Figure 2. Sea-floor spreading. Modified from NSTA/FEMA (1988).

The legend confirms the symbols used in the diagram:
- A wavy line with an arrow represents "Rising magma"
- The "X-X-X" symbol represents "Earthquake"

This helps me understand that the wavy lines shown at the mid-ocean ridge and subduction zone indicate areas where magma is rising, which is associated with volcanic activity. The "X" symbols at the mid-ocean ridge and along the subduction zone represent earthquake activity, which is common at plate boundaries due to the stress and friction between moving plates.

Now that I have examined all the key components of the diagram, I will synthesize this information to provide a comprehensive explanation of sea-floor spreading as depicted in the image.

Solution: Explanation of Sea-Floor Spreading

The diagram illustrates the process of sea-floor spreading, which is a fundamental concept in plate tectonics. Here's a step-by-step explanation of what is shown:

1. Divergent Boundary at Mid-Ocean Ridge: At the center of the diagram is the mid-ocean ridge, labeled as a "divergent boundary." This is where two tectonic plates are moving away from each other. As they separate, magma from the mantle rises to fill the gap, cools, and solidifies to form new oceanic crust. This process continuously creates new sea floor, pushing the older crust outward on both sides of the ridge.

2. Convection Currents in the Mantle: The driving force behind plate movement is shown by the convection currents in the mantle. Hot material rises from deep within the Earth, moves horizontally near the surface, cools, and then sinks back down. These currents drag the tectonic plates along with them, causing them to move apart at divergent boundaries and converge at other boundaries.

3. Subduction at Trenches: As the oceanic plates move away from the mid-ocean ridge, they eventually reach a trench, which is a convergent boundary. Here, the denser oceanic plate is forced beneath the less dense continental plate in a process called subduction. The subducting plate descends into the mantle, where it melts due to the high temperatures and pressures.

4. Volcanic Activity and Earthquakes: The melting of the subducting plate generates magma, which rises through the overlying continental plate, leading to volcanic activity and the formation of volcanic mountain ranges (as shown on the continent). Both the mid-ocean ridge and the subduction zone are areas of intense seismic activity, as indicated by the "X" symbols representing earthquakes.

5. Transform Fault: The diagram also shows a transform fault, which is a type of plate boundary where plates slide past each other horizontally. This can cause earthquakes but does not involve the creation or destruction of crust.

In summary, sea-floor spreading is the process by which new oceanic crust is formed at mid-ocean ridges and gradually moves away from the ridge, eventually being subducted under continental plates at trenches. This continuous cycle of crust creation and destruction is driven by convection currents in the mantle and is responsible for shaping the Earth's surface over geological time scales.