Illustration of sea-floor spreading showing the movement of tectonic plates and the formation of new oceanic crust.

Problem Analysis:

The task involves understanding the process of sea-floor spreading, a key concept in plate tectonics. The image provided shows a cross-section of the ocean floor, highlighting various features such as the mid-ocean ridge, ocean crust, continental crust, and mantle. The questions are designed to test comprehension of this process.

Step-by-Step Solution:

#### 1. Name and describe the feature of the ocean floor shown at A.
- Feature at A: The feature labeled as "A" is the mid-ocean ridge.
- Description: The mid-ocean ridge is a mountainous underwater range formed by divergent plate boundaries. It is where new oceanic crust is created as magma rises from the mantle and solidifies. This process is responsible for sea-floor spreading.

#### 2. Describe the process shown occurring at B and explain what results from this.
- Process at B: The process shown at "B" is sea-floor spreading.
- Explanation: At the mid-ocean ridge (labeled as "A"), magma rises from the mantle due to convection currents. As the magma reaches the surface, it cools and solidifies, forming new oceanic crust. This newly formed crust pushes the existing crust away from the ridge on either side, causing the sea floor to spread outward.
- Result: Over time, this continuous process leads to the widening of the ocean basin and the movement of tectonic plates. Older crust moves farther away from the ridge, becoming cooler and denser as it ages.

#### 3. What happens to old oceanic crust as new mantle material rises from the mantle?
- Explanation: As new mantle material rises and solidifies at the mid-ocean ridge, the older oceanic crust is pushed away from the ridge. Eventually, the old oceanic crust reaches the edge of a continent or another tectonic plate. Here, it is forced downward into the mantle at subduction zones due to its greater density compared to the surrounding mantle material. This process is called subduction.
- Result: During subduction, the old oceanic crust melts and reenters the mantle, completing the cycle of crust formation and destruction.

#### 4. Arrows are shown on the figure above the ocean floor spreading from the ridge. What evidence would you expect to find in the rocks along these arrows to support this idea?
- Evidence: The rocks along the arrows (indicating the direction of sea-floor spreading) would show a pattern of progressively older rock ages moving away from the mid-ocean ridge.
- Reasoning: New crust forms at the ridge and gradually moves outward. Therefore, the age of the oceanic crust increases with distance from the ridge. Geologists can use radiometric dating techniques to confirm this pattern, which supports the idea of sea-floor spreading.

#### 5. What is happening at C, and why does it occur?
- Feature at C: The feature labeled as "C" is the continental margin or the boundary between the oceanic crust and the continental crust.
- Explanation: At "C," the oceanic crust meets the continental crust. This boundary is significant because it marks the transition between two different types of crust:
- Oceanic crust: Thinner, denser, and composed mainly of basalt.
- Continental crust: Thicker, less dense, and composed mainly of granite.
- Why it occurs: The difference in density and composition between oceanic and continental crust is a result of their formation processes. Oceanic crust is formed at mid-ocean ridges from magma, while continental crust is primarily formed through volcanic activity and sedimentary processes over long periods. The contrast in properties leads to the distinct boundary observed at "C."

#### Building Vocabulary:
1. A device that scientists use to map the ocean floor is called an _______.
- Answer: Sonar (Sound Navigation and Ranging).
- Explanation: Sonar uses sound waves to measure the depth and topography of the ocean floor, allowing scientists to create detailed maps.

2. The fracture zone where the plates separate is called a(n) _______.
- Answer: Divergent plate boundary.
- Explanation: At a divergent plate boundary, tectonic plates move apart, allowing magma to rise and form new crust.

3. The process that continuously adds new material to the ocean crust is called _______.
- Answer: Sea-floor spreading.
- Explanation: Sea-floor spreading describes the continuous creation of new oceanic crust at mid-ocean ridges.

4. The pressure by which the ocean floor sinks into the mantle is called _______.
- Answer: Subduction.
- Explanation: Subduction occurs when older, denser oceanic crust sinks into the mantle at convergent plate boundaries.

5. A chain of underwater mountains along which sea-floor spreading occurs is called _______.
- Answer: Mid-ocean ridge.
- Explanation: Mid-ocean ridges are mountain ranges located at divergent plate boundaries where sea-floor spreading occurs.

Final Answer:

\boxed{
\text{1. Mid-ocean ridge; 2. Sea-floor spreading; 3. Old oceanic crust subducts; 4. Rocks get progressively older; 5. Continental margin}
}