Chapter 5 sediments

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Answers to STUDY BREAK Questions

Essentials 5th

Chapter 5


1. What is sediment?
Sediment is particles of organic or inorganic matter that accumulate in a loose, unconsolidated form.

2. Why are very few areas of the seabed completely free of sediments?
The marine processes that generate sediments are widespread. Sediment particles may consist of the remains of once-living organisms, bits of windblown dust, volcanic ash, etc.

3. The ocean is more than 4 billion years old, yet marine sediments are rarely older than about 180 million years. Why?
As you learned in Chapter 3, tectonic processes form and destroy the seabed over time. Because of subduction, seabed older than about 180 million years is rare.

4. What types of particles compose most marine sediments?
Most marine sediments are made of finer particles: sand, silt, and clay.

5. Which particles are most easily transported by water?
The smaller the particle, the more easily it can be transported by streams, waves, and currents.

6. How do well-sorted sediments differ from poorly sorted sediments?
Sediments composed of particles of mostly one size are said to be well-sorted sediments. Sediments with a mixture of sizes are poorly sorted sediments. Sorting is a function of the energy of the environment—the exposure of that area to the action of waves, tides, and currents.

7. What are the four main types of marine sediments?
Marine sediments are separated into four categories by source: terrigenous, biogenous, hydrogenous (or authigenic), and cosmogenous.

Terrigenous sediments are the most abundant. As the name implies, terrigenous sediment originates on the continents or islands near them. They are carried to the ocean in rivers and streams, or by winds as blowing dust, and dominate the continental margins, abyssal plains, and polar ocean floors.

Biogenous sediments, the next most abundant, consist of the hard remains of once-living marine organisms. The siliceous (silicon-containing) and calcareous (calcium carbonate-containing) compounds that make up these sediments of biological origin were originally dissolved in the ocean at mid-ocean ridges or brought to the ocean in solution by rivers. Biogenous sediments are found mixed with terrigenous material near continental margins, but are dominant on the deep ocean floor.

Hydrogenous sediments are minerals that have precipitated directly from seawater. The sources of the dissolved minerals include submerged rock and sediment, leaching of the fresh crust at oceanic ridges, material issuing from hydrothermal vents, or substances flowing to the ocean in river runoff. The most prominent hydrogenous sediments are manganese nodules, which litter abyssal plains, and phosphorite nodules, seen along some continental margins. Hydrogenous sediments are also called authigenic (authis = in place, "on the spot") because they were formed in the place they now occupy.

Cosmogenous sediments, which are of extraterrestrial origin, are the least abundant. These particles enter the Earth's high atmosphere as blazing meteors or as quiet motes of dust. Their rate of accumulation is so slow that they never accumulate as distinct layers -- they occur as isolated grains in other sediments, rarely constituting more than 1% of any layer.

8. Which type of sediment is most abundant?
Terrigenous sediments are the most abundant. The largest terrigenous deposits form near continental margins.

9. Which type of sediment covers the greatest seabed area?
Biogenous sediments, though their total volume is less than that of terrigenous sediments.

10. Which type of sediment is rarest? Where does this sediment originate?
Cosmogenous sediments, which are of extraterrestrial origin, are the least abundant.

11. Do most sediments consist of a single type? (That is, are terrigenous deposits made exclusively of terrigenous sediments?)
Most sediment deposits are a mixture of biogenous and terrigenous particles, with an occasional hydrogenous or cosmogenous supplement. The dominant type gives its name to the mixture.

12. How do neritic sediments differ from pelagic ones?
Neritic sediments consist primarily of terrigenous material. Deep-ocean floors are covered by finer sediments than those of the continental margins, and a greater proportion of deep-sea sediment is of biogenous origin. Sediments of the slope, rise, and deep-ocean floor that originate in the ocean are called pelagic sediments.

13. Are neritic sediments generally terrigenous or biogenous?
The bulk of neritic sediments are terrigenous; they are eroded from the land and carried to streams, where they are transported to the ocean.

14. What is lithification? How is sedimentary rock formed?
Neritic sediments can undergo lithification: They are converted into sedimentary rock by pressure-induced compaction or by cementation.

15. Can you think of an example of lithified sediment on land?
Much of the Colorado Plateau, with its many stacked layers easily visible in the Grand Canyon, was formed by sedimentary deposition and lithification beneath a shallow continental sea beginning about 570 million years ago.

16. Why are Atlantic sediments generally thicker than Pacific sediments?
When averaged, the Atlantic Ocean bottom is covered by sediments to a thickness of about 1 kilometer, while the Pacific floor has an average sediment thickness of less than 0.5 kilometer.

17. How do turbidity currents distribute sediments? What do these sediments (turbidites) look like?
A turbidity current is a dilute mixtures of sediment and water that periodically rushes down the continental slope. The resulting deposits (turbidites) are graded layers of terrigenous sand interbedded with the finer pelagic sediments typical of the deep-sea floor.

18. What is the origin of oozes? What are the two types of oozes?
The organisms that contribute their remains to deep-sea oozes are small, single-celled, drifting, plantlike organisms and the single-celled animals that feed on them. The silica-rich residues give rise to siliceous ooze, the calcium-containing material to calcareous ooze.

19. What is the CCD? How does it affect ooze deposition at great depths?
At the calcium carbonate compensation depth (CCD), the rate at which calcareous sediments are supplied to the seabed equals the rate at which those sediments dissolve. Below this depth, the tiny skeletons of calcium carbonate dissolve on the seafloor, so no calcareous oozes accumulate.

20. How do hydrogenous materials form? Give an example of hydrogenous sediment.
Most hydrogenous sediments originate from chemical reactions that occur on particles of the dominant sediment. The most famous hydrogenous sediments are manganese nodules.

21. How do evaporites form?
Evaporites are hydrogenous deposits that include salts that salts precipitate as water evaporates from isolated arms of the ocean or from landlocked seas or lakes.

22. How are sediments studied?
Cameras are used to visualize the bottom, and direct samplers (clamshell, piston corers) are used to obtain specimens. Reflected sound can image strata beneath the surface covering.

23. How have studies of marine sediments advanced our understanding of plate tectonics?
The discovery that marine sediments are comparatively young (compared with terrestrial sediments) is a prime proof of the tectonic theory. Remember what happens at subduction zones!

24. Would you say the “memory” of the sediments is long or short (in geologic time)?
Because the deep sea sediment record is ultimately destroyed in the subduction process, the ocean's sedimentary "memory" does not start with the ocean’s formation as originally reasoned by early marine scientists.

25. How might past climate be inferred from studies of marine sediments?
Scientists now have instruments capable of analyzing very small variations in the relative abundances of the stable isotopes of oxygen preserved within the carbonate shells of microfossils found in deep sea sediments. These instruments allow them to interpret changes in the temperature of surface and deep water over time.

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