Sedimentary and metamorphic rocks
FOCUS QUESTIONS
How are secondary rocks formed? Give examples. How are metamorphic rocks formed? Give examples. A rock is an aggregate of one or more minerals.
SEDIMENTARY ROCKS
Sedimentary rocks are formed from The accumulation of sediments The deposition and lithification of weathering products There are three basic types of sedimentary rocks.
Clastic sedimentary rocks such as breccia, conglomerate, sandstone, siltstone, and shale are formed from mechanical weathering debris.
Chemical sedimentary rocks such as rock salt, iron ore, flint, some dolomites, and some limestones form when dissolved materials precipitate from solution.
Organic sedimentary rocks such as coal, some dolomites, and some limestones form from the accumulation of plant or animal debris.
SEDIMENTARY ROCK FORMATION
Metamorphic rocks ‘morph’ (meaning form) and ‘meta’ (meaning change). A metamorphic rock is a result of a transformation of a preexisting rock. The original rock is subjected to very high heat and pressure, which cause obvious physical and/or chemical changes. They can be formed: By pressures deep inside the Earth By tectonic processes such as continental collisions When they are heated up by an intrusion of hot molten rock called magma from the Earth’s interior. Uplift and erosion help bring metamorphic rock to the Earth’s surface.
FOLIATED METAMORPHIC ROCKS
Foliated metamorphic rocks are formed within the Earth’s interior under extremely high pressures that are unequal, occurring when the pressure is greater in one direction than in the others (directed pressure). This causes the minerals in the original rock to reorient themselves, with the long and flat minerals aligning perpendicular to the greatest pressure direction. This reduces the overall pressure on the rock and gives it a stripped look. Foliated metamorphic rocks are identified on the basis of their texture:
SLATE
Formed at very low temperatures and pressures, rocks break along nearly perfect parallel planes; used in pool tables and as roofing material.
PHYLLITE
Low to intermediate temperatures and pressures; slightly more crystallized, which gives the rock a shiny appearance; layers may also be wavy or crinkled.
SCHIST
Intermediate to high temperatures and pressures; crystals are larger with the grains aligned in parallel to subparallel layers.
GNEISS (NICE)
Very high temperatures and pressures; coarse-grained texture of alternating light and dark mineral bands.
NON-FOLIATED METAMORPHIC ROCKS
Non-foliated metamorphic rocks are formed around igneous intrusions where the temperatures are high but the pressures are relatively low and equal in all directions (confining pressure). The original minerals within the rock recrystallise into larger sizes and the atoms become more tightly packed together, increasing the density of the rock. They are identified on the basis of their composition.
QUARTZITE
Composed of the mineral quartz; metamorphosed sandstone
MARBLE
Composed of the mineral calcite; metamorphosed limestone
EXAMPLES OF METAMORPHIC ROCKS
Anthracite is a type of coal with a high carbon count, few impurities and with a high luster (meaning it looks shiny).
Marble is a metamorphic rock that is formed from the sedimentary rock limestone.
Quartzite is a metamorphic rock that is formed from the sedimentary rock sandstone.
Slate is a metamorphic rock that is formed from the sedimentary rock mudstone.
Granulite is a metamorphic rock that is formed from the igneous rock basalt.