Illustration of sea-floor spreading, depicting the formation of new oceanic crust at mid-ocean ridges and the movement of tectonic plates.

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 "A" is the mid-ocean ridge.
- Description: The mid-ocean ridge is a mountainous underwater range where new oceanic crust is formed through volcanic activity. Magma rises from the mantle at the ridge and cools to form new crust, which then spreads outward on either side. This is the primary site of sea-floor spreading.

#### 2. Describe the process shown occurring at B and explain what results from this.
- Process at B: At point "B," magma is rising from the mantle into the gap created by the separation of tectonic plates. This process is called igneous activity or volcanism.
- Result: As the magma rises, it cools and solidifies, forming new oceanic crust. This continuous addition of new crust causes the existing crust to move away from the ridge, leading to the phenomenon known as sea-floor spreading.

#### 3. What happens to old oceanic crust as new mafic material rises from the mantle?
- Explanation: As new oceanic crust forms at the mid-ocean ridge (point A), the older crust is pushed outward and eventually moves toward the edges of the ocean basin. When the old oceanic crust reaches the edge of a continent, it is subducted back into the mantle at a subduction zone. Here, the denser oceanic crust sinks beneath the lighter continental crust, melting and recycling back into the mantle.

#### 4. Arrows are shown on the figure above the ocean floor spreading from the ridge. What evidence would you expect to find to support this idea?
- Evidence: Several lines of evidence support the idea of sea-floor spreading:
1. Magnetic Stripes: The ocean floor has symmetrical magnetic stripes parallel to the mid-ocean ridge. These stripes result from the Earth's magnetic field reversals over time, with new crust forming and preserving the orientation of the magnetic field at the time of its formation.
2. Age of Crust: The oceanic crust is youngest at the mid-ocean ridge and becomes progressively older as you move away from the ridge. This pattern supports the idea that new crust is continuously being formed at the ridge.
3. Volcanic Activity: Active volcanoes and hydrothermal vents are commonly found along the mid-ocean ridge, indicating ongoing magma upwelling.
4. Seismic Activity: Earthquakes are more frequent near the mid-ocean ridge due to the movement and fracturing of the crust as it spreads.

#### 5. What is happening at C, and why does it occur?
- What is happening at C: At point "C," the oceanic crust is being subducted beneath the continental crust.
- Why it occurs: Subduction occurs because oceanic crust is denser than continental crust. As the oceanic crust moves away from the mid-ocean ridge, it cools and thickens, becoming denser. When it collides with a continent, the denser oceanic crust sinks into the mantle at a subduction zone. This process recycles the oceanic crust back into the mantle, completing the cycle of crust formation and destruction.

#### Building Vocabulary:
1. A device that scientists use to map the ocean floor is called an _echosounder_ (or sonar).
2. The process that continuously adds new material to the ocean floor is called _sea-floor spreading_.
3. The pressure by which the ocean floor sinks into the mantle is called _subduction_.
4. A chain of underwater mountains along which sea-floor spreading occurs is called a _mid-ocean ridge_.

Final Answer:

\[
\boxed{
\text{1. Mid-ocean ridge; 2. Igneous activity forming new crust; 3. Old crust is subducted; 4. Magnetic stripes, age of crust, volcanic activity; 5. Subduction due to density difference.}
}
\]