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«4-1 ES 106 Laboratory # 4 THE DYNAMIC OCEAN FLOOR (Sea Floor Topography and Paleomagnetism) Introduction One of the most significant scientific ...»

4-1

ES 106 Laboratory # 4

THE DYNAMIC OCEAN FLOOR (Sea Floor Topography and Paleomagnetism)

Introduction

One of the most significant scientific revelations of the 20th Century is the fact that the ocean

basins are geologically young, ephemeral features. Based upon this discovery, a revolutionary theory

called plate tectonics has been developed that helps to explain and interrelate earthquakes, mountain

building, and other geologic events and processes.

The theory of plate tectonics is the foundation used by Earth scientists to help explain the origin of mountains and continents, the occurrence of earthquakes, the evolution and distribution of plants and animals, as well as many other geologic processes. Using information from the ocean basins, including topography, age, and mechanisms of their evolution, Earth scientists have developed the exciting theory called plate tectonics. Plate tectonics states that Earth’s surface is broken in to rigid slabs of lithosphere called plates. The plates are separating at mid-ocean ridges, where new ocean crust is forming. Along the plate margins, earthquakes are generated as plates slide past each other, collide to form mountains, or override each other causing deep-ocean trenches. This laboratory examines some of the lines of evidence that have been used to verify this comprehensive model of the way Earth scientists view our dynamic Earth.

Goals and Objectives

• Locate and describe the general features of the ocean basins with an emphasis on locating and describing the mid-ocean ridge system and deep-ocean trenches

• Determine the rate of sea-floor spreading that occurs along a mid-ocean ridge by using paleomagnetic evidence and determine age of ocean basin 4-2 Name_____KEY_______________

Lab Day/Time_________________

Pre-lab Questions – Complete these questions before coming to lab.

1. Write a brief statement that describes each of the following ocean floor features:

A. Continental shelf FLOODED EDGE OF CONTINENTAL CRUST. SHALLOW

PORTION OF OCEAN.

B. Continental slope TOPOGRAPHIC DROPOFF OF CONTINENTAL SHELF TO

DEEP OCEAN FLOOR, WHERE THE CONTINENTAL CRUST TRANSITIONS TO

OCEANIC CRUST.

C. Abyssal plain FLAT DEEP OCEAN FLOOR COVERED WITH ABYSSAL CLAYS

AND BIOGENIC SEDIMENT

D. Seamount UNDERSEA VOLCANIC PEAK THAT MAY BE ASSOCIATED WITH

THE OCEANIC RIDGE SYSTEM.

E. Deep-ocean trench TOPOGRAPHIC DEPRESSIONS ON SEA FLOOR CREATED

BY CONVERGENT LITHOSPHERIC PLATES, AND THE SUBDUCTION OF

–  –  –

Various features are located along the continental margins and ocean basin floor. Study the maps (the World Ocean Floor map, the Pacific Ocean map, and the Mid-Atlantic Ridge map) and refer to your textbook as needed to answer the following questions.

1. On the world map provided on the following page, draw the global mid-ocean ridge system in red.

2. Locate and label the following deep-ocean trenches. Draw a blue line to represent the trench and label with the letter of each trench on the map.

Deep Ocean Trenches:

Puerto Rico, Cayman, Peru-Chile, Aleutian, Kuril, Japan, Mariana, Tonga, Kermadec, Java

3. What is the approximate average ocean depth along the continental shelves bordering North America?

LESS THAN 150 m

4. Approximately how high above the adjacent ocean floor does the Mid-Atlantic Ridge rise?

1500 TO 2000 m

5. Approximately how deep are most of the deep-ocean trenches (give an average depth for a sampling of trenches)?

6000-10000 m

–  –  –

7. How would you describe the general topography of the abyssal plains? What do you suppose the abyssal plains are composed of?

VERY FLAT—COMPOSED OF SEDIMENT OVERLYING OCEANIC CRUST

8. What do seamounts represent? What is the generally accepted explanation for the numerous seamounts that dot the Pacific Ocean deep-ocean basin floor?

SEAMOUNTS ARE UNDERSEA VOLCANOES. THERE ARE HOT SPOTS OF RISING

MAGMA FROM THE MANTLE THAT RELEASE LAVA TO THE SEA FLOOR. SINCE

THE PACIFIC IS RIMMED BY DEEP OCEAN TRENCHES, THAT CATCH

TERRIGENOUS SEDIMENT, THEY ARE NOT BURIED BELOW SEDIMENT.

Part B – Paleomagnetism and Sea Floor Spreading

The critical evidence for sea-floor spreading is based on studies of changes in the Earth’s magnetic field through time. Some minerals in igneous rocks (e.g. magnetite) become aligned with the Earth’s magnetic field at the time of their formation. From detailed paleomagnetic and geochronological studies, geologists have discovered that the polarity of the Earth’s magnetic field has periodically reversed, meaning that the north magnetic pole becomes the south magnetic pole and vice versa.

The sequence of reversals occurring in the past several million years has been dated with the use of radiometric techniques; these are represented in Figure 1.





–  –  –

Activity: Calculate rate of spreading and age of ocean basins.

As tectonic plates separate along a mid-ocean ridge, magma from the mantle rises to the surface and creates new ocean floor. As the magma cools, the minerals assume a magnetic orientation equal to the prevailing magnetic field. The plates continue to separate and if Earth’s magnetic field reverses polarity, new material forming at the ridge is magnetized in the opposite direction. This process results in magnetic striping of the ocean crust running parallel to the mid-ocean ridge.

Earth scientists can measure the magnetic striping by towing a device called a magnetometer behind a ship. The magnetometer records the strength of the magnetic field in a given location.

Figure 2 shows magnetic records for the North Pacific Ocean basin and the South Atlantic Ocean basin. Where the rocks have the same magnetic polarity as the present-day field, we find stronger than average magnetic field (represented as a peak called a positive anomaly); where the rocks preserve reverse polarity, we measure weaker than average magnetic field (represented as a trough called called a negative anomaly).

Using the known time scale of magnetic reversals, we can determine the age of a magnetic anomaly. By dividing the distance from the ridge crest to the magnetic anomaly by the age of the magnetic anomaly, we can determine the spreading rate at the ridge.

4-7 Figure 2: Magnetic anomalies (the peaked curves) recorded perpendicular to spreading centers (i.e., mid-ocean ridges) in the major ocean basins reveal a similar sequence of magnetized rocks. Note that these figures show half of the ocean basins with the “0” on the right representing the location of the spreading center.

Using Figure 2 to answer the following questions:

1. Using the distance scale in Figure 2 which ocean basin has undergone the greatest amount of spreading in the last 50 million year, the Pacific or the Atlantic? __PACIFIC__________________

2. How far in kilometers has the left side of the South Atlantic Ocean basin spread in 50 million years? ____920 _______________________ km

3. How far in kilometers has the left side of the North Pacific Ocean basin spread in 50 million years?

__________1940_________________ km The distances in the questions above are for only one side of the ocean basin (in this case to the left of the ridge). Assuming that the ridge spreads equally on both sides, the actual distance each ocean basin

has opened would be twice this amount. With this in mind, answering the following:

–  –  –

5. Determine the spreading rate in centimeters per year for the two areas shown in Figure 2. (show formulas for calculations, with units) a. North Pacific (3900 km/50,000,000 yr) x (1000m/km)x(100 cm/m) = 7.8 cm/yr b. South Atlantic

–  –  –

The rate you calculated above for the South Atlantic Ocean can be used to determine the age of both the South Atlantic Ocean and North Atlantic Ocean basins. Essentially, you will estimate how many millions of years ago the North Atlantic and South Atlantic Ocean basins began to form.

6. On the large wall map posted in the lab room, measure the seaward edges of the continental shelves from eastern North America near North Carolina to northwestern Africa at Mauritania (20°N latitude). Determine the distance in centimeters in the real world.

16 cm * 38,931,000 =622,896,000 cm

7. To determine the age of the ocean basin, divide the distance in centimeters separating the continental shelves by the rate of sea-floor spreading for the Atlantic Ocean basin.

Age of the North Atlantic Ocean basin: ______164 million years______________ years (show formulas for calculations, with units) 622,896,000 cm /(3.8 cm/yr) 4-9

8. Now determine the distance between South Africa and South America (in centimeters) by measuring from the eastern edge of the continental shelf of Brazil directly east to Africa.

(show formulas for calculations, with units)

13.5 cm * 38,931,000 =525,568,000 cm

9. As you did above, use the rate of sea-floor spreading for the South Atlantic basin to calculate the age of the South Atlantic basin.

Age of the South Atlantic Ocean basin: ______138 million years_________________ years (show formulas for calculations, with units) 525,568,000 cm/(3.8 cm/yr)

10. Jurassic rocks (represented by basalt dikes and lava flows) occur in New Jersey. These are interpreted to represent the rocks that formed when North America and Africa were rifted apart.

Refer to the diagram of the rifting process in your textbook (Figure 8.11, Tarbuck and Lutgens, 11th ed., page 227), why would you expect to find volcanic rocks associated with the onset of rifting?

BASALTIC MAGMA RISES FROM THE MELTING CAUSED BY PRESSURE RELEASE

AT THE DIVERGENCE OF LITHOSPHERE AT THE OCEANIC RIDGES.

11. Is your calculated age consistent with this geologic data? Refer to the geologic time scale on the last page of the lab manual. Explain your answer.

–  –  –

Watch the brief video snippet of pillow lavas erupting near Hawaii on the computer.

1. Study the sample of pillow lava. In the space below, sketch and describe the sample.

FINE GRAINED, DARK VOLCANIC ROCK WITH GLASSY SURFACE, AND A

DISTINCTIVE RIM WITH WHITE STRUCTURES PERPENDICULAR TO SURFACE

SHOWN ON BROKEN SIDE.

2. Interpret how the pillow lava formed.

PILLOW LAVA FORMS BY THE QUENCHING OF LAVA BY SEA WATER TO COOL IT

IMMEDIATELY. THE RIND OF PILLOW LAVA OFTEN IS GLASSY, BECAUSE IT

COOLED SO QUICKLY THAT IT DID NOT CREATE MINERALS.

–  –  –

POST-LAB ASSESSMENT

1. In the space here, sketch a general profile of an ocean floor between two continents illustrating and mid-ocean ridge and a deep-ocean trench. Label each and show relative plate motions with arrows.

2. Imagine that you were to take a deep diving submersible to a mid-ocean ridge. What would be the chemical composition of the lava rocks that you would find (basalt, andesite, or rhyolite)? What shapes would the lava have?

PILLOW BASALT

3. A geologist claims to have found a sequence of rocks that was formerly a piece of ocean crust up in a mountain range. What types of rocks would you expect to find in the sequence? What types of rocks would be on the bottom of the sequence? What types of rocks would sit on the top of the sequence?

PILLOW BASALT AT BOTTOM, PELAGIC SEDIMENTS ON TOP

4. If an ocean is opening at a rate of 5 cm/yr, how wide will the ocean be in 125 million years? Give your answer in kilometers. (Show formulas for calculations, with units) 5 cm/yr x 125,000,000 yr = 625,000,000 cm x (1 m/100 cm) x (1 km/ 1000 m)=6250 km





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