Metamorphic Rocks and Conclusion of Sedimentary

Rocks

The deal with sedimentary rocks

• The bigger the grains, the higher the energy of the depositional environment in which it was formed.

• The smaller the grains, the quieter the environment in the rock formed.

• SEDIMENTARY ROCKS ARE SO USEFUL BECAUSE THEY RECORD THE EARTH’S HISTORY. THEY TELL US WHAT KIND OF ENVIRONMENT WAS WHERE AND WHEN.

Stratification

• Since sedimentary rocks are formed by deposition of sediment, they form layers of rock.

• Stratification occurs when you get distinct layering in rocks (you deposit something different on top of something else).

• Principle of Uniformitarianism and Suposition – each layer is uniform in composition, older layers are on bottom, newer layers on top.

• If there is discontinuity in the layers, geologists term it an unconformity.

How can you get an unconformity?

• Erosion of a layer just deposited.

• Faulting (normal or reverse)

• For reverse faulting, your older rocks would get pushed on top and mess up the order.

Cool Sed rock features

• Cross-bedding – formed from changes in wind direction, or change in direction of water current.

• Ripple marks – you can definitely tell which way water or wind moved

• Mudcracks – formed when mud dries up and shrinks

• Geodes- water within sediments leaches into cavities of rocks precipitating calcite or quartz crystals

Metamorphic Rocks

• Metamorphism (in the geological sense)- the changing of one rock into another by inducing heat and/or pressure.

• Two types of metamorphism: contact, and regional

Contact Metamorphism

• When rocks are changed by heat

• How this happens:

1.) rocks that are subducted and come in contact with magma

2.) rocks happen to come into contact with magma that is migrating towards the surface (some new volcanism)

Regional Metamorphism

• Rock change because they undergo some kind of process that induces a lot of pressure

• How this happens:

1.) rock in a subduction zone, the deeper you go, the more pressure you encounter

2.) rock in a fault zone (lots of pressure there)

3.) compressional forces via plate tectonics, isostatic adjustment, remember folding?????? Ductile deformation?

Chemical Alteration

• Hydrothermal – pretty much water eruptions- water comes into contact with magma, the magma boils it an sends it back to the surface enriched with chemicals from the subsurface and magma chamber

How you can tell if a rock is formed by contact or regional

metamorphism.• One word: FOLIATION• Foliated rocks have visible parallel banding. A lot

of the time these bands are wavy, warped, because after all, they were formed under pressure. So, FOLIATED ROCKS ARE FORMED BY REGIONAL METAMORPHISM.

• Non-foliated – most of the time are rocks formed by contact metamorphism

• An example: marble – it’s a non foliated rock formed by regional metamorphism