14Sedimentary Rocks Clastic – rock fragments - sediments Organic – shells – dinosaur bonesChemical – salt – Dead SeaLayers!!!Ripples in Stream bottoms!
15Sedimentary rocks cover about three-quarters of the earth's surface Clastic sedimentary rocks are made up of rock fragments, called sediments, that have been eroded from one place and moved to another by water, ice, wind, or gravity. In time, the sediments are buried, compacted and cemented together to form sedimentary rocks. Examples include conglomerate, sandstone, and shale.Other sedimentary rocks are “chemical” or “organic”.Chemical sedimentary rocks are formed by the precipitation or evaporation of minerals from solution in an ancient seawater. Examples include limestone, gypsum, and halite (rock salt).Organic sedimentary rocks are formed by the accumulation of dead plant and animal matter which is then compacted together. Examples include peat, coal and oil
23Metamorphic Rocks Contact or Thermal Metamorphism and Regional Metamorphism
24Thermal metamorphic rocks / Contact metamorphic rocks. This type is often associated with igneous rocks.Because of the high heat, the “regular” rock (known as country rock), near the hot rock is often altered.The pressure involved is usually negligible.Alteration of the rock is generally localized (close to the hot rock) in what is known as a metamorphic aureole or halo.x = Slight MetamorphismX = Intense Metamorphism^ = Igneous rock
25Regional metamorphic rock is usually the result of extreme pressure Regional metamorphic rock is usually the result of extreme pressure. These are commonly associated with mountain building events and are often associated with plate boundaries at continental margins.Pressure squeezes the mineral grains of sedimentary rock together. This eliminates the pore spaces, and fluids.With more pressure, the mineral grains will form a tightly interlocking mosaic.With even more pressure, the crystals may reform into fewer but larger grains.This process is known as recrystallization. The recrystallized mineral may remain the same as it was, or it may be a new, more dense mineral.
26Classification of Metamorphic Rocks Metamorphic rocks can be subdivided into foliated and nonfoliated types.Foliated means there are mineral grains aligned parallel to each other. This results in a layered appearance.A Nonfoliated metamorphic rock generally consists of equaldimensional grains. You can’t see layersA rock can also be identified by its metamorphic grade. This is simply a description of the overall intensity of metamorphism the rock was subjected to. What this implies is that a low-grade metamorphic rock shows textural or mineralogic evidence of having been subjected to low pressures and/or temperatures.
31SEDIMENTARY TO METAMORPHIC The following is a very general progression from sedimentary rock to metamorphic rock based primarily on pressure.SURFACE MUD5 km DEEP SHALE (sedimentary)10 km DEEP SLATE (low grade metamorphic)15 km DEEP SCHIST (Garnet appears)20 km DEEP GNEISS (high grade metamorphic)25 km DEEP HORNFELS (Cordierite appears)