BIG unit assignment


D: Sedimentary rocks and processes:



o How does each of the following types of sedimentary rocks form?

Clastic rocks: are formed by broken fragments of pre-existing rocks and minerals.
Chemical rocks: are precipitated from solution, from evaporation.


o Describe the distinguishing features of and identify the following as to which type of sedimentary rocks they are (make a table):





Conglomerate


A sedimentary rock containing abundant, rounded pebble, cobble, and boulder-sized particles.


Breccia


A sedimentary rock containing abundant angular pebble, cobble, and boulder-sized particles.


Sandstone


A sedimentary rock made up of sand-sized particles, typically quartz.


Siltstone


Shale


Sedimentary rock made up of clay-sized particles, typically fissile (fractures easily on bedding planes).


Limestone


A sedimentary rock made up predominantly of calcite.


Chert


a sedimentary rock type consisting of chemically or biologically precipitated silica


Gypsum


Gypsum rocks are sedimentary rocks made up of sulfate mineral and formed as the result of evaporating sea water in massive prehistoric basins. It is very soft and is used to make Plaster of Paris, casts, molds, and wallboards.


Rock salt


Coal


3. Diagram and describe the following sedimentary features and state the sedimentary environments they would form in. (make a chart).



-sedimentary structures (stratification, cross bedding, ripple marks, mud cracks, graded bedding, varves)


-fossils and organic structures.


stratification


The most important feature of sedimentary rocks, is layering that reflects changing conditions during deposition. Texture and composition are fairly uniform within a bed. Layering is usually horizontal or sub-horizontal.


cross bedding


A sedimentary rock texture characterized by overlapping and cross-cutting bedding, typical of Aeolian sands.


ripple marks


Wave like patterns in sedimentary bedding due to movement of water.


mud cracks


Polygonal cracks caused by contraction of mud as it dries, indicate shallow water deposition, and can be used as a top of bed indicator.


graded bedding


Represents a progressive decrease in grain size upward through a bed, and can be used as a top of bed indicator. Grading can be caused by rapid deposition from turbidity currents.


Varves


Metamorphic rocks form from deformation. This can result from heat and pressure.


One way this can happen is when rocks are next to molten material and the edge of the rock gets baked from the heat. The result is that the minerals get larger that were bordering the molten material. This is called CONTACT metamorphism.


REGIONAL metamorphism takes place over a very large area. This happens in places that are under intense pressure and temperature changes. Typically this is found where plates collided. Metamorphism is rated from low to high. Low grade would be areas that had little temperature or little pressure and high grade would be intense temperature and pressure changes. Rocks include from low to high grade: slate, phyllite, schist, and gneiss.


While being heated, the atoms start to move. The minerals start to arrange themselves perpendicular to bedding. As a result, the rocks tend to show bands or foliation (lined up like a deck of cards). Massive crystals may also form and grow under these conditions.


The larger the minerals, the more intense heat were involved. Since certain minerals can only form at certain temperatures, by looking at the minerals present in an area, scientists get an idea of what the temperature was when the rock formed. These minerals are called INDEX minerals. They include: chlorite, biotite, muscovite, garnet, staurolite, kyanite, and sillimanite.