Transcript Unit 4
REVIEW What is WEATHERING? Def: The breakdown of rocks at or near Earth’s surface There are 2 types of WEATHERING: 1. CHEMICAL 2. PHYSICAL CHEMICAL WEATHERING EXAMPLES: 1. OXIDATION (rust) CHEMICAL WEATHERING EXAMPLES: 2. WATER– the universal solvent Slowly dissolves rocks. Will happen faster when there’s acid present (remember the fizzing) physical WEATHERING EXAMPLES: 1. FROST ACTION 2. PLANT ROOTS 3. ABRASION FACTORS THAT AFFECT THE RATE AND TYPE OF WEATHERING 1. EXPOSURE (rocks need to be exposed to weathering agents in order to erode) 2. PARTICLE SIZE (the smaller the particles, the greater the surface area) 3. MINERAL COMPOSITION (different minerals weather at different rates [more on this later]) 4. CLIMATE (the temperature and the amount of water present will determine the amount and type of weathering that can occur) So that’s WEATHERING, what’s EROSION? Weathering occurs IN PLACE Involves the TRANSPORT of sediments. EROSION The process by which sediments are obtained and transported. Erosion involves a transporting system with several components: 1. AGENT OF EROSION- (e.g. stream, glacier, wave, current, wind, or human activity). 2. SEDIMENTS BEING MOVED 3. DRIVING FORCE The DRIVING FORCE behind most types of EROSION is GRAVITY GRAVITY EROSION •Gravity always pulls down on all objects. •Friction opposes the force of gravity, keeping objects in place •Usually these two forces are in EQUILIBRIUM (balance), however sometimes….. •The force of friction decreases, and gravity overtakes friction When this occurs, bad things can happen…… Oh no! Gravity erosion can happen very quickly or very slowly, depending on the conditions. “Soil creep” happens very slowly (less than 1 cm per year). Soil gradually moves downhill Landslides can be quite destructive: Rate = 1 mm/day to 100 km/hr Houses can be totally covered by sand and mud, like this: Mud Flows are usually caused by the friction holding the rock in place being reduced by the presence of water. THE PROBLEM WITH LANDSLIDES AND MUDSLIDES IS THAT THEY ARE OFTEN UNPREDICTABLE AND CAN BE VERY DAMAGING TO LIFE AND PROPERTY This landslide tragically took the life of many unsuspecting woodland creatures. The final form of gravity erosion occurs when rocks weather at a higher elevation and fall to a lower elevation. This is called, (you guessed it), a Rock Fall WATERSHEDS / DRAINAGE BASINS Def: The area of land drained by any one stream REMEMBER: a stream is running water that’s confined to a channel---this channel can be any size: MEDIUM LARGE SMALL Q:Where does water go once it hits the surface of the earth? A: Some gets absorbed into the ground (infiltration) some flows along the surface (runoff) The rivers of New York State obtain their water from very distinct regions. The state is divided into the following DRAINAGE BASINS (WATERSHEDS): These watershed regions are pretty small because the streams they feed into are pretty small What Drainage basin is Washingto nville in? The US is divided into watersheds also: THE MISSISSIPPI RIVER WATERSHED is the biggest (OBVIOUSLY) The UNITED STATES is split into two LARGE drainage basins. This side, all water flows to ATLANTIC OCEAN This side, all water flows to PACIFIC OCEAN STREAMS Running water is the most common agent of erosion on Earth’s surface. THE WATER CYCLE! HOW DOES THE WATER GET INTO A STREAM? Water always flows from HIGH elevation to LOW elevation. Along the way, this water combines with other water, so the size of the stream gets bigger and bigger. B A The smaller streams are called tributaries. Different streams flow at different velocities (speeds) WHY? GRADIENT! B A C Would a stream flow faster from A to B or from A to C? The speed at which a stream flows will determine what size sediments it can carry • Fast moving water can carry larger sediments (duh) Big Small Slow Fast Water flows at different speeds within the same stream. WHY? FRICTION The water that is in contact with the sides and bottom of the stream channel will be slowed by friction and will therefore move SLOWER. Water flows differently around curves than it does in a straight line Because of this, water flows at different speeds within the same stream. Moves slowly Movesaround faster around the inside the outside This means that around the outside of curves, since the water is moving faster, it will be able to erode more and larger particles. The opposite is also true, since the inside of curves are moving more slowly, some of the particles the water carries will be deposited (dropped). Here’s a picture: DEPOSITION EROSION Streams that exist in hilly or mountainous regions slowly erode sediment over time The goal of every stream is to make the land FLAT. This takes time (as in MILLIONS of years) The FLATTER the land, the OLDER the stream YOUNG streams flow over a steep gradient and tend to be relatively small. As they flow they carve out a V-shaped valley Notice the VERY steep sides Waterfalls indicate a YOUNG stream As time goes by, the stream erodes away the land and makes it flatter. When the gradient is low, the shape of the stream changes and meanders form: MEANDER faster around the outside of the erode more sediment, gradually Because water moves meander it is able to carving out a bigger and bigger meander: Eventually, water breaks However, the channel full of water is still there, so across the it becomes an OXBOW LAKE banks and cuts off the old channel. This process creates an OXBOW LAKE Why “Oxbow”? THE GRAND CANYON WAS FORMED BY STREAM EROSION Eventually all streams flow into the ocean. This part of a stream is called a DELTA