Chapter 2: The Sea Floor

 The Water Planet

The Geography of the Ocean Basins

The Structure of the Earth

Internal Structure

Continental and Oceanic Crust

The Origin and Structure of the Ocean Basins

Early Evidence of Continental Drift

The Theory of Plate Tectonics

Discovery of the Mid-ocean Ridge

Significance of the Mid-Ocean Ridge

Creation of the Sea Floor

Sea-Floor Spreading and Plate Tectonics

The Dynamic Mantle

Geologic History of the Earth

Continental Drift and the Changing Oceans

The Record in the Sediments

Climate and Changes in Sea Level

The Geological Provinces of the Ocean

Continental Margins

The Continental Shelf

The Continental Slope

The Continental Rise

Active and Passive Margins

Deep-Ocean Basins

The Mid-Ocean Ridge and Hydrothermal Vents

 

Box Reading: "Hot Spots" and the Creation of the Hawaiian Islands

 

 

Chapter Summary

Chapter 2 summarizes the basic aspects of the geology of the sea floor and points out its relevance to marine biology:

• Basic structural features of the ocean basins and the earth are briefly described.
• Origin of the ocean basins
• Theory of plate tectonics (Alfred Wegener)
• Early evidence for continental drift, the discovery and significance of the mid-ocean ridges, the creation of the sea floor,
• Mechanisms that explain Plate tectonics
• Trenches, island arcs, and faults.
• "Hot spot" Origin of the Hawaiian Islands
• Geologic history of the earth
• Concerns about climate change.
• Introduction to the basic types of marine sediments.
• Geologic provinces of the ocean
• Active and passive continental margins
• Hydrothermal vents, the deep-sea hot springs that serve as home to a most fascinating type of marine life.

Answers to Critical Thinking Questions

1. The process of plate tectonics is occurring today in the same way as in the past. Can you project future positions of the continents by looking at a map of their present positions and the positions of the mid-ocean ridges (see Fig. 2.6)?

North and South America will be farther west toward the Pacific, Europe and Asia farther southeast, Australia farther northeast, and Africa farther east. The Pacific is therefore expected to be narrower and the Atlantic wider. The map in Figure 2.10 actually indicates the direction of plate movement but it's better if students deduce these movements by the location of the mid-ocean ridges as indicated on the map in Figure 2.6.

2. Why are most oceanic trenches found in the Pacific Ocean?

The Pacific Ocean is shrinking and plates are descending below surrounding plates along its edges, hence the creation of trenches.

3. Scientists who study forms of marine life that lived more than approximately 200 million years ago usually have to obtain fossils not from the sea floor, but from areas that were once undersea and have been uplifted onto the continents. Why do you think this is?

Part of what was the sea floor at that time is now above the sea level.

4. What are some of the major pieces of evidence for the theory of plate tectonics? How does the theory explain these observations?

Evidence includes: the fitting together of the coasts of the continents on the opposite sides of the Atlantic Ocean, the similarity of geologic formations and fossils found on the opposite sides, a geologically active mid-ocean ridge running along the central Atlantic between the opposite coasts, bottom sediments that get thicker the farther one travels from the ridge, and rocks on the sea floor on one side of the ridge show magnetic bands that are mirror images of rocks found on the opposite side of the ridge. All of these observations are explained by sea-floor spreading from the mid-ocean ridge.