Download Environmental Science 105

Oceanography 101
POST-LAB
Saunders
Ocean Sediments & Bathymetry
Name:___________________________
Objective: At the end of this lab, students will understand the basic features of rocks and
minerals of the ocean, and the principles of magnetism and plate tectonics by:
 Analyzing and classifying sediments viewed under the microscope
 Analyzing a deep sea core sample to determine the source and formation of its sea floor
sediments
 Using the bathymetric chart to create a transect along Hood Canal in Puget Sound in
order to help explain the state of pollution there
1. Sediments (to be done in lab)
(1.1) At each table should be a set of sediment slides containing various sediments from around
the world. Examine the prepared sediments slides and determine their characteristics by
filling out the table below.
Slide
A
B
C
Description
Type
Sorting
Oceanography 101
POST-LAB
Saunders
D
E
F
G
(1.2) After you have completed the table, check your results against a key provided by your
instructor. Do you think Libya has much river input in its sediments? Explain.___________
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(1.3) Given what we’ve learned about sediments and their sources, would you guess that the
sediments of Puget Sound, which has 16 major and many minor rivers draining into it, are
more terriginous, biogenous, hydrogenous, or cosmogenous?
Explain.____________________________________________________________________
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2. Core Sample
(2.1) By studying the sediments on the ocean floor, we can gather clues about what surface
conditions were like at various times in the Earth's past when the sediments were deposited.
Using those clues, we can try to reconstruct what might have happened during the Earth's
past. As the particles pile up on the ocean floor, they begin to exert pressure on the
sediments below. What do you think will happen to these sediments over time?
Explain.__________________________________________________________
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(2.2) Below is a 6 m long core sample taken from the deep sea floor off Vancouver Island. The
core sample shows fine grained mud layers alternating with sandier layers. The black layer at
the bottom of the core is mainly volcanic ash (after Adams, 1990). What is the age of the
oldest sediment found in this core sample?____________________
Oceanography 101
POST-LAB
Saunders
(2.3) How might you explain the dark layer at the bottom of the core sample? Be specific!_____
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(2.4) How many alternating sand-mud layers do you count?______________________________
(2.5) How can you explain these alternating course, sandier layers with fine particle mud layers?
(HINT: Note that this sample core was taken in a submarine canyon)___________________
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Core sample
site
(2.6) Given the location of the core sample and its proximity to the Cascadia Subduction Zone,
these alternating sandy-mud silt deposits are probably caused by megathrust earthquakes. A
megathrust earthquake occurs where one tectonic plate is forced under (subducts) another.
Oceanography 101
POST-LAB
Saunders
Due to the shallow dip of the plate boundary, these earthquakes are among the world's
largest, with magnitudes (Mw) that can exceed 9.0. So how many megathrust earthquakes
have we had in this region over the past 7770 years?______________________
(2.7) What might be the significance of the thickness of each layer?_______________________
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3. Hood Canal Bathymetry
INTRODUCTION: If you haven’t heard the news lately, Hood Canal is in trouble. Large inputs
of non-point and point source runoff and have lead to low oxygen environments especially in the
deeper basins of Dabob Bay and Hoodsport. Let’s examine the bathymetry of Hood Canal to see
if it can lend any clues as to why this particular part of Puget Sound is so impacted.
(3.1) Obtain a map of Hood Canal. What is the scale on the map?_________________________
(3.2) What unit are the depth measurements?______________
(3.3) Obtain a straight edge and a pencil. Starting out near the opening of Hood Canal (near
Admiralty Inlet) draw a light pencil line down the center of the canal using a straight edge,
changing direction as the canal varies. Continue drawing a center line the entire length of Hood
Canal all the way to Lynch Cove. How many nautical miles long was your line?_________
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(3.4) If you were to divide your line into 30 increments, how many nautical miles would each
increment be?__________________________________________________________________
Fathoms
(3.5) Using the graph chart below, plot the bathymetry of Hood Canal along your line with a 30
increment setting (i.e. divide your line into 30 equal points and plot each point with the depth in
fathoms).
surface
Oceanography 101
0
POST-LAB
10
Saunders
20
30
Increments Along Transect
(3.6) Describe the overall shape of Hood Canal’s bathymetry.____________________________
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(3.7) What is the origin of the Hood Canal Basin (i.e. how did it form?)____________________
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(3.8) Are there any features of Hood Canal’s bathymetry that your group can see that might
influence its circulation? Explain.__________________________________________________
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(3.9) Locate on your bathymetric drawing of Hood Canal a inlet named Dabob Bay. This area
has been in the news recently as a place in trouble because of pollution and nutrient runoff from
the surrounding farmlands and urban areas. Is there anything that you can see from its
bathymetry that might indicate it has a particular susceptibility to these problems? Explain.
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