ES 9.2 Sea-floor Spreading

9.2 Sea-Floor Spreading

During the 1800s, scientists discovered large undersea mountain ranges in the middle of the Atlantic Ocean.

This discovery motivated scientists to map the ocean floor.

Key Terms:
sonar, deep-ocean trenches, mid-ocean ridge, rift valley

​Exploring the Ocean Floor

Exploring the Ocean Floor

9.2 Sea-Floor Spreading

Mapping of the ocean floor showed that the mountain range in the Atlantic, a mid-ocean ridge, formed a long chain of mountains extending throughout all of Earth's oceans. ​

◆ Earth's mid-ocean ridge system forms the longest feature on Earth's surface.

Exploring the Ocean Floor

9.2 Sea-Floor Spreading

Earth's ocean floors are made of extrusive igneous rocks (basalt).


◆ Earth's mid-ocean ridge system forms the longest feature on Earth's surface.

Sea-floor Spreading

9.2 Sea-Floor Spreading

In 1963, after these ocean floor maps had been published, a geologist named Harry Hess proposed a hypothesis of sea-floor spreading.

process of sea-floor spreading, new ocean floor forms along Earth's mid-ocean ridges and slowly moves outward across ocean basins.

Sea-floor Spreading

9.2 Sea-Floor Spreading

In the process of subduction, ocean floor returns to the mantle as it sinks beneath a deep ocean trench. The areas in which this happens are called subduction zones.

Subduction

9.2 Sea-Floor Spreading

Sea-floor Spreading

9.2 Sea-Floor Spreading

In the process of subduction, ocean floor returns to the mantle as it sinks beneath a deep ocean trench. The areas in which this happens are called subduction zones.

Evidence for Sea-floor Spreading

9.2 Sea-Floor Spreading

◆ Evidence for sea-floor spreading included magnetic strips in ocean-floor rock, earthquake patterns, and measurements of the ages of ocean floor rocks.

Sea-floor Spreading

9.2 Sea-Floor Spreading

Alternating strips of rock having normal polarity and rocks having reverse polarity. These strips matched on either side of mid-ocean ridges, meaning the rock moved outward from the ridges. This supported the idea of sea-floor spreading.

9.2 Sea-Floor Spreading

Sea-floor Spreading

9.2 Sea-Floor Spreading

The age of ocean floor rocks is youngest along the mid-ocean ridge and increases with age as you move away from the ridge. This also supports the idea of sea-floor spreading.

9.2 Sea-Floor Spreading

Review Key Concepts:

9.2 Sea-Floor Spreading

1. Describe mid-ocean ridges and deep-ocean trenches.

2. Explain what occurs during sea-floor spreading.

3. List the evidence for sea-floor spreading.

4. What is a Wadati-Benioff zone?

Review Key Terms:

9.2 Sea-Floor Spreading

• sonar

• deep-ocean trench

• mid-ocean ridge

• rift valley

• sea-floor spreading

• subduction

• paleomagnetism