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Linn Benton Community College *
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Course
108
Subject
Geology
Date
Feb 20, 2024
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Pages
8
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Name ______
Sage Clark
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Lab 2—Geology of the Seafloor
GS108
Key Ideas
To become familiar with seafloor bathymetric features and their relationships to tectonic
boundaries.
Objectives
a) Identify seafloor features such as seamounts, ridges, trenches, and abyssal plains
b) Calculate seafloor spreading rates
c) Compare and contrast relief of mountains on the seafloor and on land
Procedure
This lab has one part.
●
Part 1: You will use the NOAA Ocean Digital Atlas online mapping tool to
locate and identify geologic features of the Atlantic Ocean Basin.
●
Part 2: You will calculate seafloor spreading rates at two locations in the
Atlantic Ocean and determine a reason for why they are different.
●
Part 3: You will use the NOAA Ocean Digital Atlas online mapping tool to
locate and identify geologic features of the Pacific Ocean Basin.
●
Part 4: You will compare and contrast the elevations of Mauna Kea and Mt.
Everest.
Submission
Please submit a PDF to Moodle by Sunday at 11:59 pm.
Criteria
This lab is worth 10 points and will be graded using a rubric on Moodle.
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Background
What do you imagine is on the bottom of the deep ocean floor? Do you picture it as a
big, smooth bathtub with green slime on the bottom and perhaps a few giant fish
swimming around in the dark, freezing cold? The ocean floor has this and more! In fact,
all of the major landforms that we find on the continents (mountains, valleys, plains,
gigantic canyons, etc.) are found on the ocean floor as well. It is anything but smooth!
Mountains Under the Sea
There are 2 common types of mountains on the
seafloor, both volcanic:
mid-ocean ridges/rises
and seamounts
.
Mid-ocean ridges
are long, linear volcanic ridges
formed where the seafloor fractures into 2 plates of
oceanic crust that spread away from each other.
The cracks between the diverging plates of
lithosphere are filled with magma that rises from the
mantle beneath mid-ocean ridges, and solidifies into new oceanic crust along the crest
of the mid-ocean ridge. Repeated cycles of cracking, spreading, melt injection, and
solidification of new crust along mid-ocean ridges over long periods of time (tens to
hundreds of millions of years) produces large plates of oceanic lithosphere that floor
entire ocean basins. For this reason, we say that the seafloor is "born" or "created" at
the mid-ocean ridges. Rises have gentler slopes and are not as steep.
Seamounts
Seamounts
are submerged volcanoes on the
seafloor. They may be active or extinct, and may
occur as isolated peaks, as groups, or as linear
chains. These regions of activity may be
associated with mid-ocean ridges, as is the case
with Axial Seamount to the left (source: NOAA),
with hot spots in the mantle (as is the case with
Loihi Seamount, which will one day become the
newest island of Hawaii).
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Other Features Under the Sea: Trenches, Fracture Zones, Abyssal Plains
Subduction zones are places where an oceanic plate is bending down into the mantle
and sinking (subducting) beneath another oceanic or continental plate. A very deep
trench is formed parallel to the zone of collision. Here in the Pacific Northwest, there is
the Cascadia Subduction Zone that separates the Juan de Fuca Plate from the North
American Plate. We also have the Juan de Fuca Ridge, a spreading center or
mid-ocean ridge near us, acting as the boundary between the Juan de Fuca Plate and
the Pacific Plate. (Image: Oregon Department of Geology and Mineral Industries)
In referring to mid-ocean ridges
and subduction zones we have
really been talking about major
plate tectonic boundaries.
Mid-ocean ridges are places where
new plates are formed and
subduction zones are places where
plates are destroyed. A transform
fault is a third type of plate
boundary. This is where plates
slide past each other.
The San Andreas fault in California is such a
place (see USGS photo, left). Or on a more
detailed scale, portions of the mid-ocean ridge are
often separated by smaller transform faults that
form where the diverging plates are slipping
laterally past each other. The rugged topography
resulting from the deformation of the seafloor
along transform faults is carried away with the
spreading plates and
forms inactive extensions
of the transform faults, called
fracture zones
.
The seafloor also has huge, flat, deep areas where sediments
have buried the rough volcanic terrain that was created at
mid-ocean ridges. These places are called
abyssal plains
.
In this lab, you will use a very beautiful map that depicts what
Earth would look like if all the water were drained out of the
oceans. The map will show you all the above features. Study the
map carefully and answer the questions below.
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PART 1. EXPLORING THE SEAFLOOR
Materials
●
NOAA Ocean Digital Map
o
To see names of certain features, make sure to
zoom in
o
Make sure to turn off all layers to make viewing
easier
→
o
Click on the “Basemap” drop-down menu in the
upper right corner of the screen, and select the
GEBO 2023 (NOAA NCEI Visualization)
basemap.
Procedure
Use the map to answer these questions.
Part 1. The Atlantic Ocean Basin
1.
What is the name of the major bathymetric feature of the Atlantic Ocean basin?
The name of the major bathymetric feature is Mid Atlantic Ridge.
2.
What kind of mountain range is it? (mid-ocean ridge or seamount chain)?
The kind of mountain range it is is a mid-ocean ridge.
3.
What type of tectonic boundary does it represent?
The tectonic boundary it
represents is a divergent boundary.
4.
What is the name of the large island where this feature rises above sea level?
The name of the large island where the feature rises above sea level is Iceland.
5.
Locate and name the two trenches along the outer margin of the Atlantic Ocean.
One trench that is on the outer margin of the Atlantic Ocean is the Puerto Rico
Trench. The Puerto Rico Trench joins the Lesser Antilles Island arc in the eastern
Caribbean. The second trench is the South Sandwich Trench. It is located west
of the Mid-Atlantic Ridge between South America and Antarctica.
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6.
What type of plate boundary do these trenches represent?
The Puerto Rico
Trench represents a transform boundary. The South Sandwich trench represents
a convergent boundary.
7.
Are these trenches an unusual feature of the Atlantic Ocean or are they fairly
widespread compared with the Pacific Ocean?
The Atlantic Ocean trenches are less common than in the Pacific Ocean. This is
because the Pacific plates move faster than the Atlantic Ocean causing there to
be more trenches and subduction zones.
Part 2. Spreading Rates
Calculating Spreading Rates at Mid-Ocean Ridges
You observed the Mid-Atlantic Ridge, a divergent boundary in the Atlantic Ocean Basin.
Mid-ocean ridges are very active features. Some are spreading apart at the same rate
that your fingernails grow. In geologic time this is very fast! It is possible to calculate the
spreading rate of one side of a mid-ocean ridge by dividing distance by time
(Distance/Time = Rate). This is called a half-spreading rate. Multiply it by 2 to get the full
spreading rate for both sides of the ridge. Here is an example to practice with:
An example of how to calculate the full seafloor spreading rate
Assume an ocean began forming 120 million years ago and has been spreading
steadily ever since. The distance from the crest of the mid-ocean ridge to the nearest
continent is 2400 km.
What is the FULL seafloor spreading rate in cm/yr?
Steps
Rate = Distance/Time
Distance = 2400 kilometers (km)
Time = 120 million years
Rate = 2400 km/120 million years
= 20 km/million years
It turns out that,
(km/million years = millimeters (mm)/year),
so,
20 km/million years = 20 mm/year
FULL rate = 20 mm/year * 2 = 40 mm/year
5. If the South Atlantic began forming 135 million years ago, and has been spreading
steadily ever since, calculate the FULL seafloor spreading rate in mm/year at the
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following locations using the above example calculation and replacing distance values
with the distances in the following questions:
(a) the equator near the Romanche Fracture Zone (19 degrees South) (~3500 km
away from Africa). Show your work and include units in mm/yr.
Distance= 3500 km
Time= 135 million years
Rate= 3500 km /135 million years
= 25.9km/million years = 25.9mm/year
25.9mm/year * 2 = 51.8 mm/year
(b) Buenos Aires (35° S latitude) (~4000 km away from the Mid-Atlantic Ridge). Show
your work and include units of mm/yr.
Distance= 4000 km
Time= 135 million years
Rate= 4000 km / 135 million years
= 29.6 km/million years = 29.6 mm/year
29.6 mm/year * 2 = 59.2 mm/year
6. Propose a hypothesis for why the spreading rates are different on the Mid-Atlantic
Ridge.
If I were to come up with a hypothesis of why the spreading rates are different on the
Mid-Atlantic Ridge because of the different depths it has along the ridge. In different
areas it is a different depth so the deeper it is the faster it spreads compared to the less
depth areas.
Part 3. The Pacific Ocean Basin.
1. The
western
Pacific basin is characterized by which kind of mountains: mid-ocean
ridges or seamounts?
The western Pacific basin is characterized as a seamount.
For 2 and 3, use the Data Viewer Map and
this figure
to answer the questions.
2. Name a specific location where would you expect to find the youngest oceanic crust
in the Pacific Ocean?
The location where you would find the youngest crust in the
Pacific ocean is near mid-ocean ridges so where the light blue is shown on the map
along the mid-ridge.
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a) Why would you expect to find the youngest oceanic crust there?
You would expect to
find the youngest oceanic crust there because it is where new seafloor is being formed.
3. Where would you expect to find the oldest oceanic crust? (Name the trench or
plateau that this old crust would be closest to)
You would expect to find the oldest
oceanic crust by the trenches. Which on the map you would find them along the land in
the darker blue almost back lines. One trench you would find the oldest crust is the
Mariana Trench.
4. Find 2 areas (one along the edge of a continent and the other away from a continent)
where trenches occur. Name them:
One of the areas that is along the edge of the continent South America is the trench
Cedros Trench. An area that you could find them away from the continent along
subduction zones or by small mountain ridges. For example, one could be Hook Ridge.
5. The Pacific Ocean is rimmed by high volcanic and seismic activity. There are active
volcanoes in the Cascades of our Pacific Northwest, in the Aleutian Islands, and in the
Andes, but almost none within California. Explain why California is so "fortunate” not to
have many volcanoes and how this relates to its plate boundary?
California is considered fortunate to have a low occurrence of volcanoes due to its
location along a transform plate boundary. Having a transform plate boundary it is less
active because they are typically not associated with the same level of volcanic activity
as a seamount or Mid-ocean ridge.
6. Find the Hawaiian-Emperor Seamounts, which are a chain of volcanic islands and
seamounts formed from a hot spot. The hot spot is sitting on the edge of the Big Island
of Hawaii presently, causing the nearly continuous volcanism there.
6a. If the older Hawaiian-Emperor Chain islands and seamounts are farther away from
Hawaii and the hot spot is not moving, in which direction is the Pacific Plate moving
(choose cardinal directions: NE,SE,SW, or NW)?
The Pacific Plate is moving NW.
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6b) Find one other island-seamount chain in the Pacific Ocean basin where volcanic
activity might be occurring that is NOT related to subduction and trenches and name it.
Another island-seamount chain in the Pacific Ocean basin that is active but not moving
is called Line island.
Part 4. Comparing and Contrasting Mauna Kea and Mount Everest.
7. Find Mauna Kea, which lies on the northern part of the Big Island of Hawaii.
To find Mauna Kea, zoom into the Big Island, Hawaii. Mauna Kea is located on the
northeastern part of the island. You will have to Zoom in to find it.
Once you find Mauna Kea, click your mouse once, and notice the elevation above sea
level in meters (m) in the lower left part of the map.
To find the base elevation of Mauna Kea, zoom out to the eastern part of the island. Try
to locate a point on the abyssal plain that is flat and featureless between Hohonu
seamount and Wini Seamount.
Click on your mouse to record the elevation below sea level (note that the negative
symbol just means below sea level).
a) Calculate the true height (relief) of Mauna Kea by adding its height above sea level to
its base below sea level in meters (m). (You can disregard the negative sign because it
just denotes Mauna Kea’s base is below sea level). Show your work.
Above sea level= 13,796 ft (4,205m)
Below sea level= 19,700 ft (6,000m)
4205m + 6000m= 10,205meters
b) Mt Everest is 8440 m above sea level. If we ignore where the base of each mountain
is located, which is technically the tallest mountain on Earth, Mt. Everest, or Mauna
Kea? Explain your answer.
Looking at both Mt Everest and Mauna Kea the tallest mountain is Mauna Kea. Mauna
is the highest because if we are ignoring the water levels then it is taller than Mt
Everest. Mt Everest is 8,440 meters tall and Mauna Kea is 10,205 meters tall.
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