Lab handout Lab#2: Marine Geology
As you visit stations 1-7 around the lab room, work together to answer the questions in the lab
manual and on this sheet. Note stations 1 and 7 of Exercise 2 in your lab manual will not be
covered in this lab and Station 4 is optional. We will go over the major points during the last 5
minutes of class. These questions will not be graded---they are for you to study from.
Station 1--Contour maps (not Lab manual station 1)
a. Define these key words:
Datum
The level to which elevation and depth are referenced
Gradient The steepness or slope of an incline, calculated as (elevation change/horizontal
distance).
Relief
The maximum elevation range for a given map or area on a map.
b. Do contour lines that are closer together represent a steeper or shallower slope?—Steeper
c. Is the color contour map hanging above the computer a map of 2 peaks or 2 depressions?
-- 2 peaks.
Why are the larger contour numbers interpreted to be closer to the surface of the ocean?
-- Because on a bathymetric map, bigger numbers are defined as deeper depths.
Station 2
d. Figure 2-B tells you that a range of earthquake depths, from shallow to deep, is characteristic
of subduction zones. Use this information and figure 2-A to determine at least 2 locations
where subduction is occurring.
Some examples are: West of South America, NE of New Zealand, Off Japan
e. Note the occurrence of earthquakes between India and China--in fact a 6.5 earthquake
occurred in China early this year. What do you think is happening in this region? (Hint:
What geologic feature is found here?)
India(on the Arabian Plate) collided with Asia (Iran Plate) to form the Himalaya
Mountains. Pressure still exists between these plates as India continues to move into
Asia.
f. In the text you read about hot spots--now look at the Hawaiian Islands and Emperor
Seamounts. What can island chains formed by a hot spot tell you about the direction of plate
movement?
The hot spot chain, from oldest island or seamount to youngest, tracks the direction of the
plate. For example, the pacific plate is currently moving toward the NW.
Station 3
g. Find the East Pacific Rise and the Mid Atlantic Ridge (both spreading ridges) on the map of
bedrock geology. Comparing the patterns on the seafloor around these ridges, what can you
say about their relative spreading rates?
The East Pacific Rise is spreading faster than the Mid-Atlantic ridge. (note: more crust
is formed each time period at the East Pacific Rise)
h. Look at the plate boundaries map on p. 9 of your lab book. Is plate movement causing the
Atlantic Ocean to become smaller or larger?
Larger--The Mid Atlantic Ridge is spreading the ocean while there is no significant
subduction occurring.
Pacific Ocean?
Smaller--Subduction around the Pacific outweighs spreading of
East Pacific Rise.
Station 5
i.
Why does oceanic crust tend to "sink" as it moves away from the spreading centers?
As the crust cools it becomes heavier. It also accumulates sediment, which makes the
crust heavier.
j.
Would you expect to find a thicker accumulation of sediment at spreading centers or
subduction zones? Why?
Subduction Zones. Spreading centers are the formation areas for new crust, which have
not had time to accumulate sediments--like older crust at subduction zones.
k. Does crystal grain size increase or decrease as you go deeper into the lithosphere at a
spreading ridge? Why?
Increase. The more slowly rock cools, the larger the crystal grain sizes are. Rocks
deeper in the earth will cool more slowly.
Station 6
l.
Why do most metamorphic rocks form at subduction zones, but not at spreading centers?
Metamorphic rocks are formed by heat & pressure, transformed from sedimentary rocks
or igneous rocks. The heat and pressure needed are more present at subduction zones.
m. Why do explosive volcanoes occur near subduction zones?
Volcanic eruptions of recycled oceanic crust and sediments are explosive and produce
ash, gas, and steam.
n. What role does the relative density play at subduction zones?
The denser plate (oceanic crust) will sink below less dense plate( continental crust)
Station 7--Geology off the Washington Coast. (not station 7 from the lab manual)
o. Subduction is occurring of the coast----Speculate as to why there is no trench visible.
It is filled with sediment.