Download LAB 4-3: Seafloor Spreading

Name: ___________________
Date: __________
Earth Science – 2 hours
Period: __________
LAB 4-3: Seafloor Spreading
INTRODUCTION:
Nearly three-quarters of the earth’s lithosphere lies beneath the ocean. As a result, much of it is
hidden from direct view making it difficult for scientists to study. Recent advances in technology
along with underwater research vessels have allowed scientists to create detailed maps of the ocean
floor and make many new discoveries about the deep ocean. These maps and discoveries have led to a
better understanding of the movement of the earth’s tectonic plates under the ocean. As you have
already learned, the earth’s crust is broken up into a large number of tectonic plates that are moving in
relation to one another. The focus of this lab is to examine the sea floor of the Atlantic Ocean where
two tectonic plates are moving apart creating a divergent plate boundary. Divergent plate boundaries
are commonly found on the sea-floor underneath the earth’s oceans. Along divergent boundaries,
magma from the asthenosphere is rising to the surface as the plates move apart. The magma cools and
solidifies to form new oceanic crust which piles up to create an underwater mountain range called a
mid-ocean ridge. Along the center of this mid-ocean ridge is a rift valley, a narrow valley that forms
where the plates are separating. By studying the ocean floor in detail, scientists have found patterns in
the age as well as magnetic orientation (minerals in the rock will align with the earth’s magnetic poles
as the rock hardens) of the rocks on either side of the mid-ocean ridge. These observations have led
scientists to the theory of sea-floor spreading which explains the mechanism behind Alfred Wegener’s
theory of continental drift.
In order to fully understand what is going on during sea-floor spreading, it is important to
discuss the features of the ocean floor. The earth consists of two types of crust, continental and
oceanic. These two types of crust have different characteristics including rock type and density. In
general continental crust is composed of granite, has a low density and is generally found above sea
level. Oceanic crust is composed of basalt, has a high density and is generally found below sea level.
Along the coastal areas of continents there is a region where the continental crust is covered with
water. This region is called the continental shelf. At the edge of the continental shelf is the continental
slope, a steep downward slope where ocean depth increases dramatically. The base of this slope
represents the boundary between continental and oceanic crust. Beyond the continental slope consists
of a vast, flat area of the deep-ocean basin called an abyssal plain. Abyssal plains are the flattest
regions on earth and cover about half of the deep-ocean basin. There are parts of the deep ocean basin
that are not entirely flat. These areas are often associated with current or ancient volcanic activity.
Submerged volcanic mountains called seamounts can be found scattered along the ocean floor. If these
seamounts are tall enough they will rise above the ocean surface and form oceanic islands.
Objective:
This lab will teach you how we can use observations about the rock on either side of a
mid-ocean ridge to learn how this plate boundary actually works.
Materials:
 Colored Pencils: Red, Orange, Yellow, Green, Blue, Purple, and Brown
 Excel file titled “LAB4-3atlanticOceanProfile2013.xlsx” (Available on Haiku and on the
coursework drive)
Seafloor Spreading Lab Vocabulary:
Directions – choose a term from the word bank that correctly matches each definition. Write
that term on the line provided next to the proper definition.
Word Bank
Term:
Seamount
Continental Shelf
Mid-Ocean Ridge
Continental Slope
Rift Valley
Oceanic Island
Divergent Plate Boundary
Magnetic Orientation
Abyssal Plain
Definition:
_____________________: A seamount that extends above the surface of the ocean.
_____________________: An underwater mountain range that forms along divergent plate
boundaries as magma rises to the surface and solidifies.
_____________________: The area along the coast of continents where the continental crust is
covered by ocean water.
_____________________: A submerged volcanic mountain that can be found in the deep-ocean basin.
_____________________: A narrow valley that forms along the center of a mid-ocean ridge marking
the location where the plates are separating.
_____________________: A pattern that has been discovered in igneous rocks resulting from the
minerals aligning with the earth’s magnetic poles as the rock hardens.
_____________________: A vast, flat area of the deep-ocean basin.
_____________________: A steep downward slope at the edge of the continental shelf
where the ocean depth increases dramatically.
_____________________: A location on earth where two tectonic plates are moving
apart. These are commonly found under the earth’s oceans.
Procedure A: Atlantic Ocean Profile
The map below shows the path a research ship took across the Atlantic Ocean. The ship sailed
from the East Coast of the United States to Portugal. Along the way the ship measured the depth
of the ocean and even a measurement of elevation of an island in the Azores. The data table
below shows some of the data collected by this ship.
Distance from US
in kilometers
0
120
200
400
490
620
680
720
2000
2500
2900
3000
3090
3100
Elevation/Depth
in kilometers*
0.0
-0.2
-2.7
-3.7
-3.7
-4.6
-1.8
-4.6
-4.6
-4.0
-2.7
-1.8
-4.0
-2.4
Distance from US
in kilometers
3200
3500
3600
3650
4025
4050
4100
4125
4500
5000
5300
5800
6000
6075
6100
Elevation/Depth in
kilometers*
-2.9
-3.5
-3.7
-3.7
-4.0
-2.7
0.5
-2.2
-4.6
-5.0
-4.4
-3.7
-2.7
-0.2
0.0
*Note: Negative numbers indicate depth below sea level. Positive numbers indicate elevation
above sea level.
Procedure A: Atlantic Ocean Profile (Continued)
Complete the following steps in order:
 Have one group member logon to a computer. Once logged in go to the “Coursework”
drive, open the “Science” folder and the click on the folder titled “Donohue”. Open the file
titled “LAB4-3atlanticOceanProfile2013.xlsx”. (There will also be a copy placed on the
Haiku site on the class notes page)
 Transfer the elevation/depth data from the chart in your lab to the chart on this sheet.
Watch the profile of the ocean floor form as you do this. It would be wise to have one
group member read the data, one group member enter the data and the others check to
make sure the data is entered correctly. NOTE: The profile is vertically exaggerated!!!
 Print out a copy of this sheet for every member in your group.
 Use a BLUE colored pencil and darken the 0 km elevation on your profile. This line
represents sea level. Lightly shade (still in BLUE) everything below this line but above
your graph line to show where the water is.
 Use a BROWN colored pencil to shade everything below your graph line to represent the
Earth's crust.
 Label the following features on your profile:
- Continental Shelf (There will be two of these to label)
- Continental Slope (There will be two of these to label)
- Seamount (Submerged)
- Seamount (Island)
- Mid-Ocean Ridge
- Rift Valley
- Abyssal Plain (There will be two of these to label)
Procedure B: Southern Mid-Atlantic Ridge
Use the map below to answer the questions that follow. The map shows a portion of the Atlantic
Ocean floor located beneath South America and Africa. Isolines show the approximate age, in
millions of years, of the ocean-floor bedrock on each side of the Mid-Atlantic Ridge. Points A, B,
C, and X represent locations on the ocean floor.
B
C
Age of bedrock
(MYA)
Color Code
Temperature
0-10
RED
Warmest
10-40
ORANGE
40-68
YELLOW
68-83
GREEN
83-118
BLUE
118+
PURPLE
Coldest
1) On the diagram above, arrows pointing up ( ) represent the current orientation of our
magnetic field, while arrows pointed downward ( ) indicate reversed polarity. On the
diagram above, draw the correct pattern of magnetic polarity on the east side of the MidAtlantic Ridge.
2) Shade the diagram above to indicate the age of the ocean-floor bedrock according to the
chart above. The abbreviation MYA stands for millions of years ago.
3) Draw arrows on the diagram to indicate the direction the plates are moving on either side of
the Mid-Atlantic Ridge.
4) Describe the pattern of both the magnetic orientation and the bedrock age that can be
observed on both sides of the Mid-Atlantic Ridge.
5) On the grid below, construct a line graph of bedrock age by plotting the age of the bedrock
shown by each isoline that crosses line ABC. Points A and C are plotted on the grid. Connect
the plots from A to C with a line. BE PRECISE!!! THIS IS SCIENCE AND ACCURACY COUNTS!!!
C
6) Estimate the age of the ocean-floor bedrock at location X. ______________________________________
7) Explain why the age of the ocean-floor bedrock increases as the distance from the MidAtlantic Ridge increases.
8) Draw a line on the graph below to show the general temperature pattern from point A to C.
Ocean-Floor
Bedrock
Temperature
A
B
C
9) The Mid-Atlantic Ridge separates pairs of crustal plates, such as the South American Plate
and the African Plate. Identify one other pair of crustal plates separated by the Mid-Atlantic
Ridge.
10)What type of plate boundary is the Mid-Atlantic Ridge? ____________________________________________
Procedure C: Mid-Atlantic Ridge Cross Section Analysis
Use the cross section below to answer the questions that follow. The cross section represents a
pattern of magnetic field reversals preserved in the igneous bedrock of the ocean crust east of
the Mid-Atlantic Ridge.
1) Which cross section best represents the magnetic field patter west of the Mid-Atlantic
Ridge
2) Identify the name of the igneous rock and the density of the rock that makes up the
bedrock of the Atlantic Ocean-floor.
Name of Rock:
__________________________________________
Density of Rock:
__________________________________________
3) On the diagram below, draw arrows on either side of the ridge to indicate the motion of
the plates.
4) On the diagram below, draw arrows in the box beneath the cross section to represent the
convection current that is present beneath the Mid-Atlantic Ridge.