<P> There is also a tendency for metasomatism between the igneous magma and sedimentary country rock, whereby the chemicals in each are exchanged or introduced into the other . Granites may absorb fragments of shale or pieces of basalt . In that case, hybrid rocks called skarn arise, which don't have the characteristics of normal igneous or sedimentary rocks . Sometimes an invading granite magma permeates the rocks around, filling their joints and planes of bedding, etc., with threads of quartz and feldspar . This is very exceptional but instances of it are known and it may take place on a large scale . </P> <P> Regional metamorphism, also known as dynamic metamorphism, is the name given to changes in great masses of rock over a wide area . Rocks can be metamorphosed simply by being at great depths below the Earth's surface, subjected to high temperatures and the great pressure caused by the immense weight of the rock layers above . Much of the lower continental crust is metamorphic, except for recent igneous intrusions . Horizontal tectonic movements such as the collision of continents create orogenic belts, and cause high temperatures, pressures and deformation in the rocks along these belts . If the metamorphosed rocks are later uplifted and exposed by erosion, they may occur in long belts or other large areas at the surface . The process of metamorphism may have destroyed the original features that could have revealed the rock's previous history . Recrystallization of the rock will destroy the textures and fossils present in sedimentary rocks . Metasomatism will change the original composition . </P> <P> Regional metamorphism tends to make the rock more indurated and at the same time to give it a foliated, shistose or gneissic texture, consisting of a planar arrangement of the minerals, so that platy or prismatic minerals like mica and hornblende have their longest axes arranged parallel to one another . For that reason many of these rocks split readily in one direction along mica - bearing zones (schists). In gneisses, minerals also tend to be segregated into bands; thus there are seams of quartz and of mica in a mica schist, very thin, but consisting essentially of one mineral . Along the mineral layers composed of soft or fissile minerals the rocks will split most readily, and the freshly split specimens will appear to be faced or coated with this mineral; for example, a piece of mica schist looked at facewise might be supposed to consist entirely of shining scales of mica . On the edge of the specimens, however, the white folia of granular quartz will be visible . In gneisses these alternating folia are sometimes thicker and less regular than in schists, but most importantly less micaceous; they may be lenticular, dying out rapidly . Gneisses also, as a rule, contain more feldspar than schists do, and are tougher and less fissile . Contortion or crumbling of the foliation is by no means uncommon; splitting faces are undulose or puckered . Schistosity and gneissic banding (the two main types of foliation) are formed by directed pressure at elevated temperature, and to interstitial movement, or internal flow arranging the mineral particles while they are crystallizing in that directed pressure field . </P> <P> Rocks that were originally sedimentary and rocks that were undoubtedly igneous may be metamorphosed into schists and gneisses . If originally of similar composition they may be very difficult to distinguish from one another if the metamorphism has been great . A quartz - porphyry, for example, and a fine feldspathic sandstone, may both be metamorphosed into a grey or pink mica - schist . </P>

Can rocks only change on the earth's surface