1)) In the Mississippi River delta in south Louisiana shallow subsurface imaging
ID: 236282 • Letter: 1
Question
1)) In the Mississippi River delta in south Louisiana shallow subsurface imaging of the sediments reveal buried channels beneath delta sediments that are less than 10,000 years, and these channels extend offshore to the edge of the continental shelf. What is a simple explanation for these buried channels?
They represent giant flood events before the delta was formed and before the river formed.
They represent channels of the Mississippi river delta when sea level was lower, during the ice age.
The river must have been bigger in the past, cutting a channel clear to the edge of the shelf below sea level.
Turbidity currents cut a submarine canyon on the shelf before the delta buried them.
2)
Abyssal plains are very flat because ________.
basalt volcanos on the seafloor produce little terrain
sediments accumulate to cover the seafloor topography
they have flat faults, they form flat, planar topography
they are not really flat; it is just that there is no data so they are shown as flat
3))Scientists infer that chemical weathering of crustal rocks is not the only source of some minerals, such as chlorine, bromine, sulfur, and boron because ________.
Answers:
those minerals have the wrong isotopic composition for weathering products of crustal rocks
there are more of those minerals in the oceans than can be derived from crustal rocks alone
some of the minerals are only found in meteorites and must have an extraterrestrial source
the minerals are highly reactive and are not found as compounds that would come from crustal material
4))
Multibeam provides the best modern technique for obtaining seafloor bathymetry, so why hasn't it been used for the entire ocean?
Answers:
It is too expensive, requiring many ships many years to survey the entire ocean.
It doesn't work in deep water.
It doesn't work where there is ice.
Oceanographers simply haven't had time to process all the data.
Abyssal plains are very flat because ________.
Explanation / Answer
1. The present Mississipi River delta formed during the last 7000 years in a dynamic process known as delta cycle. The river carries sediments from the interior of continents and deposit them over the warm waters of Gulf of Mexico in a period of 1000-2000 years which represents a delta cycle. With continued growth in delta lobe, the river path become difficult and longer. As a result the river changes its course and abondon the old lobe by cutting a shorter route to the gulf. These abondoned lobes buried with the overlying sediments and then a new lobe forms with a river's new route and this cycle continues. So this indicate that the river must be bigger in the past, cutting a channel clear to the edge of the shelf.
2. Abyssal plains are comparatively flatter part of the ocean basins and have slope ratio less than 1:1000 and found at depths ranging from 13,000-16,000 feet. Since abyssal plains are made up of the sediments eroded and transported from the continents over a considerable period of thousands of years. These layers of sediments have covered any irregularites in the ocean basin beneath. So these are the sediments that accumulate to cover the sea floor topography.
3. Sulphur, chlorine, bromine and boron are highly reactive minerals and are form compunds with other minerals. They are rarely found in the free state. These source of these minerals is mainly the ocean and the sea water. So there are more of these minerals in the oceans that can be derived from crustal rocks alone.
4. In multibeam technique, by taking a lot of measurements from a single place ,the depth of the sea bed is estimated. In this technique the time taken by the sound waves to return is measured. It is useful in taking depth sounding immediately lying below the ship. To produce a map of the entire sea floor one has to tediously contour the depth of the area i.e. line connecting an equal depth. So this technique is too expensive, requiring many ships many years to survey the entire ocean.
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