KNS 1102 Engineering Geology
Unit 4 : Rock Cycle & Rock Types
Semester 2 Session 2012/2013
What is Rock Cycle?
Show interrelationships between 3 types of rock; igneous, sedimentary & metamorphic rock, through internal and external processes. involve superficial processes such as weathering, transportation, and deposition; and internal processes such as magma generation and metamorphism. Plate movement is the mechanism responsible for recycling rock materials and therefore drives the rock cycle. Geologists recognize 3 major groups of rocks: 1. Igneous, sedimentary, and metamorphic. 2. Each group contains a variety of individual rock types that differ from one another on the basis of composition or texture (the size, shape, and arrangement of mineral grains).
Further Explanation! • http://www.classzone.com/books/earth_scienc e/terc/content/investigations/es0602/es0602p age02.cfm
Rocks
• ROCK IS AN AGGREGATE OF ONE OR MANY ROCK FORMING MINERALS • THREE MAIN TYPES 1. IGNEOUS ROCKS 2. SEDEMENTARY ROCKS 3. METAMORPHIC ROCKS • ROCKS AND SOILS ARE DESTROYED AND FORMED IN A CYCLIC PROCESS (ROCK CYCLE)
The Three Basic Types of Rocks
Type of rock and source material
IGNEOUS Melting of rocks
Rock-forming process
Crystallization
Example
Coarsely crystallized granite SEDIMENTARY Weathering and erosion of Exposed rocks Deposition, burial, and lithification Cross-bedded sandstone METAMORPHIC Rocks under high temperatures and pressures
Recrystallization neocrystallization
Gneiss
My name is Rock. My family members are… Igneous – forged by fire
Sedimentary – layered storytellers
Metamorphic – changed rocks
What is Magma Made of ?
• All magmas contain Si and O
– Upon cooling, bond together into silicon-oxygen tetrahedrons
• More silica (i.e. felsic), more viscous (harder to flow, thicker)
• Also contain varying amounts of other elements like Na, K, Al, Ca, Mg, Fe, etc… • Dry magmas – no volatiles • Wet Magmas – up to 15% volatiles • Volatile content strongly effects the viscosity (ability to flow)
– More volatiles, less viscous (easier to flow or more fluid)
Magma Movement
• If magma did not move, no extrusive/volcanic rocks would ever have formed • Magma rises because:
– hotter and less dense than the surrounding rock and therefore buoyantly rises. – the weight of the overlying rock (lithostatic pressure) literally squeezes the magma out.
• Analogy: Think of stepping on a tube of toothpaste to force it out, or mud squishing through your toes when you step in a puddle
• Viscosity affects a magma or lava’s ability to flow
– Controlled by:
• Temperature (high temp - low viscosity) • Volatile content (more volatiles – less viscous) • Silica content – silica tends to form silica-oxygen tetrahedrons that bond with each other to make long chains that ultimately resist flow (more silica – more viscous)
• Volatiles: A substance that can easily change into a gas at relatively low temperatures (H2O, CO2, etc…). • The addition of volatiles at depth (mainly H2O) seeps into rocks and helps break bonds (aids in melting).
What is Bowen’s Reaction Series
The order of crystal formation from magma as cooling occurs (how igneous rocks formed)
My question?...
• Why do igneous rocks (volcanic & Plutonic) exhibit different types of composition? • How does a single magma source produce chemically different types of igneous rocks?
Bowen’s Reaction Series
• Bowen’s had confirmed that as magma cools, minerals will crystallize out in a predictable sequence from laboratory experiments. • The sequence called ‘Bowen’s Reaction Series’ • Made up of 2-trends;
a. Discontinuous reaction series b. Continuous reaction series
Bowen’s Reaction Series
• • • In order to understand the melting and solidifying of magma we need to understand Bowen’s reaction series. – Bowen figured this out by melting rocks in an oven, letting them cool, and watching what minerals crystallized This series outlines the order in which minerals form in a cooling melt Also applies in reverse order to rocks that are partially melted
• •
Discontinuous series (different minerals form) and Continuous series (Plagioclase only) So, a melt gets less mafic as it cools; In heating, the first minerals to melt are felsic.
Explanation!
• Bowen reaction series can describes why different rocks are formed which depend on the composition of magma itself, where cooling process is happen and also the rate of cooling. • As magma cools, minerals crystals grow larger and some of them settle. • The crystals that remain suspended in the liquid react with the remaining melt to form a new mineral at a lower temperature. • The process continues until the entire body of melt is solidified. • It can be classified into 2 groups: 1. Discontinuous Ferromagnesium reaction series which; Minerals formed are having different chemical composition and crystalline structure
2. Continuous Plagioclase feldspar reaction series which; Minerals formed are having different chemical compositions but similar crystalline structures
Magma
As the super-hot magma cools, it solidifies to form an igneous rock.
How does this happen??
Kilauea Volcano in Hawaii is currently erupting, sending molten rock across the landscape and into the ocean. Every day, new rocks are forming as the lava cools and solidifies.
Some magma that never makes it to the surface solidifies relatively slowly because it takes it a long time to cool inside the hot Earth.
INTRUSIVE IGNEOUS ROCKS -coarse grained
OR
Most commonly magma spills are from a volcanic eruption. As the magma moves quickly from the super-hot conditions of Earth's interior to the much cooler environment at the surface, it cools and solidifies rapidly.
EXTRUSIVE IGNEOUS ROCKS -fine grained
Igneous Rock Types
• In general, there are two basic types of igneous rocks
– Extrusive/Volcanic: Igneous rocks that form due to the freezing of melts above the surface of the Earth
• Includes rocks made of volcanic ash (pyroclastics)
– Intrusive/Plutonic: Form by freezing of melts below the surface of the Earth.
Igneous Rocks
• Also can be classified according to their mineral content. FELSIC or acidic MAFIC or basic
Light coloured Quartz
This remind me of Bowen’s reaction series
Dark coloured Olivine,pyroxene amphibole & biotite
Extrusive Igneous Rock Environments
• Explosive eruptions generally occur when source magma is:
– – – High in silica (felsic-intermediate) Low temp High in volatiles
•
Effusive eruptions generally occur when source magma is:
– – – Low in silica (mafic) High temp Low in volatiles Fluid lava flows Fire fountains (if volatiles), lava tubes
•
These volcanoes form
– – Lava domes Ash clouds and ash flows
•
These volcanoes form
– –
Hawaii
Cascades NW USA
Subduction and Volcanism
Subduction creates volcanism 1- The down-going slab has lots of volatiles (e.g. H2O). At depth, these volatiles are heated and are squeezed from the rock and migrate into the asthenosphere above the plate. 2- The addition of volatiles, as we now know, changes the melting point of rocks and causes the asthenosphere to melt above the sinking plate. 3- The sinking plate may partially melt too, but most melting occurs in the asthenosphere above the slab.
Intrusive Igneous Rock Environments
• Magma rises by percolating between grains and/or by forcing open cracks in the subsurface • The magma that doesn’t reach the surface of the Earth cools into intrusive igneous rocks
– Country rock or wall rock: The pre-existing rock that magma intrudes into – Intrusive contact: The boundary between the igneous intrusion and the wall rock
• Tabular intrusions: Dike, Sill, Laccolith (pseudo-tabular, or sheet-like) • Non-tabular intrusions: Pluton, Batholith, Stock
Mt. Rushmore is carved out of a granitic igneous intrusion
Dikes and Sills
• Dikes: igneous intrusions that cut across layering, i.e. discordant • Sills: igneous intrusions that follow layering, i.e. concordant
Dikes in the Sierra Nevada Batholith
•
Near Ruby Lake, CA @ ~12,000 ft
Laccoliths
• Laccolith: a dome-like sill that bends the layers above it into a dome shape
Non-Tabular Intrusions: Plutons
• Pluton: Irregular blob-shaped discordant intrusions that range in size from 10’s of m, to 100’s of km
• Batholith: A pluton that is 100 km2 in surface exposure • Stock: A pluton that is <100 km2 in surface exposure
The Sierra Nevada Batholith
Effects of Intrusions
• Dikes form in regions of crustal stretching • Sills may cause uplift at the surface of the Earth
Effects of Intrusions
• Dikes form in regions of crustal stretching • Sills may cause uplift at the surface of the Earth
Scotland was stretched during the Cenozoic
La Sal Mountains, Utah were uplifted by a laccolith
Effects of Intrusions
• Plutons disrupt the surrounding layers of rock and may cause crustal stretching above • Plutons grow by stoping: opening cracks and assimilating xenolithic blocks in the melt
Cooling of Magma and Lava
• Magma cools for several reasons
– Removal of volatiles – It rises to a cooler location and has time to cool
• Cooling depends very much on the geometry (surface area) of the intrusion. Tabular-shape = fast cooling Spherical shape = slow cooling
– Cooling times vary from days minutes to millions of years
OBSIDIAN (VOLCANIC)
VERY FINE GRAINED
GRANITE (ACID INTRUSIVE ROCK)
COARSE GRAINED
Sedimentary rock… I am the Layered Storytellers
• made up of SEDIMENTS eroded from igneous, metamorphic, other sedimentary rocks, and even the remains of dead plants and animals. These materials are DEPOSITED in layers, or strata, and then are squeezed and compressed into rock. Most fossils are found in sedimentary rocks.
SEDIMENTS
Undergo LITHIFICATION
Sedimentary rocks
The process by which sediments become compacted and cemented together into a sedimentary rock.
Taman Negara Mulu
Batu Cave
Weathering and Erosion
• Sediment – loose fragments of rocks or minerals broken off of bedrock, minerals that precipitate directly out of water, and shells of organisms. • Deposition / Sedimentation – occurs when sediment settles out as winds/water current die down, or as glaciers melt. • Lithified – sediment that has been cemented together by geologic processes to form a rock • Rocks are broken down and turned into sediments by two main processes
– Physical Weathering
• Plumbers snake
– Chemical Weathering
• Liquid Drain-O Highly weathered sandstone in Bryce Canyon N.P., UT
How do Sedimentary Rocks Form?
The following applies to Clastic/Detrital rocks; biochemical and chemical sed rocks are different
• 1) Erosion – mobilizes particles by weathering, most commonly by rainfall & gravity. • 2) Transportation – Occurs when currents of wind, water, and moving ice (glaciers) transport particles to new locations (downhill or downstream) • 3) Deposition – sediment is deposited when the transporting current slows to the point that it can no longer carry its load. • 4) Burial – As layers of sediment accumulate, the layers accumulate in sedimentary basins. Older, previously deposited sediments are compacted. These layers remain at depth until either erosion or tectonic processes act on them. • 5) Diagenesis & Lithification – Refers to the physical and chemical changes that lithify sediment into rock. Includes pressure, heat and chemical reactions
SEDIMENTS – the layered storyteller origin
Detrital sediments
• Form from broken pieces of rock • sediments deposited from the remains of plants and animals
Organic Sediments
Chemical Sediments
• form when a solid chemical comes out of a solution of water
The Sedimentary Stages of the Rock Cycle
Weathering breaks down rocks. Erosion carries away particles. Transportation moves particles downhill.
Delta Desert Playa lake
Glacier
Deposition occurs when particles settle out or precipitate.
Sedimentary rocks Metamorphic rocks Plutons
Burial occurs as layers of sediment accumulate.
Diagenesis lithifies the sediment to make sedimentary rocks.
SEDIMENTS – who are they?
• MECHANICALLY FORMED: DETRITAL OR CLASTIC SEDIMENTS. MADE UP OF BROKEN PIECES OF MINERALS AND ROCKS, VOLCANIC ERUPTIONS OR SEA SHELLS. GRAIN SIZE >200mm (BOULDER) 200-60 COBBLE, 60-2 GRAVEL, 2-0.06 SAND, SILT AND CLAY. SHAPE MAY BE ROUNDED, SEMI-ROUNDED OR ANGULAR. • CHEMICALLY FORMED: INORGANIC REACTIONS OF DISSOLVED MATERIALS IN GROUNDWATER, SEAWATER, LAKES ETC. (Na, Cl, Ca ETC.) • ORGANIC SEDIMENTS: SKELETONS OF ANIMALS, CORALS, PLANT REMAINS OR VEGETABLE MATTERS.
NAMES OF SEDIMENTS
NAME OF SEDIMENT
ALLUVIUM (ALLUVIAL) AEOLINE (WIND) LACUSTRINE (LAGOONAL) GLACIAL COLLUVIUM LITTORAL SHALLOW SEA SEDIMENT DEEP SEA SEDIMENTS
DEPOSITION ENVIRONMENT
RIVER (FLUVIAL) DESERT OR COAST LAKE POLAR/MOUNTANEOUS GRAVITY FALLS/LANDSLIDES MARINE/COASTAL AND DELTAS MARINE SHALLOW SEA MARINE DEEP SEA
Sedimentary rocks classified into:
CLASTIC
• Detrital sediments form rocks • Eg: Shale, Sandstone, Conglomerate • organic and chemical sediments • Eg: Limestone, Coal
NONCLASTIC
• A clastic/detrital rock forms in five stages:
1- Weathering/Erosion 2- Transportation
Clastic Sedimentary Rocks
3- Deposition
4- Burial 5- Lithification and Diagenesis
(compaction+cementation)
Grain size is reduced as sediment is transported
Classifying Clastic Sedimentary Rocks
Clastic/Detrital rocks are classified by: 1. Clast size 2. Clast composition 3. Angularity and Sphericity 4. Sorting 5. Type of cement
CLASTIC SEDIMENTARY ROCKS
COMPOSITION GRAIN SIZE FEATURES NAME
Mainly quartz, feldspar, rock fragments and clays
Gravel (> 2 mm)
Rounded grains
Angular grains
Conglomerate
Breccia
Sand (0.0625 – 2.00 mm)
Mostly grains of Sandstone quartz, feldspar, rock Subgroups are named fragments or mixed according to features with much silt and clay
Silt (0.0039 – 0.0625 mm) Clay (less than 0.0039 mm)
Nonfissile (compact)
Siltstone
Fissile (splits easily)
Nonfissile (compact) Fissile (splits easily)
Shale
Claystone Shale
NON-CLASTIC SEDIMENTARY ROCKS
MINERAL COMPOSITION Calcium Carbonate (CaO3) GRAIN SIZE Gravel to clay FEATURES Shells or shell fragments cemented ROCK NAME Skeletal Limestone (subgroup name according to grain size) Chemical Limestone (subgroup name according to grain size) Dolostone Chert Rock salt Rock Gypsum Bituminous Coal
Spherical grains or deposited crystals
Dolomite (CaMg(CO3)2 Quartz (SiO2)
Halite (NaCl) Gypsum (CaSO42H2O) Plant fragments
All sizes
Commonly altered from limestone Crystalline or Layers, lenses, amorphous nodules All sizes Crystals deposited as inorganic chemical precipitates All sizes or dense with Black and non-porous conchoidal fracture
Sedimentary Rock – my story
CONGLOMERATE (CLASTIC)
Conglomerate could be thought of as a giant sandstone, containing grains of pebble size (greater than 4 mm) and cobble size (>64 mm).
SHALE (NON-CLASTIC)
Shale is a claystone that is fissile, splitting in layers. Shale is usually soft and does not crop out unless harder rock protects it.
Metamorphic rock a.k.a Changed Rocks
Metamorphic rocks
– produced when sedimentary or igneous rocks are transformed by heat and/or pressure. – Metamorphic rocks are either foliated or nonfoliated. – These extreme conditions are only present deep inside the Earth. – E.g. Marble, Slate, Quartzite, Schist, Gneiss
How Do We Identify Metamorphic Rocks?
1- Metamorphic Textures – grains are interlocked and grew in place. Many different types of metamorphic textures 2- Metamorphic Minerals – Certain minerals only grow under metamorphic temperatures and pressures. - Called a metamorphic mineral assemblage, or metamorphic facies 3- Foliation – The alignment of platy minerals or alternating layers of light (felsic) and dark (mafic) minerals.
A foliated Outcrop
of Gneiss
Formation of Metamorphic Textures
How do metamorphic textures form?
• •
•
•
•
Recrystalization – changes the shape and size of grains, but the same mineral remains. E.g. Sandstone may recystallize into quartzite. See (a) Phase Change – When a mineral keeps the same composition but the atoms arrange into a new form (polymorph). E.g. quartz (SiO2) may change to coesite (SiO2). Metamorphic reaction/neocrystallization – The result of chemical processes that decompose minerals and produce new minerals. Happens through diffusion of atoms through solid crystals. Very slow process. See (b) Pressure Solution – Mineral grains dissolve where their surfaces are in contact. Occurs when rock is squeezed in one direction more than the others, at low temps, and usually in the presence of water. Usually zig-zag shaped and common in carbonates. See (c) Plastic Deformation – At high temps, minerals can behave like soft plastic and become squished or stretched. Takes place without forming cracks and without changing the composition of the minerals. See (d)
What Causes Metamorphism?
1. 2. Heat - Increased heat allows chemical
bonds to break easier.
Pressure – high pressures cause
minerals with ‘open’ lattices to collapse, forming more dense crystals. Most metamorphic rocks form at 40-100 km depth where pressures are 10,00030,000 times greater than the surface of the Earth.
A ‘nice’ sample of gneiss
3. 4.
Differential Stress – When forces are
not equal in all directions, minerals may deform and change shape.
Hydrothermal Fluids – More than
just water, hydrothermal fluids are solutions that chemically react with minerals.
Recrystallized limestone becomes marble
Process involved - METAMORPHISM
• occurs when a rock is exposed to hot magma inside the Earth. • The intense heat of the magma alters the rock, CONTACT often causing its minerals to recrystallize. METAMORPHISM • The area of rock affected by contact metamorphism is appropriately known as the baked zone. • occurs during the formation of mountain ranges • As tectonic plates collide and converge, intense REGIONAL pressure deforms and alters sedimentary and igneous rocks already buried in the Earth METAMORPHISM • folds or curves in the rocks indicate the direction of the intense pressure.
Metamorphic rocks classified into:
FOLIATED
• the minerals have been flattened and pushed down into parallel layers. • Eg: Slate, Schist, Gneiss • do not display layers. • Eg: Quartzite, Marble
NONFOLIATED
Types of Metamorphic Rocks
• Metamorphic rocks are grouped into two main categories:
– Foliated Metamorphic Rocks – Non-Foliated Metamorphic Rocks
• But what exactly is foliation?
Foliation
• Foliation – The repetition of planar surfaces or layers in a metamorphic rock. Layers can be paper-thin or meters thick.
– Happens because when rocks are subjected to differential stress, platy minerals align or alternating light and dark layers form, giving the rock a planar fabric, called foliation. Note that this is different than bedding.
Slate, a foliated metamorphic rock makes nice roof shingles because its foliation creates cleavage planes that easily break
Foliation and Compression Direction
• Slaty Cleavage forms perpendicular to the compression direction, i.e. a horizontal squish will create vertical cleavage planes. Compression also commonly results in folding.
SOME METAMORPHIC ROCKS
CRYSTAL SIZE VERY FINE FINE COARSE
COARSE
FOLIATED METAMORPHIC ROKS ROCK NAME TEXTURE SLATE SLATY CLEVAGE PHYLITE PHYLLITIC SCHIST PARALLEL ARRANGEMENT OF PLATY MINERAL GRAINS GNEISS COLOUR BANDING DUE TO ALTERNATING LAYERS OF DIFFERENT MINERALS
NON-FOLIATED METAMORPHIC ROCKS ARE MANY. EG. QUARTZITE, CONGLOMERATE, AMPHIBOLITE, HORNFELS, MARBLE, GRAPHITE ETC.
SLATE
NON-FOLIATED, MADE FROM SHALE (SEDIMENTARY ORIGIN)
FOLIATIONS (E.G. MIGMATITE GNEISS)
SCHIST (NO FOLIATIONS BUT LOT OF ORDER)
How are the Rock Cycle and Plate Tectonics Related?
Heating within Earth’s interior results in convection cells that power the movement of plates, and also magma, which forms intrusive and extrusive igneous rock. Movement along plate boundaries may result in volcanic activity, earthquake and in some cases mountain building. The interaction between atmosphere, hydrosphere and biosphere contributes to the weathering of rocks exposed on Earth’s surface. Plates descending back into Earth’s interior are subjected to increasing heat and pressure, which may lead to metamorphism as well as generation of magma and yet another recycling of materials.
The rock cycle never ends!!
References
1. Harwant Singh Bujang Kim Huat, Basic Eng Geology for Tropical Terrain 2. http://www.beyondbooks.com 3. http://www.classzone.com 4. http://www.appstate.edu
THANK YOU
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