Rocks can be divided into three types as per causes of formation:
1. Magmatic rock: mainly including basalt (extrusive rock) and granite (intrusive rock).
2. Sedimentary rock: fossil, shale, sandstone, conglomerate, limestone and others.
3. Metamorphic rocks: for example, limestone is heated to form marble and shale is squeezed to form killas. Those are metamorphic rocks.
①Igneous rock is also called magmatic rock and comes from molten substances in the Earth. Under different geological conditions, the rocks are formed by cold solidification. When magma pours out of the Earth’s surface from the volcanic feeder and then solidifies into the rock, this kind of rock is known as extrusive rock or volcanic rock. Common volcanic rocks include basalt, andesite and rhyolite and so on. When the magma rises up but does not reach the Earth’s surface and solidifies into the rock in a certain depth of earth crust, this kind of rock is called intrusive rock. Intrusive rocks can be divided into plutonite and hypabyssal rock according to different invasion parts. Granite, gabbro and diorite are typical plutonites. Granite porphyry, gabbro porphyrite and dioritic porphyrite are common hypabyssal rocks. According to the chemical components, igneous rocks are divided into ultrabasic rocks (SiO2, less than 45%), basic rock (SiO2, 45%~52%), intermediate rock (SiO2, 52%~65%), acid rock (SiO 2 , more than 65%) and alkaline rock (containing special alkaline minerals, SiO 2, 52%~66%). Igneous rocks account for 64.7% of the volume of the earth’s crust.
②Sedimentary rock. In the normal temperature and pressure of Earth’s surface, bedded rocks are formed by carrying, deposition and diagenesis and composed of weathered materials, tephra, organic matters and few cosmic materials. According to the origin, they can be divided into clastic rocks, clay rock and chemical rock (including biochemigenic rock). Common sedimentary rocks include sandstone, tuff sandstone, conglomerate, clay rock, shale, limestone, dolomite, siliceous rock, ferruginous rock, phosphate rocks and so on. Sedimentary rocks account for 7.9% of the volume of the earth crust, but they are distributed widely in the crustal derm, accounting for about 75% of the land area. Seafloor is almost covered with sediments. Sedimentary rocks have two prominent characteristics: one is bedding, called bedding structure. The interface between beds is called bedding plane. Usually, the lower rock stratum is older than upper rock stratum. The other is there are remains or survival and activity traces of “Rocky” ancient organisms in many sedimentary rocks-fossils. It is the valuable information used to determine the geological age and study paleogeographic environment, known as “Page” and “Text” used for recording the history of the Earth.
③Metamorphic rock. The original rock is formed into metamorphic rock by metamorphism. According to different types of metamorphism, metamorphic rocks can be divided into five types: dynamic metamorphic rock, contact metamorphic rock, regional metamorphic rock, migmatite and metasematite. Common metamorphic rocks include mylonite, cataclasite, hornstone, killas, phyllite, schist, gneiss, marble, quartzite, amphibolites, leptite gneiss, eclogite, migmatite and so on. Metamorphic rocks account for 27.4% of the volume of earth crust.