Theory Earth's Rocks

Earth is a rocky world. As a planet of the inner solar system it formed mostly of dense material existing into primordial planet building blocks called planetesimals. Sun radiation had blown more volatile elements, like ice and gas, away from this part of the solar system. Once aggregated into a Moon-size body these primordial materials heated and differentiated, forming a core, a mantle, and a crust. Most rocks Earth originated from this process as from further long-term processes like tectonics, erosion or volcanism. Earth's surface was shaped and transformed over eons, manufacturing rocks and manufacturing some again. Nowadays ancient ocean bottoms may be seen at highest mountains' summits as volcanic ridges are creating new rocks in the middle of the oceans or water streams are taking to the sea material taken along their course

->The Oldest Earth's Rocks
Rough zircons -diamonds- found in Western Australia in August 2007 are the oldest Earth's rocks known, with an age of 4.35 billion years old! They are showing, further -with the Earth 4.6 billion years old- that the primordial crust might have taken 200 million years only to form instead of the 400 millions usually thought. The existence of rocks at such an early time, on the other hand, might too be the sign that the Earth also had an ocean -thus life maybe too!
Rocks dated between 3.8 and 4.25 billion years old have been discovered in September 2008 on the the eastern shore of the Hudson Bay, in northern Quebec, Canada. Those rocks are likely of volcanic origin
Such older rocks are rare due to the multiple changes Earth endured since that time, in terms of geology. It could help scientists to find whether the Earth's early crust did form the same way crust is still forming today at the bottom of the oceans

Igneous Rocks

Any rock Earth originated in the hot magmatic material which formed when the building blocks endured heating and eventually formed the core, the mantle and the crust of our planet. These very first rocks are termed "igneous", or volcanic, rocks. According to how their solidification process unfolded igneous rocks are further classified. Slowly cooling magma (this usually occurred at depths of thousands of feet) gave coarse-crystals rocks like granite. Such rocks are of the "macrolithic" type. Granite further may be formed when ancient rocks, made of basalt, are driven down into the planet by plate tectonics. Water of oceans combines with the basalt to form granite and the mixture is reborn through volcanic eruptions. When magma cooled more swiftly (this usually occurred near or at the surface), this gave smaller crystals and fine-grained rocks like basalt ("microlithic" rocks). When the solidification occurred abruptly this gave other types of rocks like the tuff and other light volcanic products (amorph rocks). These different types of igneous rocks may have been combined and formed further varieties like coarse-crystals rocks embedded into fine-grained rocks (porphyry) or fine-grained rocks into amorph rocks (like trachytes). Little by little Earth kept cooling and the crust crystallize in surface with giant amounts of olivine, pyroxenes, amphiboles, feldspars, metallic sulfurs and oxyds crystallizing then

Sedimentary Rocks

Our planet kept cooling and water managed to condense, with rain eventually yielding oceans, by about 4.2 billion years. Rains interacting upon grained rocks then rapidly formed the first sedimentary rocks -like sandstones and sands- as clays are stabilizing at last like crusts and thicks layers, which are the precursors to first soils. Sedimentary rocks mostly formed due to erosion processes working at the igneous rocks. Water and wind e.g. eventually weared the igneous rocks as materials were carried away, accumulating in lakes, oceans or on land. Material settled into layers of sediments and formed back rocks -the sedimentary rocks. Rock formation based upon bonds by minerals and chemicals or electric attraction. Such process might further work with animals or plants debris. It may too stop short of forming a really agregated rock, forming loose and unconsolidated rocks only. Sedimentary rocks are always found in layers parallel to Earth surface. When the layers are twisted, broken or at a high angle with the surface, this is the evidence that some geological process occurred. Sedimentary rocks are further classified into three categories: clay and shale-type rocks (based on mud), chalks (marine creatures), sand-based rocks (sandstone)

Metamorphic Rocks

Plate tectonics then is nurturing the continuous volcanism which brings rocks from depths to surface and subduction is recycling rocks towards depths, bringing to a perpetual recycling of Earth' materials. Enduring intense pressures and temperatures deep under the surface or due to volcanic processes, both igneous and sedimentary rocks may be transformed into "metamorphic" rocks (it's what the term means in Greek: "meta-morphein", "beyond-form"). Metamorphism is acting on the original rocks by making them denser and more compact only. Clays and shales are giving schists, granite is giving gneiss and micaschists, chalk yields marble, as sandstone quartzites. Oldest metamorphic rocks are often ore or gem-rich

Generally, most rocks formed from 8 elements only (oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium). These elements may be combined in various ways and they were along Earth geological history. Hence thousands of types of rocks and minerals exist on Earth. No rock earlier than 3.9 billion years ago have been found although Earth being 4.5 billion years old. This is due to all older rocks having been transformed into new rocks during Earth earliest geological times