The solidification of molten magma when it reaches the surface of the earth results in the formation of basalts and traps. This process is known as volcanic activity and plays a significant role in shaping the Earth's surface.

1. Formation of Magma:
Magma is formed beneath the Earth's surface through the melting of rocks under high temperatures and pressure. This molten material is composed of various elements and minerals, including silica, iron, magnesium, and calcium.

2. Movement towards the Surface:
Due to the differences in density, magma tends to rise towards the surface. This movement is driven by the buoyancy of the magma, as it is less dense than the surrounding rock. As the magma rises, it may encounter fractures or weak points in the Earth's crust, allowing it to reach the surface.

3. Eruption and Solidification:
When magma reaches the surface, it is known as lava. The eruption of lava from a volcano results in the release of gases, ash, and rock fragments. As the lava flows or is ejected into the air, it begins to cool rapidly.

4. Rapid Cooling:
The cooling process is crucial in the solidification of lava. As the lava is exposed to the lower temperatures of the atmosphere, its temperature drops, causing it to solidify. The cooling rate determines the texture of the resulting rock. If the cooling is rapid, fine-grained rocks like basalt are formed. Slower cooling results in coarse-grained rocks like granite.

5. Basalts and Traps:
Basalt is a type of volcanic rock that forms from the rapid cooling of lava on the Earth's surface. It is characterized by its dark color, fine-grained texture, and high content of iron and magnesium. Basalt is commonly found in volcanic regions and is a major component of oceanic crust.

Traps, also known as flood basalts, are large areas covered by extensive basalt flows. These flood basalt provinces are formed by successive eruptions over a short period of time. They can cover vast areas and are often associated with volcanic activity.

In conclusion, the solidification of molten magma when it reaches the surface of the Earth results in the formation of basalts and traps. This process, known as volcanic activity, is responsible for shaping the Earth's surface and creating various geological features.