Rock Classifications

ROCKS:Defined

Magma • molten material that forms inside earth.

Crystallization • magma cools and solidifies.• may occur beneath the surface or at the surface.

Resulting rocks are called igneous rocks (ignis = fire).

Igneous Rocks

Made by Fire

volcanic or extrusive• rocks that result when lava solidifies.• they are extruded onto the surface.plutonic or intrusive• the magma is not able to reach the surface eventually crystallizes at depth.• they intrude existing rocks.

Melt liquid portion of a magma body. composed of ions that move about freely.Crystallization random movements of the ions slow, and the ions begin to arrange themselves into orderly patterns.

cooling strongly influences crystal size. slow cooling results in the formation of large crystals. quick cooling results the formation of solid mass of small intergrown crystals.

Classifying Igneous RocksIgneous Textures

describes the overall appearance of n igneous rock based on the size, shape, and arrangement of its interlocking crystals.

i. Coarse-grained Texture- appearance of a mass of intergrown crystals, which are roughly equal in size and large enough to be identified with the unaided eye.

ii. Porphyritic Texture- has large crystals embedded with a matrix of smaller crystals.

iii. Glassy Texture- results when the ions do not have sufficient time to unite into an orderly crystalline structure.

Obsidian- similar to a large chunk of manufactured glass.Pumice

- volcanic rock that exhibits a glassy texture.

Mineral Composition depends on the chemical composition of the magma from which it crystallizes.

minerals that form at about the same temperature are found together in the same igneous rock.

Naming Igneous RocksIgneous rocks are classified by

their texture and mineral composition.Granitic Rocks – composed almost entirely of light-colored silicates – quartz and feldspar.

Granite-best-known igneous rock partly because of its natural beauty, and partly because of its abundance.

- slabs of polished granite are commonly used for tombstones, monuments, and as building stones.

Rhyolite

-extrusive equivalent of granite and is composed essentially of the light-colored silicates.

- rhyolite deposits are less common and generally less voluminous.

Rhyolite Rocks

Basaltic Rocks

-contain a high percentage of dark silicate minerals.

- because of their iron content, basaltic rocks are typically darker and denser than granitic rocks.

Basalt - a very dark green to black fine-grained volcanic rock.

Andesitic or Intermediate Composition- rocks with a composition between granitic and basiltic rocks.

Peridotite- opposite side of the compositional spectrum from granitic rocks.

Sedimentary Rocks

Sediments are loose, unconsolidated accumulations of mineral rock particles that have been transported by wind, water, ice, or gravity and redeposited.

Derived from the Latin sedimentum means settling, reference to a solid material settling out of a fluid.

Formation: weathering of pre-existing rocks 0 either by physical breakup into finer and finer fragments, or by solution. precipitation of crystals out of solution. usually, the particle are broken down further during this transport phase. sediment become lithified, or turned to rock.

Clastic Sedimentary Rocks from the Greek word klatos,

meaning ‘broken’ formed from the products of

the mechanical breakup of other rocks.

broken up into smaller and smaller pieces and ultimately, into individual mineral grains.

Economic Importance of

Sedimentary Rocks presently the source of over half the electronic power in the United States. other major energy sources (petroleum and natural gas) occur in pores of sedimentary rocks. other sedimentary rocks are major sources of iron, aluminum, manganese, fertilizer, and sand and gravel.

Classifying Sedimentary RocksDetrital Sedimentary Rocks solid particles from

weathered rocks such as igneous rocks.

the rock is called conglomerate if the sediment is rounded.

the rock is called breccia if the pieces are angular.

Sandstone is the name given to rocks when sand-grains prevail.Shale is made of very fine-grained sediment.Siltstone is sometimes difficult to differentiate from rocks such as shale which is composed of even smaller clay-sized sediment.

Chemical Sedimentary Rocks

derived from material that is carried in solution to lakes and seas.

does not remain dissolved in water indefinitely.

Limestone – the most abundant chemical sedimentary rock.

Coquina – a coarse rock composed of loosely cemented shells and shell fragments.

Chalk – soft, porous rock made up almost entirely of the hard parts of microscopic organisms.

Travertine – type of limestone that decorates taverns.

Coal quite different from other

chemical sedimentary rocks. the end product of the burial

of large amounts of plant material over extended periods

We divide sedimentary rocks into two major groups: detrital and chemical.The main criterion for classifying rocks are distinguished by their mineral composition.

Lithification of Sediment refers to the processes by

which sediments are transformed into solid sedimentary rocks.Cementation – another important means by which sediments are converted to sedimentary rock.

Features of

Sedimentary Rocks

Sedimentary rocks are particularly important evidence of Earth’s long history.

Strata – the single most characteristic feature of sedimentary rocks.

Sedimentary rocks provide geologists with evidence deciphering past environments.

Metamorphic

Rocks

produced from preexisting igneous, sedimentary, or from other metamorphic rocks.

every metamorphic rock has a parent rock – the rock from which it was formed.

Metamorphism = to change form takes place where existing

rock is subjected to temperatures unlike those in which it formed.

most changes occur at the elevated temperatures and pressures that exist in the zone beginning a few kilometers below Earth’s surface and extending into the upper mantle.

Most metamorphism occurs in two things:1. Thermal Metamorphism –

when intruded by a magma body, change is driven by a rise in temperature within the host rock surrounding a molten igneous body.

2. Regional Metamorphism – during mountain building, great quantities of rock are subjected to directed pressures and high temperatures and associated with large-scale deformation.

Agents of

Metamorphism

Heat as a Metamorphic Agent

Heat is the most important agent of metamorphism.

it provides energy to drive chemical reactions that result in the recrystallization of existing minerals and/or the formation of new minerals.

Two Sources of Heat1. Rocks experience a rise in

temperature when they are intruded by magma rising from below or contact metamorphism.

2. Rocks that formed at Earth’s surface experience a gradual increase in temperature as they are transported to greater depths.

Pressure (Stress) as a Metamorphic Content

like temperature, also increases with depth as the thickness of the overlying rock increases.

the deeper you go in the ocean, the greater you confining pressure.

the for the rock that is buried.

Differential Stress – forces that generate mountains are unequal in different directions. - differential stress acts mainly as along the plane.

Chemically Active Fluids

composed mainly of water and other volatiles.

believed to play an important role in some types of metamorphism.

hot fluids aid in the recrystallization of mineral grains by dissolving material from regions of high stress then precipitating this material in areas of low stress.

Metamorphic TexturesRock Slate – forms when shale is subjected to temperatures and pressures only slightly greater than those associated with the compaction that lithifies sediment.

Common Metamorph

ic Rocks

Foliated Rocks – reorientation of the mineral grains into layered or banded appearance. Slate is a very fine-grained

foliated rock composed of minute flakes.

a most useful rock for roof and floor tile, chalkboards, and billiard tables.

can be almost of any color, depending on its mineral constituents.

black slates contain organic material.

Schists – strongly foliated rocks, formed by regional metamorphism.- Platy and can be readily split into thin flakes or slabs.

Nonfoliated RocksMarble – a coarse, crystalline rock whose parent rock is limestone.- Composed of large interlocking calcite crystals, which formed from crystallization of smaller grains in the parent rock.

Quartzite – a very hard metamorphic rock most often formed from quartz and sandstone.

Rock Cycle• allows us to view many of

the interrelationships among different parts of the earth.• Helps to understand the origin of igneous, sedimentary, and metamorphic rocks.• See that each type is linked to the others by the processes that act within the planet.

The Basic Cycle

Reported by:Mingel P. Rico

IV-12 BSE English