A clast is a fragment of rock or mineral, ranging in size from less than a micron (too small to see) to as big as an apartment block. Various types of clasts are shown in Figure 5.12 and in Exercise 5.3. Most clasts that are smaller than sand size (<1/16 mm) are made of clay minerals.

Clastic sedimentary rocks form from the accumulation and lithification of mechanical weathering debris. Examples include: breccia, conglomerate, sandstone, siltstone, and shale. Chemical sedimentary rocks form when dissolved materials preciptate from solution.

Clastic rocks are classified by clast size. What are the size classes used to describe clastic rocks? The size classes used to describe clastic sedimentary rocks are, from smallest to largest: clay and silt (mud size); fine and coarse (sand size); pebbles, cobbles, and boulders (gravel size).

Particle size is an important textural parameter of clastic rocks because it supplies information on the conditions of transportation, sorting, and deposition of the sediment and provides some clues to the history of events that occurred at the depositional site prior to final induration.

Siltstone is made of smaller particles. Silt is smaller than sand but larger than clay. Shale has the smallest grain size….Clastic Sedimentary Rocks.

AGENTS OF METAMORPHISM – The agents of metamorphism include heat, pressure (stress), and chemically active fluids.

The three agents of metamorphism are heat, pressure, and chemically active fluids.

There are three ways that metamorphic rocks can form. The three types of metamorphism are Contact, Regional, and Dynamic metamorphism. Contact Metamorphism occurs when magma comes in contact with an already existing body of rock.

Dynamic metamorphism takes place anywhere that faulting occurs at depth in the crust. Thus, mylonites can be found at all plate boundaries, in rifts, and in collision zones.

Dynamic metamorphism is the result of very high shear stress, such as occurs along fault zones. Dynamic metamorphism occurs at relatively low temperatures compared to other types of metamorphism, and consists predominantly of the physical changes that happen to a rock experiencing shear stress.

Dynamic metamorphism involves high shear stress, high pressure, high strain, high fluid partial pressure and variable temperature.

Regionally metamorphosed rocks usually have a squashed, or foliated appearance – examples include slate, schist and gneiss (pronounced “nice”), formed by metamorphism of mudstones, and also marble which is formed by metamorphism of limestone.

Regional metamorphism: Changes in enormous quantities of rock over a wide area caused by the extreme pressure from overlying rock or from compression caused by geologic processes. Deep burial exposes the rock to high temperatures.

Regional metamorphism is metamorphism that occurs over broad areas of the crust. Most regionally metamorphosed rocks occur in areas that have undergone deformation during an orogenic event resulting in mountain belts that have since been eroded to expose the metamorphic rocks.