Lecture 3 - Geology

8/3/2019 Lecture 3 - Geology

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Course 3

PETROGRAPHY NOTIONS



The Petrography is the science that study the rocks.

With few exceptions rocks are made of dentically or different

minerals, with same or igin and structure.

The internal and external forces continue to interact, forcing substancesout of equilibrum. Therefore, the Earth has a highly var ied and ever-

changing surface. A useful aid in visualizing these relationships is the

rock cycle: igneous, metamorphic and sedimentary

-igneous rocks ±are rocks that appear to have

crystallized from magma; when magma solidif ies-sedimentary rocks ± is that have formed from

lithif ication of sediment, precipitation from

solution, or consolidation of the remains of plants

or animals

-metamorphic rocks ± transformation of

preexisting rock into new rock as

a result of high temperature, high pressure, or

both

Obs. If the temperature gets very high, the rock

melts and become magma again, completing

the cycle

Sedimentary rocks

Igneous rocks

Lithif ication

Cooling

(or cryalization)Magma

Metamorphism

Metamorphic rocks

Melting

The rock cycle



The characteristics of the rocks:

-Chemical composition ± is the same with the or iginal mater ial from

which the rock had been made- The structure ± is the spatial distr ibution of the grains-The texture - can be character ized by grain size distr ibution and

morphology of grains-The bodies are depending on the rock type:

-igneous rocks -appear as cones, dikes, sills, stock, batholiths,

columns, masive body

-metamorphic rocks ± crystalline shists or massive rock-sedimentary rocks ±form layers

IGNEOUS ROCKSIgneous rocks form when magma solidif ies. The magma is made of

silicate,water, gases, to 1100-1400 0 C temperature and 1200 atm pressure

Lava is molten rock that reaches the earth¶s surface; it contains 45-75% silica;

the more silica, the more viscous lava.

Intrusive rocks are igneous rocks that formed under ground

Extrusive rocks are igneous rocks that formed to the surface, by a volcaniceru tion

Obs. Those minerals with the highest melting temperatures crystallize from

the cooling magma before those with lower melting point (Bowen¶s reaction

ser ies)



INTRUSIVE ROCKS EXTRUSIVE ROCKS

(f) GRANITE RYOLITE

(i) SIENITE TRAHITE

(i) DIORITE ANDESITE

(m) GABBRO BASALT

A very silicic magma is silica-r ich (65-72 %), iron-and magnesium-poor

produce igneous rocks composed mostly of quartz, potassium feldspar and

plagioclase ; are classif ied as felsic (light coloured)

A mafic magma is r ich in magnesium and iron, and poorer in silicon (45-52 %) produce igneous rocks composed mostly of dark-coloured

ferromagnesian minerals; are classif ied as mafic (dark);

Ultramafic igneous rocks are composed entirely or almost entirely of

dark ferromagnesian minerals. No feldspars are present and, of course, no

quartz; Most ultramaf ic rocks are composed of coarse-grained pyroxene and/or

olivine.Chemically these rocks contain less than 45% silica.There are also intermediate igneous rocks of both felsic and maf ic

minerals;



Bodies of igneous rocks

-Intrusives that crystallize at

considerable depth= Plutonic

Ex.Most plutoni rock is inBatholiths

Is a lar ge discordant pluton

with an outcrop area greater

than 100 square

kilometers, the body is called

batholith

- Shallow intrusive structure

Ex. Laccolith A rarely found is a

blisterp-shaped

structure called a

laccolith

- to surface =Volcanic

Ex. Lode

Some intrusive rocks have solidif ied near the surface as a direct result of

volcanic act

ivity (necks-sol

idif ies w

ithin volcanoes)

Most intrusive rock is plutonic



METAMORPHIC ROCKS

The transformation of preexisting rocks into new rocks as a result of high

temperature, high pressure, or both ,thanks to tectonics movement or to

ascent of magma .

- a new recrystallization - mineralogy or chemical changes

- new mineral association -stable

- the grains are growing up, new crystals are or ientated, minerals

folliation (elongated or platy minerals)

-the new rock has a texture clearly different from that of the or iginalrock (=parent rock)

Factors controlling the characteristics of metamorphic rocks

-the mineral content of the metamorphic rock is controlled by the chemical

composition of the parent rock

-temperature-a mineral is ³stable´ if, given enough time, it does not react withor convert to a new mineral or subtance

-pressure-conf ining (static pressure= pres. litostatica) and directed pressure

(compressive, dynamic)

-The effects of fluids, such as water

Metamorphic rocks are: gneiss, mica-schist, quartzite, marble



SEDIMENTARY ROCKS

-are formed from lithif ication of sediment ,precipitation from

solution, or consolidation of remains of plants or animals

Sediment is a collective name for loose, solid particles, usually collected in

layers on the earth¶s surface

Hor izontal

layers

Layers types

Oblique layers

Curly layers

Crossing layers



Physical and

mechanical

process of sedimentary rocks

1. Weathering-erosion

-insolation-freezing-thaw

-r iver

-sea water

-wind

-glacier -plants and animals

clasts

2.Transport

-water

-wind-ice

-gravity

Rounding and sorting

3. Deposition

Acumulation

of chemical or

organic

sediment

1.2.

3.



Clastic sedimentary rocks - are formed from cemented sediment grains

that are fragments of preexisting rocks.

Obs. In most cases the sediment has been eroded and transported before

being deposited.

UNCONSOLIDATED CONSOLIDATED

(sediment) (sedimentary rock)

0 > 2 mm GRAVEL (rounded particles) CONGLOMERATE

COBBLE ( angular particles) BRECCIA

0,063<0<2 mm SAND SANDSTONE

0,004< 0<0,063 mm SILT SILTSTONE

MUD<0,004SHALE, MARLY

MUDSTONE,

CLAY

Obs. Sediments and sedimentary rocks are classif ied and def ined according tothe grain size of individual fragments



Ex. Lithif ication of sand grains to become sandstone

a) Loose sand grains are deposited with open pore space

between the grains

b) The weight of overburden compacts the sand into a tighter arrengement,

Reducing pore space

c) Precipitation of cement in the pores by ground water binds

the sand into

the rock sandstone, which has a clastic texture.

Lithification- is general term for a group of processes that cover loose

sediment into sedimentary rock.

Obs. Most sedimentary rocks are lithif ied by a combination of compaction

(a decrease in pore space) and cementation (the cement-solid mater ial, canprecipitate in the pore space and bind the loose sediment grains together to

form a solid rock).

Chemical sedimentary rocks- are those deposited by precipitation of

minerals from solution

Ex. Evapor ites- rock salt, rock gypsum, borates, potassium salt,

magnesium salt

Chemical limestone= travertine; chemical dolomite= dolostone



Or ganic sedimentary rocks ±are the rocks that accumulate from

the

remains of or ganisms

Ex. Limestone- are composed of skeletons of calcareousmicoor ganisms (calcite secreting)

Coquina- is a limestone var iety formed from the

cementation of shells accumulated on the sea floor

Chalk- is a var iety of bioclastic limestone, of tiny mar ine

or ganisms

Oolitic limestone- formed by cementation of small

spheres of inor ganic calcite

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Lecture 3 - Geology