Geology 2
Confining Pressure
where the pressure is applied equally in all directions, Confining pressure causes the spaces between mineral grains to close, producing a more compact rock with a greater density, Confining pressure does not fold or deform rocks,
Agents of Metamorphism
Chemically active fluids: act as a catalyst Aids in the exchange of ions between growing crystals Metamorphism can add or remove chemical components that dissolve in water Sources of fluids: pore spaces of sedimentary rocks Fractures in igneous rocks Hydrated minerals like clays and micas
Fluids
Clay minerals can contained up to 60% water Water is part of the crystal structure in many minerals, such as mica and amphibole When subject to low to medium temperatures, water molecules can be removed from minerals Once expelled, the water moves along the individual mineral grains and is available to transport ions At higher metamorphic temperatures, the water and fluids are driven from the rock
Ductile deformation (flattening) of mineral grains can occur in one of two ways
1.Solid-state plastic flow by intracrystalline movement within each grain 2Dissolving of ions from areas of high stress and the moving and deposition of the ions in low stress areasBoth mechanisms change the shape of the mineral grains (but the volume and overall composition remains essentially unchanged)
Igneous Rocks
95% of the earth's crust are igneous 100% of the mantle is igneous! The Earth is a huge mass of igneous rocks covered by a thin veneer of sedimentary rocks, and with a small metallic core
How Magmas Evolved
A large number of igneous rocks exist, so a large number of magmas must also exist! Magmas will change over time (evolve) Can be seen in how succeeding lava flows from a single volcano can change over time Led to the discovery of Bowen's reaction series
Foliation
A set of flat or wavy parallel planes produced by directed stress deformation Presence of platy or elongated minerals (chiefly micas and chlorite) help create the foliation, Stress deformation causes mineral grains in preexisting rocks to develop parallel, or nearly parallel, alignment
Types of Physical weathering
Abrasion Frost wedging Exfoliation or unloading Thermal expansion Plant Roots
Cooling Basltic Magma
As the magma cools, olivine will crystallize first Olivine (Fe,Mg)2SiO4 So the remaining liquid part of the magma will have less Fe and Mg than it started with, and proportionally more Na, Ca, and K (and more SiO2) If the olivine crystals are removed from the magma, the remaining magma is now richer in silica, sodium, calcium and potassium At a slightly lower temperature, pyroxene [(Mg,Fe)2SiO6] and calcium-rich plagioclase [CaAl2Si2O8] will also start to crystallize The melt becomes more silica-rich Na and K-rich This process is called magmatic differentiation A major process behind variations in magma composition
Basaltic Magma
Basalt is the composition of a magma formed from partial melting of peridotite (mantle) About 5% melt Basalt 45-55% SiO2 Lots of iron (Fe) and magnesium (Mg) Less sodium (Na), calcium (Ca) and potassium (K)
Bowen's Series
Based on the observation that magmas will crystallize in a systematic way over a range of temperatures Certain minerals will crystallize first, others later The liquid (magma) left after some of the crystals form will have a different composition
Origins of Sedimentary Rocks
Begins with weathering Creates solid particles and ions in solution from preexisting igneous, metamorphic and sedimentary rocks Weathered material is transported during erosion by water, wind or ice or by gravity during mass wasting Material is then deposited when the wind or water slows, or the ice melts It settles (settle = sediment) Ions precipitate when the chemical conditions change
Ultramafic Rocks
Between 40-45% Silica Plutonic Rock name: Peridotite Phaneritic texture Olivine and pyroxene Mantle rocks - very rarely in the crust Sometimes occur as cumulates of ferromagnesian mineralsBetween 40-45% Silica Volcanic Rock name: Komatiite Spinifex texture Olivine crystals form so fast in this very hot magma that they grow in strings that look like spinifex grass (spiky Australian grass) Same mineralogy Very rare, vary old Most at least 2.7 billion years old
Mafic Rocks
Between 45 and 55% Silica Plutonic Rock name: Gabbro Phaneritic texture, porphyritic texture is very rare Mostly made of pyroxene and Ca-rich plagioclase Olivine and amphibole are less common Makes up a large part of the lower oceanic crustBetween 45 and 55% Silica Volcanic Rock name: Basalt Aphanitic texture (sometimes porphyritic, with larger olivine or plagioclase crystals) Same mineralogy Most common volcanic rock Makes up most of the surface of oceanic crust Volcanic islands are mostly basalt (Hawaii) Also large basaltic outpourings on the continent (usually associated with hotspots)
Intermediate Rocks
Between 55 and 70% Silica Plutonic Rock name: Diorite Felsic minerals are quartz (usually 5-15%) and feldspars (mostly plagioclase) Mafic minerals are at least 25% of the rock (mostly amphibole, some biotite) Not as common as granite, but also found in mountain rangesBetween 55 and 70% Silica Volcanic Rock name: Andesite Same mineralogy Often Porphyritic (light rectangular feldspars (plagioclase and sometimes orthoclase) in a light grey groundmass) Found mostly in continental volcanic ranges associated with subduction (name comes from the Andes Mountains of South America)
Conglomerate and breccia
Both are composed of particles greater than 2 millimeters in diameter. Conglomerate consists largely of rounded gravels. Breccia is composed mainly of large angular particles.
Origin of Magma
Both the crust and mantle are made of primarily solid, not molten rock So how do magmas form?Temperature increases in the upper crust on average between 20oC to 30oC per kilometer. geothermal gradient Deeper in the crust and mantle, the geothermal gradient steepens (less temperature increase per km) Rocks in the lower crust and upper mantle are near their melting points. Additional heat may induce melting.
Chemical Weathering
Breaks down rock components and internal structures of minerals The most important agent is water. Responsible for transport of ions and molecules involved in chemical processes
Organic Sedimentary Rocks
Coal Different from other rocks because it is composed of organic material. Stages in coal formation (in order): 1. Plant material 2. Peat 3. Lignite 4. Bituminous
Rare and unusual metamorphic minerals and deposits are nearly instantly formed
Coesite: Very high pressure mineral that cannot form on Earth in normal geologic circumstances Tektites: Natural glass spherules and blobs that form from the instantly melted rock splashed out
Maturity of Detrital rocks
Concept of maturity: how long the sediments that make up the rock have been weathered The most mature rocks are made of: clays (weathered feldspars) Quartz (most resistant to weathering) Immature rocks have many unstable minerals indicating limited weathering and rapid transport and deposition Mafics, feldspars
Chemical Sedimentary Rocks
Consist of precipitated material that was once in solution Precipitation of material occurs by: Inorganic processes Evaporation, change in chemical conditions Organic processes (biochemical origin) Calcite (aragonite) shells and coral made by clams, etc... Silica shells made by diatoms
Major Enviroments
Continental Dominated by stream erosion and deposition Glacial deposits Wind deposits (eolian) Marine Shallow (to 200 meters depth) Shore to edge of continental shelf) Deep (> 200 meters depth) Past continental shelf Transitional environments Shoreline deposits Sand beaches and mud tidal flats Lagoons and deltas Build out into the ocean (marine) environment
Magma to crystalline rocks
Cooling of magma results in the systematic arrangement of ions into orderly patterns. Silicate minerals result from crystallization in a predictable order. Texture is the size and arrangement of mineral grains.
Cross Bedding
Cross-bedding is internal bedding that is tilted at an angle to the primary bedding. Cross beds are formed by a scour and fill transport process involving either wind or water
Alterations Due to Chemical Weathering
Decomposition of unstable minerals Limestone dissolution Formation or retention of stable materials Feldspars → clays Physical changes such as the rounding of corners or edges Making quartz sands
Sedimentary Enviroments
Deposition of sedimentary rocks happens in specific environments Determines the nature of the sediments deposited (grain size, shape, etc...) By studying how the rocks change through time, we can figure out how the depositional environment has changed through time
Differential Stress
Differential stress is a pressure that is applied from a direction (rather than all directions), Rocks subject to differential stress are preferentially shortened in the direction that pressure is applied ,and lengthened in the direction perpendicular to that pressure
Effects Of Pressure
Directed pressure guides the shape and orientation of the new metamorphic minerals Metamorphic minerals can be compressed, elongated and/or rotated by being forced into preferred orientationsAt low pressures, rocks are brittle and tend to fracture when subjected to differential stress At high pressures, rocks are ductile and flow like plastic Under ductile conditions, mineral grains tend to flatten and elongate when subject to differential stress
Chemical Weathering Major Processes
Dissolution Aided by small amounts of acid in the water Oxidation Any reaction when electrons are lost from one element Hydrolysis The reaction of any substance with water. A hydrogen ion attacks and replaces other ions.
Geothermal Gradient
Does not cause melting in the mantle Temperature and pressure increase as you go deeper in the mantle Increase in pressure causes the melting temperature of peridotite (mantle rock) to go up peridotite: rock made of olivine and pyroxene ± others (garnet, magnetite, ilmenite)
Abrasion
Dust and sand particles are picked up by wind (or larger particles by water) and hit a rock or another particle, causing it to break
factors affecting weathering
Factors affecting weathering Surface area Rock characteristics Rocks containing calcite (marble and limestone) readily dissolve in weakly acidic solutions. Silicate minerals weather in the same order as their order of crystallization.
Rate of Cooling
Fast cooling: lots of tiny crystals (fine-grained =Aphanitic texture Slow cooling: fewer, but larger crystals= Phaneritic texture Initial slow cooling, followed by rapid cooling= Porphoritic texture Extremely rapid cooling: magma cools so quickly, it doesn't have time to crystallize= Glassy texture Obsidian Rock
Hydrolysis
Feldspar alters to clay Feldspars = stable at high temperatures and pressures Clays are stable under conditions at the Earth's surface
Index minerals in mafic rocks
Fine-grained mafic rocks will also change compositions during regional metamorphism Especially if they have previously been hydrated during hydrothermal metamorphism (basalt) Intermediate grade mafic rocks tend to have a lot of chlorite (which is green), so they are called Greenstones Fine-grained mafic rocks will also change compositions during regional metamorphism Especially if they have previously been hydrated during hydrothermal metamorphism High grade mafic rocks tend to have a lot of amphibole (hornblende) so they are called Amphibolites
Garnets
Garnets are only found in metamorphic rock and can be used to judge the grade of the metamorphism (index mineral) Most garnet is not of gem quality Most common use of garnet is as an abrasive, such as in garnet sandpaper
Foliated Rocks - Gneiss
Gneiss are a group of high-grade metamorphic rocks, which are characterized by having medium- to coarse-grained minerals that are banded or laminated in appearance Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processesGneiss can have any kind of parent rock (protolith) White bands are usually made of felsic minerals (quartz, potassium feldspar, plagioclase) Black band are usually made of mafic minerals (biotite and amphibole - rarely pyroxene)
More types of sedimentary structures
Graded beds Ripple marks Mud cracks Fossils
Agents of Metamorphism
Heat: recrystallization results in new, stable minerals Sources of heat: nearby magma or increasing heat with depth (geothermal gradient) Pressure: increases with depth Two types of pressure: Confining pressure: forces are applied equally in all directions Differential pressure (stress): stresses are applied unequally in different directions
Metamorphic Environments: Hydrothermal Metamorphism
Hydrothermal fluids can carry dissolved calcium dioxide, sodium, silica, copper and zinc Ascending hydrothermal fluids can react with overlying rock, creating new minerals (which may have great economic value)The most widespread occurrence of hydrothermal metamorphism is along the mid-oceanic ridges As seawater percolates through the newly created crust, it is heated and chemically reacts with the mafic (Fe and Mg rich) basaltThe ferromagnesian igneous minerals, such as olivine and pyroxene, are changed into metamorphic minerals such as serpentine, chlorite and talc Ca-rich plagioclase feldspars become more Na-rich as the sea salt (NaCl) exchanges calcium for sodium
Dissolved gasses
If a magma has lots of dissolved gasses within it (volatiles), when the pressure is released the volatiles become gasses and escape Volcanic rocks have textures related to the volatile and silica content Pumice: high volatile and silica-rich Vesicular: high volatile (not high silica) Pyroclastic: debris from explosive eruption Pegmatitic Texture:If a magma has lots of dissolved gasses within it (volatiles), and pressure is not released, crystals can grow very large Ions can migrate very rapidly, so crystals can grow large
Magma: Role of Pressure
If increasing pressure will increase the melting temperature, decreasing pressure will decrease the melting temperature Decompression melting This is what happens at spreading ridges
How does Bowen's reaction series relate to the igneous rocks we see?
In general a magma of a certain composition will result in a rock of a certain composition Two end-members of magmas: Felsic: Lots of Silica (SiO2), Aluminum, Calcium (Ca), Sodium (Na) and Potassium (K) Less Magnesium (Mg) and Iron (Fe) Mafic: More Magnesium and Iron Less Silica, Calcium, Sodium and Potassium Same two main end-members in rocks!
Magma: Role of Volatiles
Increasing the volatile content (mostly water) will decrease the melting temperatureIncreasing the volatile content (mostly water) will decrease the melting temperature This happens at subduction zones where wet oceanic crust is subducted beneath either more oceanic crust, or continental crust
Fossils
Index fossils let us know when the sediments were deposited
Magmatic Differentiation
Just like we see in minerals, as melts become more silica-rich they become less dense Specific gravity of pyroxene: 3.1-3.9 Specific gravity of amphibole: 2.8-3.7 Specific gravity of feldspar: 2.77-2.55 Specific gravity of quartz: 2.65
Metamorphic facies and Index minerals
Just like with sedimentary rocks, we can determine the environment of metamorphism by looking at the rock minerals and textures Index minerals are specific minerals that appear and disappear with different pressures and temperatures They are most common in metamorphic rocks made from shales and siltstones
Black Smokers
Large amounts of metals, such as iron, cobalt, nickel, silver, gold and copper, are dissolved from the newly formed crust These hot (~350oC), metal-rich fluids rise along fractures, generating particle-filled clouds called Life is very sparse at these depths, but black smokers are the center of entire ecosystems Sunlight is nonexistent, so many organisms must convert the heat, methane, and sulfur compounds provided by black smokers into energy through a process called chemosynthesis
Common Chemical Sedimentary rocks
Limestone Most abundant chemical rock Composed chiefly of the mineral calcite Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms). Inorganic limestones include travertine and oolitic limestone.Dolostone Typically formed secondarily from limestone. Combination of calcite and dolomite minerals Chert Microcrystalline quartz Varieties include flint, jasper and agate
Detrital Rocks
Made up of fragments (clasts) of pre-existing rocks Clay minerals Quartz Feldspars Micas (biotite and muscovite) Particle size, shape and composition is used to distinguish among the various rock types. Common type :Shale- Mud-sized particles in thin layers that are called lamina Most common sedimentary rock Mostly made of clays Sandstone- Sand-sized particles Forms in a variety of environments Predominant mineral is quartz If the predominant mineral is feldspar it is called arkose
Magma and its 3 distinct parts:
Magma: completely or partially molten rock At the surface magma is called lava Three distinct parts Melt: liquid made up of mobile ions Mostly the 8 most common elements (O, Si, Al, Fe, Mg, Ca, Na, K) Solid: Silicate minerals that have already crystallized Suspended in the melt Gas: Volatiles that vaporize at surface pressures Dissolved in the liquid portion at depth: water vapor (H2O), Carbon dioxide (CO2), and sulfur dioxide (SO2)are most common
Percentage fo silica present
Magmas with more silica are more viscous Even at high (above melting) temperatures silica tetrahedra form and partial polymerization occurs Causes the magma to be thicker Has two effects: High-silica lava flows are thicker and don't travel as far than flows from low-silica lavas Volcanic eruptions tend to be more violent because of volatile content (high-silica magmas tend to also be high-volatile magmas)
Variations in Magma Composition
Magmatic differentiation Assimilation Magma mixing
Metamorphic Grade
Metamorphic rocks are classified by how much metamorphic changes they have undergone High-grade: Formed in deeper crustal regions, perhaps as deep as the upper mantle, under high temperature and/or high pressure Low-grade: Formed in shallower crustal regions under low temperature and/or low pressure
Metamorphic Rocks
Metamorphic rocks are produced from Igneous rocks Sedimentary rocks Other metamorphic rocks
Large-crystal Textures
Metamorphic rocks can exhibit a great variation in crystal size During the recrystallization process, certain metamorphic minerals, including garnet, staurolite and andalusite, tend to develop a few very large crystals
Metamorphic Textures
Metamorphism creates new textures on the rock it alters In general the grain size of crystals increase as the grade of metamorphism increases Four main criteria: The size the crystals How the mineral grain shape is changed The degree to which minerals are segregated into light and dark bands Metamorphic gradeThere are three major types of metamorphic rock textures: Granoblastic Large-crystal Foliation When we look at foliation, we will also discuss: Schistosity Gneissic texture
Metamorphism Deffinition
Metamorphism is the transition of one rock into another by temperatures and/or pressures unlike those in which in formed
Foliated Rocks - Migmatite
Migmatite is a very high-grade of metamorphic rock Migmatite is a rock at the frontier between metamorphic and igneous rocks Temperatures are just high enough to start melting the rock As a consequence, migmatite is typically very badly deformed and contorted with veins, pods and lenses of melted rock
Granitic Rocks
Most common felsic rock-type Has 70-75% Silica Plutonic Rock name: Granite Phaneritic, or porphyritic (big crystals and small crystals, but all visible to the naked eye) Over 25% quartz, 65% feldspar (plagioclase and orthoclase) Very abundant—often associated with mountain buildingMost common felsic rock-type Has 70-75% Silica Volcanic Rock name: Rhyolite aphanitic May contain glass fragments and vesicles (but similar minerals) More rareMost common felsic rock-type Has 70-75% Silica Other granitic rock-types Obsidian Volcanic Dark colored Glassy texture Pumice Volcanic Frothy appearance with numerous voids
Metamorphic Environments: Regional Metamorphism
Most common type of metamorphic environment, associated with mountain buildingDuring these dynamic events, large segments of the Earth's crust are intensely deformed along convergent plate boundaries The mountain building applies differential stress literally over a wide regional area
Foliated Rocks - Schist
Most schists have been derived from clay and mud sedimentary rocks which have passed through a series of metamorphic processes involving the production of shales, slates and phyllites as intermediate steps
Mud Cracks
Mud cracks form when mud-covered shorelines or lake bottoms, dry up. This produces an irregularly- cracked surface.
Fault Zone Metamorphism
Near the surface, rock behaves like a brittle solid So near the surface, movement along a fault zone fractures and pulverizes the rock, creating what is called fault brecciaIn contrast, at depth under higher heat and pressure, rock is ductile and flows like plastic At depth along a fault zone, the mineral structures are deformed by the ductile flow, giving the metamorphic rock a foliated or lineated appearanceThese rocks are undergoing shear stress (sliding past each other) They will become elongated/deform parallel to the stress
Stresses?
No differential stress Minerals grow coarser, and crystals interlock Very near heat source, minerals with water will be come dehydrated Clays to micas and amphibole Amphibole to pyroxene These rocks are known as hornfels
Granoblastic Rocks
Not all metamorphic rocks have foliated texture Many metamorphic rocks have a massive or coarse granular appearance and exhibit no deformation They are composed mainly of crystals that grow in equidimensional shapes Therefore they are catalogued by mineral composition, and not texture These are called granoblastic rocks
Impact Metamorphism
Occurs when an asteroid or comet impacts the Earth's surface These objects can be moving as fast as 100,000 miles per hour (~28 miles per second)In a fraction of a second, the energy of the rapidly moving object is transferred into heat energy and shock waves as it smashes into the EarthThe impacting asteroid or comet is vaporized The impacted rock is shattered, pulverized and sometimes even melted Minerals in the rock are instantly subjected to both high temperature and high pressureRare and unusual metamorphic minerals such as coesite, which are normally never found on the Earth's surface, are nearly instantly formed Staggering quantities of matter are blown into the atmosphere
Magma Mixing
Occurs when one magma body intrudes into another with a different composition The two magmas mix and the result has an intermediate composition
Metamorphic Environments: Burial Metamorphism
Occurs when thick accumulations of sedimentary strata on the ocean floor are subducted beneath another plate or burried deeply by other sedimentsLow grade metamorphism that typically begins when sediments reach a depth of 6-10 kilometers (3-6 miles) or when the temperature reaches about 200oC
Igneous Compositions Cont....
Other compositional groups Intermediate (or andesitic) composition Contain 25% or more dark silicate minerals Often associated with explosive volcanic activity Ultramafic composition Rare composition in the crust that is high in magnesium and iron Composed entirely of ferromagnesian silicates The mantle is UltramaficSilica content as an indicator of composition Crustal rocks exhibit a considerable range—45% to 70% Silica content influences magma behavior. Felsic magmas have high silica content and are viscous. Mafic magmas have much lower silica content and more fluid-like behavior.
Sediments → Sedimentary Rocks
Over time, the sediments are piled on top of each other Burial of older sediments occurs Sediments are compacted and cemented together
Oxidation
Oxygen combines with iron-bearing silicate minerals causing "rusting" Biotite, Amphibole, Pyroxene Iron oxides are red, orange, or brown in color
Phyllite
Phyllites also form during low-grade metamorphism of mud- and clay-rich sedimentary rocks Forms under slightly more heat and pressure conditions than slatevery fine grained rocks, but you can see the grains with the naked eye (barely) They usually show cleavage Micas are common minerals, but they are very small
Classification according to sorting
Poorly sorted: Particles of various sizes short transport and/or rapid deposition Turbulent streams or some glacier deposits Well sorted: all the particles are about the same size Longer transport and/or slow deposition Wind transport, wave deposits
Porphyroblasts
Porphyroblasts are metamorphic minerals that grow in a matrix of fine-grained minerals The garnets grew much faster than the matrix in this schist
Metamorphism
Progresses incrementally from low to high grade Rocks remain essentially solid during metamorphism Main types of metamorphism: Contact Regional Other types: Hydrothermal Dynamic Impact
Sedimentary Structures
Provide information useful in the interpretation of Earth's history Types of sedimentary structures Strata, or beds (most characteristic of sedimentary rocks) Bedding planes that separate strata Cross-bedding
Igneous Compositions
Remember that igneous rocks are composed primarily of silicate minerals. Dark (or ferromagnesian) silicates Olivine, pyroxene, amphibole, and biotite Light (or nonferromagnesian) silicates Quartz, muscovite mica, and feldspars Light rocks are felsic Felsic comes from the main minerals that form these types of rocks: feldspar and silica (quartz) Major constituent of continental crust Dark rocks are mafic Mafic comes from the main elements that define these types of rocks: Magnesium and Iron (Fe) Major constituent of oceanic crust and volcanic islands
Thermal Expansion
Repeated daily heating and cooling of rock; Heat causes expansion; cooling causes contraction. Different minerals expand and contract at different rates causing stresses along mineral boundaries.
Marble
Result of metamorphism of limestone or dolostone Metamorphic process causes a complete recrystallization of the original rock into an interlocking mosaic of calcite, aragonite and/or dolomite crystalsWhite in its pure form, marble is available in a beautiful variety of colors, which are caused by mineral impurities such as clay, silt, sand, iron oxides, or chert
Rippled Marks
Ripple marks form when moving wind or water causes sedimentary grains to "hop" along the bottom,Ripple marks can be either symmetrical (formed by waves sloshing back and forth), or asymmetrical (formed by water or wind flowing in one direction)
Metamorphic Environments: Contact (thermal) metamorphism
Rise in temperature caused by magma invading a host rock A zone of alteration forms in the rocks surrounding the magma: contact aureole
Exfoliation or unloading
Rock breaks off into leaves or sheets along joints which parallel the ground surface; Caused by expansion of rock due to uplift and erosion; removal of pressure of deep burial
Types of Sedimentary Rocks
Rock types are based on the source of the material. Detrital rocks—transported sediment as solid particles Chemical rocks—sediment that was once in solution Organic rocks—sediment that was produced organically (Coal primarily) Sedimentary rock comprise about 5% of the Earth's outer 12 km
Differential Weathering
Rocks do not weather uniformly due to regional and local factors. Results in many unusual and spectacular rock formations and landforms
characteristics of magma
Rocks formed from lava are extrusive, or volcanic rocks Rocks formed from magma at depth are intrusive or plutonic rocks
Plant Roots
Roots grow in cracks, forcing rocks to split
Classification according to angularity
Rounded grains: long transport time by air or water Very rounded sandstones are usually mostly quartz Angular grains: short transport time Variable mineralogy (can have feldspars, micas)
Schist
Schists are a group of medium grade metamorphic rocks, which are characterized by having medium- to coarse-grained minerals that are platy or flakey in appearanceThese platy mineral grains include micas, chlorite, talc, hornblende, graphite, and others They don't have clayThe small, platy (flakey) grains of mica in this schist can easily be seen with the unaided eyeSchist is also used as a "catch-all" term to describe the texture of a metamorphic rock To indicate composition, mineral names are For example, this is a "mica garnet schist" (Note that the garnets are of gem quality and don't have a preferential direction of growth!)
Dissolution
Several common minerals dissolve in water halite calcite Limestone and marble contain calcite and are soluble in acidic water Marble tombstones and carvings are particularly susceptible to chemical weathering by dissolution Caves and caverns typically form in limestone form a rock called travertine stalactites - from ceiling stalagmites - on ground
Graded Beds
Show a gradual change in particle size as you move from the bottom of a bed to the top.
Foliated Rocks
Slate Phyllite Schist Gneiss Migmatite
Slate
Slates are of the lowest metamorphic grade They are so fine-grained that you need a microscope to see individual minerals Parent (protolith) rock is shaleSlate can be spilt along cleavage into thin sheets, which gives it economic valueChalk blackboards were made of thin sheets of slateSome of the clays have started to turn into micas (muscovite and biotite) and chlorite (Chlorite is a green iron- and magnesium-rich platy mineral common in low-grade metamorphic rocks) All of the minerals are too small to see, and slate has a dull appearance
Climate
Temperature and moisture characteristics Chemical weathering is most effective in areas of warm, moist climates.
Igneous textures
Texture is the overall appearance of a rock based on the size, shape and arrangement of interlocking minerals Factors affecting crystal size: Rate of cooling Percentage of silica present Dissolved gasses
Diagenesis
The chemical, physical biological changes that take place after sediments are deposited Occurs within the upper few km of the Earth's crust Lithification is one process of diagenesis Natural cements that hold mineral particles together include calcite, silica and iron oxide Recrystallization occurs as well, as more stable minerals form from more stable ones
Parent Rocks
The initial composition of the parents rocks is a fourth major factor: the protolith Most metamorphic rocks have the same overall composition as the parent rock from which they formed except for the possible loss or accumulation of volatiles such as water and carbon dioxide
Physical weathering
The mechanical breakdown of larger rocks or particles into smaller pieces
Naming Igneous Rocks
The name of an igneous rock depends on both the texture and the composition Aphanitic (volcanic) and Phaneritic (plutonic) rocks with the same composition will have different names
Regional Metamorphic textures
Types of metamorphic rocks are defined by their texture and by what minerals are present. Foliation is the main texture Low grade metamorphism in usually only expressed in sedimentary rocks like shale and siltstone High grade metamorphism happens in all rock types
Quartzite
Very hard metamorphic rock formed from quartz sandstone Pure quartzite is white, but reddish/pinkish and grayish colors caused by impurities are common
Frost Wedging
Water infiltrates into cracks, then expands when it freezes, expanding the cracks over time
Chemical Weathering
Water, gases and solutions (may be acidic) react with rocks, adding or removing elements from minerals
Assimilation
When a magma body intrudes into solid rock it can melt and incorporate some of the surrounding wall rock
Lithification
process of compaction and cementation that creates sedimentary rocks
weathering
the physical breakdown and chemical decomposition of rock
erosion
the physical removal of material by water, wind, ice, or gravity
mass wasting
the transfer of rock and soil downslope under the influence of gravity
