GS 106 - Introduction to Physical Geology

16-1 through 20-1

16) Describe a meandering stream using the words and phrases lateral erosion, point bar, cutbank, and oxbow lakes.

Answer:  A meandering stream forms where gradient is low and the river can no longer cut down vertically in its channel but rather must use lateral erosion to flow across the land. Meandering rivers primarily erode the outside of their curves, called cut banks, and deposit that sediment on the inside of curves, called point bars. If the meander becomes too curvy, the stream can cut through the neck of a point bar and cut off the meander from the main river channel forming an oxbow lake.

17) Describe neap and spring tides and how they form.

Answer: Neap tides occur when there is the least difference between high and low tide. Neap tides occur when the Moon 90° out of alignment with the Sun and the Earth, and this occurs on first and third quarter moons. Spring tides occur when the Sun, Earth, and Moon are in total alignment and the increased gravitational pull creates the highest difference between high and low tide. Spring tide occur at the full and new moons.

18) Describe the 3 major ocean landforms that are created due to plate tectonic boundaries. Which of these landforms do we now find anywhere close to Oregon’s Coast?

Answer: The three major ocean landforms are mid-ocean ridges, volcanic island arcs and trenches. The mid-ocean ridge forms as plates separate and lava erupts creating ridges of new lithosphere. Trenches are a long, narrow, deep depression in the ocean floor, typically one running parallel to a plate boundary and marking a subduction zone where the overlying plate is compressing the overlying plate making a dip in the ocean floor. A volcanic island arc occurs as subduction introduces water in the mantle at oceanic-oceanic convergent boundaries. This lowers the minerals in the mantles melting point ad creates magma that is erupted at the surface to form volcanoes. Oregon does not have any volcanic island arcs associated with it.

19) How does a surface ocean current form? Be sure to use the words and phrases wind, deflection, Coriolis effect, and gyre in your answer

Answer: The water at the ocean surface is moved primarily by winds that blow in certain patterns because of the Earth’s spin, the uneven heating of Earth’s surface, and the Coriolis effect. Winds are able to move the top 400 meters of the ocean creating surface ocean currents. Surface ocean currents form large circular patterns called gyres. Gyres form as surface currents are deflected 45° from the wind current due to Ekman transport and the Coriolis effect as well.

20) What is the equation for discharge? What, specifically, are the things you would need to know to calculate discharge? What are 2 reasons why one would need to know the discharge of a river?

Answer: The equation for discharge is Q = A x V, or discharge equals channel area multiplied by stream velocity. To measure discharge you would obviously need to know how fast the stream was moving at a given point but also the width and depth of a river channel. Reasons to measure discharge include knowing whether flood or drought conditions are occurring, recreation safety and enjoyment consider fishing and swimming, water management and allocation, and to work out the load of contaminants and sediment the waterway is carrying.

16-2 through 20-2

16) Describe the formation and movement of thermohaline circulation by using the words and phrases, density, salt, ice, and conveyer belt.

Answer: Thermohaline circulation forms as warm water from the equator is brought north to and as it meets the area around Greenland. Here, the water becomes cold enough to form ice. When ice is formed the water increases in salinity. Both the cooling of the water and the increased salinity creates dense water that sinks. As this water sinks it pushes other water out of the way creating a sort of conveyer belt current that circles around the whole globe.

17) Describe how meandering streams and braided streams form.

Answer: Meandering streams form where a stream is laterally eroding in its valley because of a low gradient. These streams became very loopy, or begin meandering, so that the outer portion of the meander where the greatest amount of erosion occurs. Braided streams form due to a large amount of sediments being introduced to a stream, too much sediment for the stream to carry. As the stream channel backs up, it jumps its channel and creates a new channel. This creates a stream that flows in many shallow inter-connecting channels.

18) Name and describe the 3 types of sediment loads found in streams and give an example of the types of sediments found in the load. Which of these loads has the hardest time making it to the ocean?

Answer: The first type of sediment load is the dissolved load, which is sediments that are dissolved and part of the water. These can include things like salts and other ions. The second type of sediment load is the suspended load, which is sediment that is suspended in the water column. These can include fine-grained sediments like clays, silts, and sands. The final sediment load is a bed load where coarse-grained material jump along the bottom of the stream. These sediments can include things like gravel, cobbles and boulders.

19) Describe the process of sea floor spreading using the words and phrases convection, mid-ocean ridge, trench, and lithosphere.

Answer: Seafloor spreading occurs at mid-ocean ridges, where tectonics plates separate, causing magma to form and erupt lava on the ocean floor creating new lithosphere. As new lithosphere is created it pushed old lithosphere out of the way. Eventually the plates meet a convergent plate boundary and are subducted into the mantle, and this occurs where trenches occur. This overall movement this creates is driven by convection heat flow, where heat rises at mid-ocean ridges, and cold crust sinks back into the mantle at trenches.

20) Describe the difference between competence and capacity. Describe the capacity and competence of a headwater stream. Describe the capacity and competence of a stream near the mouth of a river.

Answer: The competence of a stream is the maximum particle size that it is able to transport. The capacity of a stream is the maximum load a river can transport. A headwater stream occurs in mountainous areas and so tends to have both high capacity and low competence because of the amount of sediments added as well as the slower velocity. The mouth of a river tend to have high competency but low capacity because even though it increases its velocity, unfortunately, most of the larger grains stay in the headwaters.