GEOL 103

Example midterm questions:

• Halite

  • Does this sample have cleavage? If so, what is the name of this type of cleavage 

  • What special property of this mineral can be used for identification 

• Gabbro

  • Is this igneous rock mafic, felsic, intermediate, or ultramafic 

    • Mafic 

  • What type of texture does it have

    • Phaneritic 

Sedimentary rock: rocks formed from sediment 

• Grains + cement = rocks 

What is sediment?

• Dictionary definition: 

  • The matter that settles to the bottom of a liquid 

• Use this definition: 

  • Material derived from pre-existing rock, from biogenic sources, or precipitated by chemical processes, and deposited at, or near, the earth's surface 

How do we make sedimentary rocks?

• Four big processes 

1. Weathering 

2. erosion/transportation 

Weathering: processes acting on Earth’s surface to break down pre-existing rock 

• Two types

  • Chemical: breakdown by chemical agents, minerals in a rock may change, processes like oxidation or dissolution 

  • Mechanical: physically breaking down rocks into smaller pieces (no change in chemistry, creates clasts: smaller fragments of rocks)

• Left with 3 products after weathering  

  • Quartz sand 

  • Clay 

  • Minerals in solution 

Erosion: transportation away from the place of weathering  

• Water: most important means of erosion 

  • Rivers, rainwater, oceans, tides

• Ice

  • Glaciers 

• Wind

  • Sand dunes

• Gravity 

  • Landslides 

Deposition: the importance of energy 

Diagenesis: putting the pieces together 

• Lithification: process where sediments compact under pressure, expel fluids, and become solid rock 

  • Compaction: pressure from overlying sediments causes reduction of pore space and removal of water

  • Cementation: precipitation of secondary minerals from solution fills pore spaces and binds grains together

  • After lithification, grains of sediment, rock fragments and fossils can be replaced by other minerals 

Types of sedimentary rocks 

Clastic: made from broken pieces of rock cemented together 

Chemical: made from minerals being dissolved into water and precipitated oit 

• Inorganic (chemical)

• Organic (biochemical)

Grain size: increase in grain sized leads to an increase in energy needed for transportation 

• Major terms: gravel, sand, silt, clay 

Coarse grains 

• Gravel and sand 

  • Particles large enough to see 

  • If some scratch glass, the rock is clastic

Fine grains 

• Silt and clay 

  • Particles too small to see

  • If the rock is dark in color, dense, and shows prominent layering the rock is clastic 

  • All other fine grained rocks are chemical or biochemical 

Well sorted sediment = little variation in grain size 

Poorly sorted sediment = wide range of grain sizes

Rounding: how smooth a grain is 

• Well rounded =  smooth surfaces 

• Sorting and rounding correspond to distance or time a grain is transported 

Breccia: more angular, not as well rounded

Conglomerate: well rounded, transported a greater distance 

Maturity: how far removed a rock is from its source, igneous minerals have a lot of amphibole

Mature quartz sandstone 

Immature arkosic sandstone 

Chemical sedimentary rocks 

• Evaporites 

• Carbonates

• chert/flint 

Evaporites

Halite 

• NaCl 

Gypsum 

Carbonates: form form chemical leaching and precipitation 

• Removing elements from minerals, deposited elsewhere 

• Can be chemical or biochemical

• Other inorganic processes can affect formation 

Biochemical sedimentary rocks: may be predominantly clastic or biogenic 

• Limestone, coquina, chalk, chert 

• Organic: formed due to decomposition of organic remains under higher temperatures 

Limestones: made of finely crushed remains of shelled marine organisms 

• Micrite 

• Fossiliferous 

Coquina: poorly cemented shell fragments 

• Shells and cement are both composed of calcite 

• Forms in warm, high energy shallow marine environments 

Chalk: soft white, porous limestone

• Made from marine microorganisms classed coccolithophores 

• Forms in quiet, deep marine waters 

Coal: fossilized plant tissue, formed in a bog 

bio

Chert: secretly quartz, can be chemical or biochemical, may form from the accumulation of silica skeletons from microscopic organism or by replacement of other rocks, minerals, and organic matter after deposition 

3/10/25 Midterm Review 

Cooling history 

• phaneritic texture: cools very slowly, time for big crystals to develop, cooling history intrusive 

  • Big crystals mean at some point it was cooling slowly 

• Obsidian: texture – glassy 

• Vesicles: cooling outside, cooling quickly, extrusive 

• No crystals: cooling quickly, cooling extrusively, no time to grow any crystals 

Clast: piece of a rock 

Biochemical and chemical rocks: processes that result in the creation of the rock in addition to the elements necessary for the formation and where they come from 

• Pink in color, reddish, halite: rock salt, forms through removing all the water from a given area, same class as rock gypsum – chemical rock 

• Bituminous coal: starts out as trees, fall over in a swamp and get buried, originally it is made from something organic – biochemical rock 

• Cochina: made from pieces of broken up shells that get cemented together – Biochemical rock (the material it is made from was originally part of an organism) 

• Chert: forming from silica or tiny little microorganisms – either chemical or biochemical