Oveview of Rocks

Igneous rocks

· form by cooling & crystallization of magma or lava

· cool on/near surface --> extrusive rock: volcanic (lava)

· cool well below surface --> intrusive rock: plutonic (magma)

· All igneous rocks have an interlocking crystalline texture.

Sedimentary Rocks

· are made of sediments transported as solid grains (clastic sedimentary rocks) or chemically precipitated out of water (chemical sedimentary rocks)

· may contain fossils

· are recognized by their layering

Metamorphic rocks

· formed by subjecting a pre-existing rock to very high pressures and/or temperatures

· form below surface

· develop interlocking crystalline texture

· may be foliated or non foliated

Rock Cycle Diagram

Igneous rocks

Igneous rock textures

Fine-grained texture: fast cooling (at or near surface)
Coarse-grained texture: slow cooling (deeper in crust)
Porphyritic texture: coarse crystals (phenocrysts) in fine-grained rock (groundmass)

Igneous Rock Textures

Porphyritic texture --> initial slow cooling (makes phenocrysts), then rapid cooling
Glassy texture -- cooling so rapid crystals can’t form. Get glass instead. Obsidian is volcanic glass.
Vesicular texture -- vesicles are rounded openings formed from escape of gasses during cooling. Pumice has this texture.

Igneous Rock Classification

	ROCK NAME
Mineral Composition	Coarse Texture	Fine Texture
lots of quartz, feldspar; fewer mafics	granite	rhyolite
no quartz, K-feldspar; lots of plagioclase, moderate mafics	diorite	andesite
Ca-rich plagioclase & mafics	gabbro	basalt
almost entirely mafics	ultramafic	---------

Grain Size and Texture

· Vitreous - glassy, no crystals

· Aphanitic - fine grained < 2mm crystals

· Phaneritic - coase grained > 2mm crystals

· Pegmatitic - very coase grained > 10mm crystals

· Vesicular - voids created by gases

· Amigdaloidal - filled voids created by gases

· Porphyritic - some large crystals (phenocrysts)in fine matrix

How do you make magma?

Magmas form in crust and mantle
Need high temperature to melt rock & form magma.
Rock composition, pressure and water content influence melting point of rocks

Influences on rock melting point

Composition -- mixtures of minerals melt at lower temperaturespressure -- high pressure environments require higher temperatures to melt rocks
water content -- presence of water in rock allows for melting at lower temperature

Magma composition

Magma composition determines igneous rock composition.
Igneous rock classification shows that magmas vary in silica content
basalts & gabbros -- low silica (SiO2) magmas -- 40-50% silica
granites & rhyolites -- high silica magmas (70% or more)

Magmatic Differentiation

Experiments by N.L. Bowen and others showed that different minerals crystallize at different temperatures
This can lead to different magma compositions.

Bowen’s Reaction Series

Fractional Crystallization

If early-formed crystals are removed from the magma, the composition of the remaining magma will be different from original composition.

Other processes that alter magma composition

partial melting of rocks -- when rocks begin to melt, the last-formed crystals (e.g. quartz, K-fsp, musc) are the first ones to melt. Resulting magma is high in silica
assimilation -- hot magma may partially melt surrounding rocks -- adds silica to magma

Types of igneous intrusions

Classified according to size & geometry
Dikes -- small intrusions that cut across surrounding rocks
Sills -- small intrusions that are parallel to surrounding rocks
Stocks -- medium sized intrusions
Batholiths -- huge bunch of stocks

Metamorphic Rocks

Metamorphic rocks are made from pre-existing rocks subjected to high temperatures and/or pressures. High T, P cause significant chemical and physical changes
usually occurs at 10-30 km below surface at temperatures 300-800oC.

Look at the Rock cycle again

Factors that influence met.

Temperature -- causes recrystallization; speeds up reactions
Pressure -- two types:
confining pressure - due to overburden
directed pressure - pressure in a particular direction; can produce foliation

Other factors that influence met.

Introduction of new chemicals --change bulk composition
Fluids -- hydrothermal fluids -- hot water solutions derived from magma, pore water, or water in minerals. Can cause metasomatism (change in bulk composition) and speed up reactions.

Types of metamorphism

Regional metamorphism -- affects large area; commonly due to mountain building; foliated rocks common

Contact metamorphism -- rocks near igneous intrusion get baked; produces non-foliated met. rocks (hornfels)

Burial metamorphism -- very low grade; results from deep burial

Types of metamorphism

Hydrothermal metamorphism -- due to hot, watery solutions; common along mid-ocean ridges.
Cataclastic metamorphism -- due to high pressure along a fault; produces distinctive rock called mylonite

Classification of metamorphic rocks

Foliated vs. non-foliated

Foliated metamorphic rocks:
slate, phyllite, schist, gneiss
---increasing grade-------->

Non-foliated rocks:
quartzite (met. sandstone)
marble (met. limestone)
hornfels (contact metamorphosed rock)

Metamorphic mineral assemblages

Types of minerals in met. rock depends on original rock composition and metamorphic grade.
Can see changes in mineral composition in contact metamorphosed zone near intrusion.
Can also see these changes on regional scale.

Sedimentary Rocks

Two types of sedimentary rocks

· clastic -- form from deposition of solid grains; classified based on grainsize: conglomerate, sandstone, shale

· chemical -- form from minerals precipitating out of water; classified based on mineral content: limestone, dolomite

The Rock Cycle

Sedimentary Environments

· places where sediments accumulate

· physical, chemical, and biological processes operating in the environment produce diagnostic features in sediments

· allows geologists to determine what types of environments existed in the past

Important sediment characteristics

· Texture -- grainsize; sorting (grainsize variability)

PARTICLE DIAMETER (mm)	NAME	CLASTIC ROCK TYPE
> 256	Boulder	VERY COARSE CONGLOMERATE
64-256	Cobble	COARSE CONGLOMERATE
2-64	Gravel	CONGLOMERATE
2-0.6	Coarse Sand	COARSE SANDSTONE
0.6-0.2	Medium Sand	MEDIUM SANDSTONE
0.2-0.06	Fine Sand	FINE SANDSTONE
0.06-0.002	Silt	SILTSTONE
< 0.002	Clay	MUDSTONE

SEDIMENT PARTICLE DESCRIPTION ROCK NAME

Gravel Rounded rock fragments. CONGLOMERATE

Gravel Angular rock fragments. BRECCIA

Sand Quartz predominant, visible grains, often thickly bedded, depositional structures such as cross-bedding common. SANDSTONE

Sand Sandstone with more than 25% feldspar grains. ARKOSE

Silt Quartz predominant, grains barely visible, gritty feel. SILTSTONE

Clay Thick beds >1cm blocky, fine mud, no particles discernable, may show polygonal cracks, composed predominantly of clay minerals and very fine quartz. MUDSTONE

Clay Laminated mudstone, fissile, splits into thin sheets. SHALE

Ÿ Sedimentary Structures -- e.g. cross bedding, mudcracks

· Bedding geometry -- shapes of beds

· Associations -- how characteristics change vertically and laterally

· Fossils -- info. about ancient life

Examples

Continental sedimentary environments

· glacial -- deposits may have wide range of grain sizes (poorly sorted); better sorted and layered if deposited by meltwater

· fluvial (river) -- migration of ripples and dunes on river bed produce cross-stratification in deposits; well sorted

Examples

Continental Sedimentary Environments

· Lacustrine (lake) -- deposition of mud thin layers (laminae) on lake bed; in arid regions may form evaporites

· Aeolian -- large wind-blown sand dunes migrate and produce thick cross-stratification in deposits

Examples

Marine (Nearshore) Sedimentary Environments

· Deltas -- where rivers meet the sea -- clastic sediments are deposited

· Beaches -- deposits of sand at the coast; waves transport sediment to produce well-sorted deposits

Examples

Marine Sedimentary Environments

· Shelf -- may be sand and mud or carbonate sediments

· Reefs -- build-up of limestone from coral skeletons

· Deep marine -- finely layered mud

Clastic Sedimentary Rocks -- classified by grainsize

· Conglomerate -- more than 30% of grains are gravel-sized (> 2mm)

· Sandstone -- grains bts. 1/16mm-2mm

· Shale -- made of mud-sized grains (<1/16 mm)

Chemical Sedimentary Rocks
classified by composition

· limestone -- made mostly of calcite

· dolomite (dolostone) -- made mostly of dolomite

· evaporites -- various mineral salts that precipitate from evaporating water

Fossils

· Sedimentary rocks may contain fossils

· Body fossils are the lithified remains of the hard parts of organisms.

· Trace fossils are disruptions of sediment by animals moving through, or may be imprints of animal’s movement (e.g. dinosaur tracks)

Diagenesis

· refers to processes that lithify sediments

· occurs after sediment is deposited

· may occur at or very near surface, but more commonly occurs after sediments are buried

Diagenetic Processes

· compaction -- due to pressure; fine-grained sediments undergo more compaction than coarse sediments

· cementation -- precipitation of minerals around sediments (commonly quartz or calcite are precipitated)

Diagenetic Processes

· recrystallization -- due to pressure, temperature changes; replacement of unstable minerals

Formation of dolomite

· Dolomite does not form at earth’s surface. Dolomite rocks form from diagenesis of limestone.

Carbonate Sedimentary Environments

· Most carbonate sediments are formed by organisms.

· Carbonate sediments accumulate primarily in warm ocean waters that are clear (free of clastic sediment input). This is where carbonate-producing organisms fluorish.

Evaporites

· rocks formed from minerals that precipitate out of evaporating water

· Example: evaporate seawater in coastal area. Will see calcite, gypsum, anhydrite, halite crystallize out of water evaporates.