The age of rocks in the seafloor provides evidence for seafloor spreading because the rocks are generally younger near the mid-ocean ridges and older as you move away from them. This age progression of the ocean floor suggests that new ocean crust is being formed at the mid-ocean ridges and then gradually moves away from the ridge as seafloor spreading occurs.

To understand how the age of rocks provides evidence for seafloor spreading, consider the following:

1. Formation of Rocks at Mid-Ocean Ridges: At mid-ocean ridges, molten rock from the Earth's mantle rises and erupts onto the seafloor. This lava cools and solidifies to form new oceanic crust.

2. Cooling and Spreading: As the new oceanic crust moves away from the mid-ocean ridge, it starts to cool and contract. This causes it to become denser and sink deeper into the ocean basin. This process is known as subsidence.

3. Age Progression: As new oceanic crust forms at the mid-ocean ridge, it pushes older crust away from the ridge. This means that the rocks near the mid-ocean ridge are generally younger than the rocks farther away from the ridge.

4. Drilling and Dating: Scientists have conducted scientific expeditions to drill into the seafloor and collect rock samples from different locations. By analyzing these samples, geologists can determine the age of the rocks using various dating techniques, such as radiometric dating.

5. Supporting Evidence: Magnetic anomalies on the seafloor, observed through measurements of the Earth's magnetic field, also support the age progression of the rocks and the pattern of seafloor spreading.

So, the age of rocks in the seafloor can be used to determine the relative ages of different parts of the ocean floor and hence provides evidence for the process of seafloor spreading, wherein new crust is created at ocean ridges and moves away on either side with time.