger Delta with long fingers of sediment reaching into the sea ex. Mississippi Delta ( p. 32) Estuarine Delta: Formed when river runs into a bay or estuary Tidal mud flats form which can be seen at low tide Sediment deposited from river outflow and from Tidal inflow The Seine River Delta (figure 2.12 on p. 32)

5 Continental Glaciers vs. Alpine Glaciers Continental Glaciers There are two types of glaciers, Alpine Glaciers and Continental Glaciers: Continental glaciers cover parts of continental land masses like Greenland a mass of ice, situated over most of a continent, which may be moving, or has moved, overland. Alpine glaciers are found high in mountain valleys, above the snow-line a mass of ice, situated on an upland, which may be moving, or has moved, overland. Differences: Location; Alpine glaciers are only found on mountain tops whereas continental glaciers are only found at the earth's poles regardless of elevation. Size; Alpine glaciers are smaller compared to Continental glaciers. Similarities: Both move and cause erosion Both change the landscape Both developed in constantly cold temperatures below freezing. Land Forms Created by Continental Glaciers Figure 2.13 on page 34 of your text shows a continental glacier and the land forms it creates through weathering and deposition. Features of Continental Glaciation Outwash plain: Like a river Delta Melt water flowing from glacier deposits silt like river deltas Silt is deposited in layers Small particles are carried further away Larger particles drop closer to the glacier Terminal Moraine: Heap or ridge of bulldozed gravel that marks the end of the forward motion of a glacier. As a glacier retreats it deposits debris/gravel Erratics: Large boulders that were transported long distances and dropped. They now sit in a region and look very much out-of-place.

6 Drumlins: Egg shaped hill Formed under glaciers Sloped or Pointy end points in direction of ice flow Formation: Ice melts under glacier Deposits of gravel are made Glacier moves forward Deposits are bull-dozed along and catches up in rough areas forming piles or drumlins. Eskers: Long deposits of eroded glacial material Formed by sub-glacial streams that deposit material like all rivers. They are sometimes known as Highways of the North because they are good for traveling on with ATV's. Evidence for Direction of Glacier Movement The gently sloped end of drumlins point in the direction of glacier movement. The terminal moraine marks the furthest extent of the glacier. The layers of silt in an outwash plain can indicate direction of glacier movement. fine particles would be at the leading edge while larger particles would have been closer to the glacier. Alpine Glaciers Alpine glaciers are like very slow moving rivers of ice flowing down high mountain valleys. create land forms by weathering and deposition typically erode the mountain beneath them into a U-shaped valley with steep sides Some alpine or valley glaciers are 1000m thick and up to 160 km long, though most are only a few km in length Features of Alpine Glaciation Cirque a circular hollow cut into bedrock during glaciation Side and back walls are steep but front wall opens downward Cirque Formation Alpine glacier freezes onto mountain valley and as is proceeds it plucks/gouges rock from the mountain top leaving the cirque shape. Arête Steep knife edged ridge between two cirques in a mountainous region.

7 Hanging Valley A high level tributary valley from which the ground falls sharply to the level of the lower, main valley. The depth of the lower valley is due to more severe glaciation. Lateral Moraines a landform deposited by a glacier or ice sheet at the side of the glacier. Terminal Moraines deposits that mark the farthest extent of the alpine glacier the same as with continental glaciers. Fjords Alpine Glaciers erode troughs and valleys in the mountain Glacier valley reaches the coast. Glacier melts and sea water floods the valley Fjords are very common in Norway and in Newfoundland

Chapter 2. Denudation: Rivers and Ice

Chapter 2. Denudation: Rivers and Ice DENUDATION: process that lowers level of land - caused by rivers, glaciers, waves & wind - involves processes of WEATHERING & EROSION Weathering Def: breakdown of