The highest mountain and deepest trench represent only a negligible fraction of the Earth's surface area. Plotting the elevation distribution of the Earth's surface, we see that most of the surface lies below 1 km above sea level and 5 km below sea level. This means that two elevations dominate the Earth's surface, which roots in the presence of material that makes up the continental areas and material that represents the oceans regions. Note that Earth's elevation distribution also demonstrates how many of us live just above sea level. There are hundreds of rock types found at the surface, but blessfully we only need to concern ourselves with about a dozen main types.
Rocks are sedimentary, igneous or metamorphic in origin.
Sedimentary rocks are deposited at the Earth's surface, whereas igneous and metamorphic rocks are formed deeper in the Earth. Gabbro is a dark-colored rock consisiting mainly of the minerals feldspar, olivine and pyroxene. Chemically, it has low Si, Na and K content, and relatively high Ca and Mg content. Isostasy The density difference between granite and gabbro has an important consequence that we can illustrate by simple experiment. The reason is that hardwood has a slightly higher density than softwood, and thus is heavier.