Second only to oxygen, silicon is the most abundant element in Earth's crust. It is found in rocks, sand, clays and soils, combined with either oxygen as silicon dioxide, or with oxygen and other elements as silicates.
Gravity is a force that attracts objects. It is also the force that makes objects fall down when you drop them. Because of gravity things on earth have a certain weight. Gravity on Earth pulls objects to the centre of the planet. Every time you throw a ball up into the air it is pulled back to Earth. This occurs in nature all the time.
Quick Answer. A constructive force creates or builds something on the earth. For instance, volcanoes are built up by constructive tectonic plate movement. Destructive forces like tornadoes and tsunamis tear down or wear away parts of the earth.