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ENGINEERING GEOLOGY

CE- 226 Lecture 02: Basic Rock Formations/Rock Cycle in Geology

THE ROCK CYCLE

• Rocks are the most common material on Earth. They are naturally

occurring aggregates of one or more minerals.

• Rock divisions occur in three major families based on how they

formed: igneous , sedimentary , and metamorphic. Each group

contains a collection of rock types that differ from each other on the

basis of the size, shape, and arrangement of mineral grains.

• The rock cycle is an illustration that is used to explain how the three

rock types are related to each other and how Earth processes change

a rock from one type to another through geologic time. Plate tectonic

movement is responsible for the recycling of rock materials and is the

driving force of the rock cycle.

TYPES OF ROCKS

• I gneous rocks are formed from magma, which has originated well below the surface, has ascended towards the surface, and has crystallised as solid rock either on the surface or deep within the Earth’s crust as its temperature fell.
• S edimentary rocks are formed by the accumulation and compaction of (a) fragments from pre-existing rocks which have been disintegrated by erosion (see Section 3.2.1); (b) organic debris such as shell fragments or dead plants; or (c) material dissolved in surface waters (rivers, oceans, etc.) or ground water (Section 5.2.2), which is precipitated in conditions of oversaturation.
• M etamorphic rocks are formed from pre-existing rocks of any type, which have been subjected to increases of temperature (T) or pressure (P) or both, such that the rocks undergo change. This change results in the metamorphic rock being different from the original parental material in appearance, texture and mineral composition

IGNEOUS ROCKS

• Igneous Rocks are differentiated on the Basis of there Constituents

and grain sizes.

1. Fine Grained Igneous Rocks (crystals are on average less than 1 mm across)
2. Medium Grained Igneous Rocks (they are between 1 and 3 mm across)
3. Coarse Grained Igneous rocks (they are over 3 mm across)

Igneous rocks are composed of interlocking crystals and are said to

have a crystalline texture.

IGNEOUS ROCKS

• Extrusive igneous rocks have formed (as lavas) by rapid cooling of

magma at the surface, and are fine grained

• Intrusive igneous rocks have formed by the slow cooling of great

volumes of magma, typically at depths of a few kilometers, within the

Earth. The heat from their enormous bulk can dissipate only very

slowly and they are normally coarse grained

EXTRUSIVE IGNEOUS ROCKS

SEDIMENTARY ROCKS

• Any rock (igneous, sedimentary, or metamorphic) exposed at the

Earth's surface can become a sedimentary rock. The forces of wind,

rain, snow, and ice combine to break down or dissolve (weather), and

carry away (transport) rocks exposed at the surface. These particles

eventually come to rest (deposited) and become hard rock (lithified).

• Sedimentary rocks tell us what the Earth's surface was like in the

geologic past. They can contain fossils that tell us about the animals

and plants or show the climate in an area. Sedimentary rocks are also

important because they may contain water for drinking or oil and gas

to run our cars and heat our homes.

SEDIMENTARY ROCKS

SEDIMENTARY ROCKS

• The four major groups of sedimentary rocks are: (a) terrigenous sedimentary rocks (sometimes referred to as clastic , which are formed from minerals or rock fragments derived from the breakdown of preexisting rocks (b) chemical sedimentary rocks, which are formed from the precipitation of salts dissolved in water; (c) organic sedimentary rocks, which are formed from the skeletal remains of plants and animals and include coal and oil; (d) limestones and dolomites, which are sedimentary rocks consisting of more than 50% carbonate, and can include chemical, clastic and biological material

NON-CLASTIC SEDIMENTARY ROCKS

Non-clastic sedimentary rocks form from chemical reactions, chiefly in the ocean. Non-clastic and clastic sedimentary rocks are the only members of the rock family that contain fossils as well as indicators of the climate that was present when the rock was formed. Non-clastic sedimentary rocks are named according to the mineral present.

1. Limestone : Composed of the mineral calcite, may contain marine fossils; formed by precipitation from water. 2. Rock salt : Composed of the mineral halite (salt); formed by evaporation. 3. Chert : Composed of microscopic mineral grains of quartz; very hard with sharp edges 4. Rock gypsum : Composed of the mineral gypsum; formed by evaporation.

CLASTIC SEDIMENTARY ROCKS

Clastic sedimentary rocks form by weathering processes which break down rocks into pebble, sand, or clay particles by exposure to wind, ice, and water. Clastic and non-clastic sedimentary rocks are the only members of the rock family that contain fossils as well as indicators of the climate (ripple marks, mud-cracks and raindrops) that was present when the rock was formed. Clastic sedimentary rocks are named according to the grain size of the sediment particles.

1. Sandstone : grains ranging in size from 2mm to 1/16 mm 2. Shale : grains lesser in size from 1/16mm 3. Breccia : coarse (2mm to 64 mm), angular grains 4. Conglomerate : coarse (64 mm to >256 mm), rounded grains

CLASTIC SEDIMENTARY ROCKS