Physical Geology Chapter 6 Part 1 The Rock

Physical Geology Chapter 6

Part 1 The Rock Cycle

Rocks are solid pieces of the earth of any size. They can be made up of one or more minerals, or they can be made up of solid organic matter.

3 Types by Origin • Igneous – “from fire”; forms by cooling of magma or lava • Sedimentary – formed from compaction and cementation of rock fragments (sediments) • Metamorphic – “changed form”; from alteration of existing rock by heat, pressure and time

Rock Cycle

• Igneous may become sedimentary, metamorphic, or igneous • Sedimentary may become metamorphic, igneous, or sedimentary • Metamorphic may become igneous, sedimentary, or metamorphic

Rock Properties • Bowen’s Reaction Series – As magma cools, certain minerals tend to crystallize first. As they form out, they remove certain elements from the magma, which changes the composition of it. Thus different minerals form at different times during the cooling and solidification, and they generally always form in the same order.

The Canadian geologist N. L. Bowen found that minerals tend to form in specific sequences in igneous rocks, and these sequences could be assembled into a composite sequence.

In Summary

Chemical Stability • Tendency to maintain original chemical composition • Based on strength of chemical bonds • Highest # of bonds between Si and O are most resistant

Physical Stability • Reduced by zones of weakness – Layer joints – Joints between crystalline formations – Joints and fractures from release of formative pressures

Part 2 Igneous Rock

• Magma is melted rock below the surface; liquid or fluid • Melting depends upon – Temperature – Pressure – Presence of fluids • Melt temperature depends upon the chemical composition of the rock • Pressure heightens melting point; release lowers • Fluids decrease some melting points

Partial Melting • Different components have different melting points • As each melts, magma composition of the magma is altered • Opposite effect of Bowens Series

Fractional Crystallization • Opposite of partial melting • Minerals crystallize and change makeup of magma chemically • In same order as Bowens Series • Longer formation time = larger crystals • Shorter time = smaller crystals • Inner/outer crystals may be different due to differences in the magma as time goes by

Igneous Rock Textures • Coarse grained – slow loss of heat by subsurface magma results in large, welldeveloped crystals (rough granite)

Igneous Rock Textures • Fine grained – rapid heat loss from superficial lava results in small, poorly developed crystals (smooth basalt)

Igneous Rock Textures • Porphyritic – magma cools somewhat slowly, then speeds up near or on surface resulting in a mixed appearance/texture

Igneous Rock Textures • Glassy – viscous magma cools quickly with few dissolved gases forming few, if any crystals Obsidian

Igneous Rock Textures • Vesicular - viscous magma cools quickly with a lot of dissolved gases forming few, if any crystals, but many bubbles (vesicles) Pumice

Igneous Rock Composition • Felsic – lots of silica; light color; has minerals feldspar, mica and quartz; e. g. , granite, rhyolite, obsidian, pumice • Mafic – rich in Fe and Mg; dark color; minerals pl feldspar, pyroxene, ferromagnesians (hornblende); e. g. , basalt and gabbro • Intermediate – intermediate silica content; plagioclase, hornblende, pyroxene, and biotite; e. g. , diorite andesite

Igneous Rock Composition • Felsic – lots of feldspar (fel); lots of silica (sic) • Mafic – rich in Fe (f) and Mg (Ma) or ferromagnesians • Intermediate – intermediate blend

Intrusive formations batholith

Extrusive formations

Some uses of Igneous rock: GRANITE: An igneous-plutonic rock, medium to coarse-grained that is high in silica, potassium, sodium and quartz but low in calcium, iron and magnesium. It is widely used for architectural construction, ornamental stone and monuments. PUMICE: An igneous-volcanic rock, it is a porous, brittle variety of rhyolite and is light enough to float. It is formed when magma of granite composition erupts at the earth’s surface or intrudes the crust at shallow depths. It is used as an abrasive material in hand soaps, emery boards, etc. GABBRO: An igneous-plutonic rock, generally massive, but may exhibit a layered structure produced by successive layers of different mineral composition. It is widely used as crushed stone for concrete aggregate, road metal, railroad ballast, etc. Smaller quantities are cut and polished for dimension stone (called black granite). BASALT: An igneous volcanic rock, dark gray to black, it is the volcanic equivalent of plutonic gabbro and is rich in ferromagnesian minerals. Basalt can be used in aggregate.

Practice questions for igneous rocks

Part 3 Sedimentary Rock

Beginnings - sediments

Lithification in this case means compaction (squeezed together tightly reducing pore space) and cementation (glued together by chemicals in the water) of the sediments into rock

Classes by origin • Chemical • Organic • Clastic

Chemical • Dissolved minerals • Precipitate to layers as evaporites or chemical reaction products • Examples: halite, gypsum Bonneville Salt Flats, Utah

Organic • Remains of living things • Heavy deposits • Examples: coal, chalk and organic limestone

Clastic • Imported fragments that are lithified • By fragment size – Conglomerate (breccia) – large fragments cemented by small – Sandstone – quartz fragments (sand) – Shale – clay sized fragments usually compacted into flat layers

C l a s t I c

Characteristics of Clastic Sediments • Sorting – tendency of currents to separate sediments according to size • Angularity – collisions during movement of particles tends to round them off. Little movement = angular; more movement = more rounded/smooth; great movement = most rounded/smooth particles

very angular sub-angular rounded well-rounded Angularity Patterns

Sedimentary Rock Features • • Stratification Cross bedding Graded bedding Ripple marks Mud cracks Fossils Concretions

Stratification – in layers or beds

Cross beds – slanting layers

Graded beds – bed of sediments assorted by size

Ripple marks & Mud cracks

Fossils

Concretions and geodes

• Sedimentary rocks have great economic importance • Oil, natural gas, coal, and uranium, our major energy resources, are formed in and come from sedimentary rocks. • Sand gravel for construction come from sediment. • Sandstone and limestone are used for building stone. • Rock gypsum is used to make plaster. • Limestone is used to make cement. • Salt is used for flavoring. • Phosphate-bearing sedimentary rocks are used for fertilizer. • Quartz sand is used to make glass.

Practice questions on sedimentary rocks

Part 4 Metamorphic Rock

Metamorphism • The process by which heat, pressure, or chemical processes change one type of rock into another – Change into other minerals – Change in size or shape – Separate into bands – Change mineral/chemical composition by addition/subtraction of materials • Most deep within the crust – Contact with hot magma – Regional from heat/pressure of tectonic forces

Metamorphic Classification • Foliated – Extreme pressure causes crystals to realign or regrow in parallel bands – Minerals of different compositions separate to produce a series of bands • Non-foliated – No banding – Original rock has basically one mineral so no separation or banding – Original rock has round/square grains that cannot realign when subjected to further stress

A rock that contains parallel- or sub-paralleloriented mineral grains is said to be foliated Foliation promotes cleavage in rocks (they break easily along parallel planes).

• If mineral crystals are disc or pencil shaped, their parallel orientation is easily discernible and the rock is clearly foliated. If all mineral crystals are equant (length, width, and height are equal) there can be no discernible parallel orientation. • Nonfoliated rocks - mineral crystals are equant in shape. Made of calcite = marble (metamorphosed limestone) Made of quartz = quartzite (metamorphosed sandstone)

Uses for metamorphic rocks - • Slate is used to make roofing tiles and in earlier periods of our history was used as "blackboards" in classrooms. As a resource, slate is nonrenewable, but common. Man-made roofing tiles are a suitable substitute. • Marble is used as a building stone and as ornamental rock, such as for carving statues. As a resource, marble is nonrenewable, but common. Man-made building stones are a suitable substitute.

Practice questions on metamorphic rock