Sea Floor Spreading in the Atlantic Ocean

SEA FLOOR SPREADING IN THE ATLANTIC OCEAN Background Sea floor spreading is one component of the theory of plate tectonics. According to this concept, new sea floor rock (basalt) is added to the edges of the North American, South American, Eurasian, and African plates. The result of the addition of new sea floor rock at the Mid-Atlantic Ridge (MAR) is that the Atlantic Ocean between these sets of continents is widening. North and South America move farther away from Eurasia and Africa each year. The youngest sea floor rocks are found in the middle of the ocean because that is where new sea floor is added. As you move away from the MAR, the sea floor rocks become increasingly older. The oldest sea floor (the first Atlantic sea floor formed that is still preserved) is found closest to the continents. One way that geologists can recognize the strips of sea floor basalt created at, and subsequently moved away from, the MAR is by collecting and determining the magnetic properties and ages of rocks from the ocean floor. Materials: (per student) • • • Ruler with mm subdivisions Calculator Copies of map, worksheets, and geologic time scale (ESRT p. 8 & 9) Objective: Using ocean depth data you will construct an ocean bottom profile. Using the profile, maps of the ocean floor and of Earth’s tectonic plates you will identify features of the ocean bottom in regions of diverging plate boundaries. 1  Procedure A: Construct an ocean bottom profile on the graph titled “North Atlantic Ocean Bottom Profile: using the ocean depth data provided. The horizontal distance scale is: one block = 150 km. The vertical depth scale is: one block = 0.5 km. Label the following ocean floor features: Mid-Atlantic Ridge, Rift Valley, continental shelf, deep ocean floor, and seamounts. Referring to the ESRT (page 5), draw arrows representing the directions in which the sea floor is moving. Depth (km) Distance (km) Depth (km) Distance (km) 0 0 - 3.5 3500 - 0.2 120 - 3.7 3600 - 2.7 200 - 3.7 3650 - 3.7 400 - 4.0 4025 - 3.7 490 - 2.7 4050 - 4.6 620 + 0.5 4100 - 1.8 680 - 2.2 4125 - 4.6 720 - 4.6 4500 - 4.6 2000 - 5.0 5000 - 4.0 2500 - 4.4 5300 - 2.7 2900 - 3.7 5800 - 1.8 3000 - 2.7 6000 - 4.0 3090 - 0.2 6075 - 2.4 3100 0 6100 - 2.9 3200 Sea Floor Spreading Lab 2 Sea Floor Spreading Lab 3                                 Procedure B: Complete the following procedures on the “ map provided. ” Age of the Atlantic Sea Floor 1. Study area: North Atlantic Ocean Using the “84” line to the left of the Mid-Atlantic Ridge, record its age below and carefully measure the distance it has moved from the mid-ocean ridge where it formed. 2. Sea floor age: _______84_______million years (My) 3. Distance to the Mid-Atlantic Ridge ____833__________kilometers (km) Using the age of the rock and its distance from the MAR, calculate the half-rate of sea floor spreading, the velocity at which one strip of this rock has spread away from the MAR. Sea Floor Spreading Lab 4 4. Calculated (velocity) of sea floor spreading (distance / time = velocity): ____________________km per My 5. Calculated total rate (velocity) of sea floor spreading (2 X half-rate = total spreading rate): ____________________km/My 6. Total present day distance between North America and Africa (measured between points A and B): km 7. Calculated age of the North Atlantic Ocean (total distance / total velocity = time) : _____________________My 8. Geologic Period during which the North Atlantic began to open (use page 9 of ESRT): 9. Convert the total sea floor spreading rate from step #5 above to units that are easier to "imagine". This can be done simply by filling in the spaces below and performing the multiplication. Check this calculation by making sure that units "cancel out" to correctly yield the units desired (this procedure is known as dimensional analysis). (____km/My) X (____mi/km) X (____ft/mi) X (____in/ft ) X (____My/yr) = _____in/yr 10. How much has the distance (in inches) between North America and Africa increased since you were born? 11. How much does the distance (in feet) increase during the average lifetime of an American (~82 years)? Sea Floor Spreading Lab 5  !       $    ! "# $# %## #   &   $$ $ ! Sea Floor Spreading Lab     $    6 '() * * * *    $ + $+ + +      ,$! + $+ + + -$.# /#  0#* 1.! + $+ + + . 2*# $#  Sea Floor Spreading Lab 7  # $   30*4! + 5 $+ &5 + ,65 + 165 7$$! + $+ + + (. " .. . 8$  & ' , . . . . 90*   &  '   Sea Floor Spreading Lab 8