Final answer:
Lighter elements such as silicates rise towards the surface, eventually erupting as molten rock, or magma, and forming extrusive igneous rocks upon cooling. This process is a result of the earth's planetary differentiation where lighter materials ascend while heavier metals sink.
Step-by-step explanation:
The lighter elements that typically rise towards the surface and erupt in volcanoes as molten rock are primarily silicate materials. These elements, which are less dense than other materials in the earth's interior, include a variety of minerals and volcanic glasses. During the process known as planetary differentiation, heavier metals sank toward the core of the earth, causing lighter silicates to rise to the surface.
When heated within the earth, these silicates eventually become magma. This magma can then rise through the crust, either through cracks or volcanic structures, and eventually is released as lava during volcanic eruptions. When the lava cools on the earth's surface, it forms extrusive igneous rocks, which are typically fine-grained due to rapid cooling.