Sediments and Soils; Igneous Rocks

Types of crystalline Igneous rocks

1. Ultramafic, mafic, intermediate or felsic? 2. Coarse or fine grained?

Sedimentary Rock Process

1. Weathering: older rocks that are broken apart; particles that have been mechanically transported or chemically precipitated from solution or secreted by organisms. 2. Transport/travels: greater the distance of transport, the smoother or more rounded grains become. Smoother grains means the rock has travelled a lot. Travelling in fast water causes grains to become bigger (sorting). 3. Deposition: glacial deposition is the continual deposition of all sizes of particles which leads to poor sorting. Stream deposition is the most efficient, particles get sorted into different sizes by water. 4. Lithification: conversion of sediment into rock. Compaction is the reduction in volume of sediments resulting from weight of newly deposited sediments above. Cementation is a process by which precipitates bind together the grains of a sediment, converting it into a sedimentary rock.

How was the canyon formed?

In order to yield a salt layer that is 2 km thick, the entire Mediterranean must have dried up several times, with the sea refilling after each drying event. When the Mediterranean Sea dried up, the Nile River flowed down into a deep lowland and in the process carved a canyon. Later when the sea refilled with water, the river could no longer cut down, and the flooded canyon filled with sand and gravel brought in from upstream.

Why did the Mediterranean Sea dry up?

Only 10% of its water comes from rivers, so for the sea to remain full, water must flow in from the Atlantic Ocean. 6 million years ago- northward drifting African Plate collided with the European Plate, forming a natural da, separating the Mediterranean from the Atlantic. When global sea levels dropped, water stopped flowing over this dam from the Atlantic and the Mediterranean evaporated. Salt dissolved in water precipitated to form a solid deposit of halite and gypsum on the floor of the resulting basin and the pre-Nile canyon formed. 5.5 million years ago- Mediterranean rose to present level and gravel, sand, and mud carried by the Nile River accumulated in the pre-Nile canyon.

Halite and Gypsum

Types of salt that precipitate from seawater when the seawater dries up.

Why does the texture pf nonfragmental igneous rock largely reflect its cooling rate?

mineral crystals grow when atoms diffuse through melts and attach to crystal seeds. at high temps seeds constantly form and dissolve because het causes diffusion to occur rapidly. only some seeds are successful enough to grow into small crystals. most of these end up dissolving in the melt again. relatively few crystals grow large enough to survive until the melt cools at a level at which crystals stop dissolving. if a melt cools very rapidly it can become solid before seeds have grown into crystals so resulting rock has a glassy texture. if the melt does not cool rapidly enough to form glass it will develop a fine grained aphanitic texture. many crystals in the resulting rocks will have grown from seeds but none of those crystals will have had enough time to grow large. if a melt cools slowly successful crystals have time to grow large before the rock solidifies completely. as the large crystals are growing new seeds and small crystals form but because the melt remains hot they dissolve into the melt again before they can grow large.

Turbidity Current

- A moving submarine sediment suspension. - Sediment deposited on submarine slope tend to be unstable. - Earthquake or storm might disturb sediment and cause it to slip downslope. - Sediment mixes with water to produce murky, turbulent cloud. - cloud denser than clear water flows downslope like an underwater avalanche. - Downslope: turbidity current slows and sediment that it carried settles out. - Larger grains sink faster through a fluid so the coarsest sediment settles out first. - Finer grains accumulate on top. - Finest sediment, clay, settles out last. - Forms a graded bed: a layer of sediment in which grain size varies from coarse at the bottom to fine at the top.

Volcano

- A vent/opening from which melt that originates inside the Earth emerges onto the planet's surface or rises into the air during an episode called a volcanic eruption. - can also apply to the hill or mountain built from the products of an eruption - a mountain formed by the accumulation of extrusive volcanic rock

Sediment

- An accumulation of loose mineral grains, such as boulder, pebbles, sand, silt, or mud that are not cemented together. - Consists of loose fragments of rocks or minerals broken off bedrock and shells/shell fragments. - Materials came from weathering/physical and chemical breakdown of pre-existing rocks. - form a veneer/cover over bedrock. - cover's thickness ranges from 0km-20km in rapidly sinking basins. - Can transform into soil which may serve as a substrate for plants.

Where does weathering happen the fastest?

- At edges. - Even faster at corners of broken blocks. - Weathering attacks a flat face from only 1 direction, an edge from 2 directions, and a corner from 3 directions. - As a result over time edges of blocks become blunt and corners become rounded. - In rocks which do not contain distinct layer or fabrics that can influence weathering rates (e.g. granite), rectangular blocks transform into a rounded shape.

Sedimentary Rock Types

- Breccia: coarse sedimentary rock consisting of angular fragments. Are created when large blocks of rock tumble off a cliff and slam into other blocks already at the bottom. The impact shatters the blocks producing a pile of angular clasts. Fragments are cemented together before being transported very far. - Conglomerate: burial and lithification of rounded clasts. Storm causes clasts to be carried away by a turbulent river. In moving water, clasts bang into each other and into riverbed which shatters them into smaller pieces and breaks off their sharp edges. Clasts are carried downstream while gradually becoming rounded pebbles/cobbles. Water slows and clasts stop moving and form a mound/bar of gravel. - Arkose: gravel stays put for long time and undergoes chemical weathering. Cobbles/pebbles break apart into individual mineral grains and eventually produce mixture of quartz, feldspar, and clay. Clay is fine so flowing water easily picks it up and carried downstream. This leaves sand containing mixture of quartz and feldspar grains which may be buried and lithified. - Quartz sandstone: overtime feldspar grains in sand continue to weather into clay. During events that wash sediment downstream, sand loses feldspar and becomes durable quartz grains. Some sand makes it to sea where waves carry it to beaches while others in desert dunes. Is buried and lithified. - Siltstone: some silt and clay accumulate in flat areas bordering streams called floodplains and become submerged during floods. The rest settles in a wedge called delta at the mouth of river, lagoons, or mudflats along the shore. Silt is lithified. - Shale: mud that is finely laminated. Breaks into thin sheets. - Mudstone: mud that is fairly massive. Does not easily split into sheets.

Why are Sedimentary rocks important?

- Constitute only 7% of the Earth's crust. - Cover approximately 75% of the Earth's surface. - Archive the physical, chemical, and biological records of their sedimentary environments (e.g. oceans). - Contain the entire record of biological life (fossils). - Have an economic significance: agriculture, construction of roads, oil, water.

Describing texture in Igneous rocks

- Crystalline texture: forms when a melt solidifies and the minerals in some rocks grow and interlock like pieces of a jigsaw puzzle; crystals interlock because the rock does not solidify instantly but instead grow at different rates and times; as crystals grow they interfere with each other; e.g. a fast growing crystal may surround a slower growing crystal partly or entirely and later forming crystals fill in gaps between earlier forming crystals; coarse-grained/phaneritic rocks have crystals large enough to identify with the naked eye; fine-grained/aphanitic rocks have crystals that are too small to identify with the naked eye; Porphyritic rocks have larger crystals called phenocrysts surrounded by a mass of fine crystals called groundmass. - Fragmental texture: form from pyroclastic debris and consist of chunks or shards that are packed, welded, or cemented together after they have solidified. - Glassy texture: rocks made of solid mass of glass or tiny crystals surrounded by glass; these rocks typically fracture conchoidally (curved or like a clam shell).

Physical Weathering: Animal Attack

- For burrowing creatures. - Can move rock fragments. - Humans have become perhaps the most energetic agent of physical weathering on the plant. - Excavate quarries, foundations, mines or roadbeds by digging and blasting, shatter and displace large volumes of rock that might otherwise have remained intact for million of years.

Physical Weathering: Thermal Expansion

- Heat of an intense forest fire breaks a rock causing outer layer to expand. - On cooling, layer contracts which generates force in the rock sufficient to break off the outer part of the rock. - Heat of Sun's rays sweeping across dark rocks in a desert may overtime cause cobbles on the ground surface to fracture into thin slices.

Chemical Weathering: Oxidation

- In rocks, transform iron bearing minerals such as biotite and pyrite into a rusty brown mixture of various iron-oxide and iron-hydroxide minerals. - Causes iron-bearing rocks to rust.

Physical Weathering: Jointing

- Natural crack in rock - formation of joint separates one piece of rock into 2 separate pieces. - Almost all rock outcrops contain joints. - Large granite plutons typically split into onion-like sheets along exfoliation joints that lie parallel to the mountain face - Sedimentary rock layers tend to break into rectangular blocks bounded by joints on the sides and layer boundaries above and below. - Breaks bedrock into many separate blocks- when exposed on a slope eventually tumble downslope, fragmenting into smaller pieces as they fall. Resulting chunks may collect in an apron of talus or may be carried away by rivers or glaciers at the base of a cliff. - Form when rock that had been buried deeply in the crust rises towards the Earth's surface. As erosion strips away overburden, the rock becomes cooler and the pressure squeezing it decreases. Rock changes shape slightly but enough to cause hard rock to crack.

Where are Sedimentary rocks deposited?

- Ocean, underwater - Freshwater (lakes and rivers) - By glaciers - By land slides - By wind

How do physical and chemical weathering work together?

- PW speeds up CW. - CW takes place at the materials surface, so the overall rate at which CW occurs depends on the ratio of surface area to volume. - The greater the SA the faster the volume of the whole material can chemically weather. - When PW breaks a large block of rock into smaller pieces, the SA increases so CA weathering happens faster. - CW speeds up PW by dissolving away grains or cements that hold a rock together by transforming hard minerals (e.g. feldspar) into soft minerals (e.g. clay) and causing minerals to absorb water and expand. Can make rock weaker so can disintegrate easily.

Why study sedimentary structures?

- Provide important clues that help geologists understand the depositional environment in which sediments accumulated. - E.g. fossil types tell us whether sediment was deposited along a river or in the deep sea because different species of organisms live in different environments.

Types of pyroclastic Igneous rocks

- Pyroclastic debris: accumulation of fragments when the volcano erupts and spews out clots or droplets of lava as well as glass shards (broken up walls of vesicles in pumice), larger fragments of pumice, and other broken up chunks of recently formed igneous rocks. - Pyroclastic rock: when the debris becomes consolidated into a solid mass due to either still-hot clasts welding together during accumulation or due to cementation by minerals precipitating from groundwater long after accumulation; have a fragmental texture and we can distinguish among rock types based on grain size.

Organisms Role in Chemical Weathering

- Roots of plants, fungi, and lichens secrete organic acids that help dissolve minerals in rocks in order to extract nutrients. - Microbes eat minerals by plucking molecules off of a minerals surface and use the energy from the molecules chemical bonds to supply their own life force.

Clastic Sedimentary Rock

- Sedimentary rock consisting of cemented-together derived from the weathering of preexisting rock. - E.g. Breccia Conglomerate, sandstone, Siltstone, shale. - Sandstone: consists of loose clasts that have been stuck together to form a solid mass. The clasts can consist of individual minerals or chinks of rock. Held together with carbonate or quartz cement. Feels gritty with small grains of quartz. - Shale: platy, fine-grained rock, splits along bedding planes, breaks/splits in thin layer. - Conglomerate: smooth, rounded grains, water transport, must be cemented to form.

Organic Sedimentary Rock

- Sedimentary rock formed from carbon-rich relicts of organisms e.g. coal and oil shale. - Organic debris gets eaten by other organisms or decay at the Earth's surface. - In some environments, organic debris settles along with other sediment and eventually buried. - When lithified, organic-rich sediment becomes OSR. - Provides fuel of modern industry and transportation as organic chemicals can burn to produce energy. - Coal: black, combustible rock. Contains 40%-90% carbon. Remainder is clay or quartz. Forms when plant remains have been buried deeply and long enough for the material to become compacted and lose volatiles (hydrogen, water, CO2, ammonia). As volatiles seep away, the concentration of carbon increases. - Oil shale: contains clay and 15%-75% organic material called kerogen. Kerogen comes from fats and proteins that made up the living part of plankton and algae. Tiny organisms that do not immediately rot or get eaten mix with clay minerals in mud. Mud gets buried and lithified to form shale which transforms into kerogen. Presence of organic material makes oil shale black.

Chemical Sedimentary Rock

- Sedimentary rock made up of minerals that precipitate directly water solution. E.g. Dolostone, Chert. - Have crystalline texture formed partly during their original precipitation and partly when new crystals grow at the expense of old ones through recrystallization. - Some crystals are coarse while others are too small to see. - Evaporites: thick salt deposits that form as a consequence of precipitation from saline water. Occurs where saltwater becomes supersaturated so it can't keep all dissolved ions it contains in solution. Happens because evaporation removes water from a water body faster than the rate at which new water enters. Takes place in desert lakes and along margins of restricted seas. Thick deposits of salts form when large volumes of water evaporate. Type of slat mineral in evaporite depends on amount of evaporation. E.g. 80% evaporation of water forms gypsum, 90% forms halite precipitates. - Travertine/chemical limestone: a rock composed of crystalline calcium carbonate CaCO3 formed by chemical precipitation from groundwater that has seeped out at the ground surface in either hot or cold water spring or on walls of caves. Happens when groundwater degasses so some of CO2 that had dissolved in groundwater bubbles out of solution as removal of CO2 encourages precipitation of carbonate. Also occurs when water evaporates which increases the concentration of carbonate. Various species of microbes live in environments in which travertine accumulates so biological activity may contribute to precipitation process. When produced at springs forms terraces and mounds that are very thick (e.g. Mammoth Hot Springs). Grows on walls of caves where groundwater seeps out and forms growth forms called speleothems. Has distinctive layering so used for construction. - Dolostone: carbonate rock that contains mineral dolomite CaMg[CO3]2 containing equal amounts of calcium and magnesium. Forms by chemical reaction between solid calcite and magnesium bearing groundwater. Take place beneath lagoons along shores soon after limestone forms or after limestone is buried deeply. - Chemically precipitated chert: in New York State, outcrops of limestone contain layers/nodules/lenses/lumps of black chert/flint. Used to create sharp-edged tools (arrowheads or scrapers) because of the way it breaks. Formed when microscopic quartz crystals gradually replaced calcite crystals within a body of limestone long after the limestone deposited called replacement chert. Can be black, white, red, brown, green, gray, depending on impurities it contains. Petrified wood forms when silica-rich sediment lime ash for volcanic eruption buries trees. Silica dissolves in groundwater and later precipitates as microcrystalline quartz within wood and gradually replaces wood's cellulose. Chert deposit retains shape of wood cells and growth rings within it. Agate precipitates in concentric ring inside hollows in a rock and end up with a stripes caused by variations in content if impurities.

Classification of Clastic Sedimentary Rock

- Size: diameter of fragments or grains making up the rock. Coarsest to finest: boulder, cobble, pebble, sand, silt, mud. - Composition: makeup of clasts in sedimentary rock. May be composed of rock fragments or individual mineral grains. Sand consists mostly of quartz grains. - Angularity: degree to which clasts have smooth surfaces or sharp corners and edges. - Sphericity: how closely the shape of a clast resembles a sphere. - Sorting: the proportion of clasts in a rock that are the same size. Well sorted means almost all clasts are the same size. Poorly sorted means there is a mixture of clast size. - Character of cement: in some rocks cement consists predominately of quartz while in others it consists of calcite. - Formation: form in layers, distinctive rock units that can be mapped over a large area. E.g. Lockport formation.

Chemical Weathering: Hydration

- The absorption of water into the crystal structure of minerals causes some minerals (e.g. clay) to swell. - Expansion weakens rock.

Chemical Weathering

- The many chemical reactions that alter or destroy minerals when rock comes in contact with water solutions or air. - Minerals undergo chemical weathering at different rates. - E.g. granite contains quartz that does not change so it is resistant to chemical weathering.

Bowen's Reaction Series

- The sequence in which different silicate minerals crystallize during the progressive cooling of a melt - In a cooling mafic melt, olivine and calcium-rich plagioclase form first. The plagioclase reacts with the remaining melt, and new plagioclase, containing more sodium (Na), grows. This new plagioclase may replace or grow around the earlier-formed plagioclase. Meanwhile, some olivine crystals react with the remaining melt to produce pyroxene. Some of the early olivine and pyroxene crystals become isolated from the melt, effectively extracting iron and magnesium, so the remaining melt becomes progressively enriched with silica - As the melt continues to cool, plagioclase continues to form. Laterformed plagioclase has more sodium than earlier-formed plagioclase, pyroxene crystals react with the melt to form amphibole, and then some amphibole reacts with the remaining melt to form biotite. All the while, some newly formed crystals become isolated and don't exchange atoms with the melt—some crystals may, in fact, sink and accumulate at the bottom of the melt. As more crystals form, the remaining melt becomes progressively more felsic. Finally, at temperatures between 650°C and 850°C, only about 10% of the melt remains, and this melt has a high silica content, so when the last melt finally freezes, only quartz, potassium feldspar (K-feldspar), and muscovite can form from it - The discontinuous reaction series refers to the sequence of olivine, pyroxene, amphibole, biotite, K-feldspar/muscovite/quartz, in that each step yields a different kind of silicate mineral. - The continuous reaction series refers to the sequence from calcium-rich to sodium-rich plagioclase, in that each step yields a different version of the same mineral

Chemical Weathering: Dissolution

- Water flows over or through rock and slowly dissolves minerals since water is a solvent. - Affects salts and carbonate minerals. - Silicate minerals slightly dissolve.

Chemical Weathering: Hydrolysis

- Water reacts chemically with minerals and breaks them down to form other minerals. - E.g. hydrolysis reaction transform feldspar and many other silicate minerals into clay.

Physical Weathering: Salt Wedging

- Weakens rock so when exposed to wind and rain, the rock disintegrates into separate grains. - Arid climates - Dissolved salt in groundwater precipitates in open pore spaces in rock, forming crystals that push apart the surrounding grains. - Occurs along seacoast where salt spray percolates into rock and then dries.

Differential Weathering

- When rocks in an outcrop undergo weathering differences. - E.g. graveyard: inscriptions on some headstones are sharp and clear but those on other stones have become blunted or have even disappeared because minerals in different stones have different resistances to weathering. - Granite: igneous rock with high content of resistant quartz retains inscriptions the longest. - Marble: metamorphic rock composed of soluble calcite dissolves away rapidly in acidic rain. - Cliffs composed of layers of different rock types develop a stair-step or sawtooth shape.

Physical Weathering: Frost Wedging

- When water trapped in a joint freezes, it forces the joint open and may cause the joint to grow. - Helps break blocks free from intact bedrock.

Describe the exception to the standard cooling rate-grain size relationship

- a very coarse grained igneous rock called pegmatite doesn't form by slow cooling. - contains crystals up to tens of cm across and typically occurs in dikes. - dikes generally cool quickly so the coarseness of the rock may seem surprising - pegmatite becomes coarse because it forms from water rich melts in which atoms can diffuse so rapidly that large crystals grow very quickly.

Weathering

- combination of phenomena that corrode and break up solid rock, eventually transforming it into sediment/loose debris. - produces ions that dissolve in surface water or groundwater. - may look discolored or rough compared to fresh rock

Describing color in Igneous rocks

- dark or light? - gray, pink, white, or black? - igneous rocks contain visible mineral grains each with a different color - color reflects the rocks composition but can also be influenced by grain size and by the presence of trace amounts of impurities; e.g. presence of a small amount of of iron oxide gives rock a reddish tint

Forming igneous rocks at mid-ocean ridges

- divergent plate boundaries - most igneous rocks present at the Earths surface are a product of seafloor spreading - Igneous activity happens at mid ocean ridges because as seafloor spreading takes place, oceanic lithosphere plates drift away from the ridge - hot asthenosphere rises to keep the resulting space filled and undergoes partial melting due to decompression - partial melting of mantle periodite yields mafic magma which rises into the crust and collects in a magma chamber; some cools slowly in the magma chamber to form massive gabbro; some intrudes upwards to fill vertical cracks that appear as newly formed crust splits apart - magma that cools in the cracks forms basalt dikes - magma that rises as far as the seafloor and extrudes as lava forms pillow basalt flows.

Distinguish between wet and dry melts

- dry: do not contain volatiles - wet: contain volatiles; 15% dissolved volatiles including H2O, CO2, N2, H2, SO2; come out of the Earth at volcanoes in the form of gas

Porphyritic rocks

- form when a melt cools in 2 stages 1. melt cools at depth slowly enough for phenocrysts to form 2. melt erupts and the remainder cools quickly so fine grained groundmass forms around the phenocrysts.

Vesicles

- forms when a rapidly cooling lava freezes while it still contains a high concentration of gas bubbles, these bubbles remain as open holes

How do molten rock differ from one another?

- in terms of the proportions of chemicals they contain - 4 major compositional types depending on the proportion of silica relative to the sum of magnesium oxide and iron oxide in the melt. - Mafic melts: contain a relatively high proportion of magnesium oxide and iron oxide compared to silica. - Ultramafic melt: even proportion of magnesium oxide and iron relative to silica. - Felsic/silicic melts: high proportion of silica compared to magnesium oxide and iron oxide. - Intermediate melts: composition is partway between that of mafic and felsic melts.

What is the structure of molten rock?

- is liquid, so molecules do not lie in an orderly crystalline lattice - occurs in clusters or chains that can move with respect to one another - water makes up about half of the gas erupting at a volcano, so molten rock contains the molecules that become water and air.

Large igneous provinces (LIPs)

- large volume of igneous rock or huge eruptions of felsic ash - form when the bulbous head of a mantle plume first reaches the base of the lithosphere - more -partial melting can occur in a plume head - large quantities of hot mafic magma forms in the plume head; when this magma reaches the surface huge amounts of lava spew out of the ground - if the plume head lies beneath a rift, thinning of the lithosphere causes even more decompression which leads to even more melting - On land: hot mafic lava that erupts in LIPs has such low viscosity that it can flow tens to hundreds of km across the landscape, forming a vast sheet of basal; process may repeat hundreds of time yielding thick stacks of thin broad lava flows called flood basalts

Products of hot spots

- mantle plumes: columns or streams of hot mantle rock rising from deeper in the mantle. - plume hypothesis: plume itself does not consist of magma, rather its composed of solid rock that is hot enough to be soft so that it can flow plastically at rates a few cm a year. - when hot rock of a plume reaches the base of the lithosphere, decompression causes partial melting within the hot rock. - partial melting of ultramafic/peridotite generates mafic/basaltic magma. - at oceanic hot spots, mafic magma erupts at the surface and solidifies to form basalt - continental hotspots: part of the mafic magma erupts to form basalt but some transfers heat to the continental crust which then partially melts producing felsic magmas that erupt to yield rhyolite.

Lava

- melt that has emerged at the surface - molten rock that has flowed out onto the Earths surface - volcano: lava pools around the vent or moves downslope as a syrupy red-yellow stream called a lava flow 1. base of volcano: lava flows slow, but continues to advance, engulfing roads, houses, vegetation along the way 2. as it cools, its surface darkens and forms a hardened rind that occasionally cracks open to reveal the hot, sticky mass oozing within 3. lava flow stops moving and within days or weeks cools 4. once red hot melt becomes hard dark-grey rock

Magma

- melt underground - molten rock beneath the Earths surface - not all makes it to the surface - some solidify underground

Hawaiian volcano

- near the vent, lava has a temp of 1150 deg C - moves swiftly around 1- to 60 km/h

Forming igneous rocks at rifts

- rifts: places where continental lithosphere stretches horizontally - result of stretching: lithosphere thins vertically; asthenosphere undergoes decompression. - partial melting due to decompression produces basaltic magma which rises into the crust - some magma makes to the surface and erupts as basalt; some becomes trapped in crustal magma chambers and transfers heat to the crust - partial melting yields felsic magmas that erupt explosively as rhyolite - result: volcanic rocks in a rift generally include lava flows of basalt and layers of rhyolitic tuff.

Intrusive Igneous Rocks

- rock formed by the solidification of magma underground - greater volume of igneous rock formed by the solidification of magma underground after it has pushed its way or intruded into pre existing wall rock.

Igneous rock

- rock formed by the solidifying/freezing of a melt/hot molten rock (magma or lava) - makes up entire oceanic crust and much of continental crust, so can be found at many places on the Earths surface - 2 main categories: extrusive and intrusive

Extrusive Igneous Rock

- rocks that form from lava that freezes above ground, in contact with air or water after it erupts or extrudes - e.g. rocks that solidify within a lava flow and rock made from cemented together fragments of pyroclastic debris that blasted out of a volcano and into the air - Pyroclastic debris: fragmented material that sprayed out of a volcano and landed on the ground or sea floor in solid form; occurs in many sizes, from very fine particles called volcanic ash, to coarser chunks.

What is magna and lava (molten rock) made out of?

- silica: silicon + oxygen - varying portions of aluminum, calcium, sodium, potassium, iron, and magnesium

How does subduction trigger melting?

- some minerals in oceanic crust rocks contain volatile compounds - at shallow depths these volatiles are bonded to other elements within mineral crystals - when subduction caries crust down into the hot asthenosphere, the crust warms up, and at a depth of about 150 km it becomes so hot that volatiles separate and diffuse up into the overlying hot ultramafic rock of the asthenosphere as molecules of H2O or CO2. - addition of volatiles causes flux melting of periodite - only part of original ultramafic rock melts since silica goes into the melt. - process yields mafic magma, some of which rises to form basaltic sills and dikes in the crust, some travel all the way to the surface to extrude as basaltic lava - continental volcanic arcs: not all mantle derived basaltic magma rises directly to the surface; some gets trapped at the base of the continental crust; some resides in magma chambers deep in the crust - fractional crystallization and assimilation: cause some of the magma to become progressively more felsic - very hot mantle derived magma transfers heat into the adjacent continental crust which causes partial melting of the continental crust - some of the new melt becomes assimilated in the rising melt and some rises on its own - as magma cools, it undergoes fractional crystallization - much of continental crust has a mafic to intermediate composition, resulting magma is intermediate to felsic; as magma rises it either cools higher in the crust to form plutons in magma chambers or rises to the surface to erupt as lava

Why do melts of so many compositions form in the Earth?

- source rock composition: composition of melt reflects the composition of the solid from which it was derived; not all melts form from the same source rock so not all melts have the same composition. - Partial melting: process by which only part of an original rock melts to produce magma; the melting in a rock of minerals with the lowest melting temps while other minerals remain solid; due to temp and pressure conditions in Earth only 2% to 30% of a source rock can melt to produce magma at a given location; temp at sites of magma production never get high enough to melt the entire source rock and magma tends to migrate away from the site of melting before all of the original rock has melted; magmas formed by PM are more felsic than the source rock from which they were derived because more silica enters the liquid as melting begins than remains behind in the still solid source; e.g. PM of an ultramafic rock produces a mafic magma. - Assimilation: as magma sits underground before solidifying completely it may incorporate chemicals dissolved from the wall rock or from blocks that detached from the wall and sank into the magma; process of magma contamination in which blocks of wall rock fall into a magma chamber and dissolve. - Magma mixing: originally distinct magmas mix to yield a new different magma; e.g. mixing felsic magma with mafic magma could produce intermediate magma.

Plutons

- tend to be composed of coarse grained rock - plutons that intrude into hot wall rock at great depth cool particularly slowly and tend to have larger crystals than plutons that intrude into cool wall rock at shallow depth

Flood Basalts

- vast sheets of basalt that spread from a volcanic vent over an extensive surface of land - may form where a rift develops above a continental hotspot and where lava is particularly hot and has low viscosity - make up the bedrock of the Columbia River Plateau in Oregon and Washington, the Parana Plateau in southern Brazil, the Karoo region of southern Africa, and the Deccan region of southwestern India.

Products of subduction

- volcanic arc: chain of volcanoes that forms on the overriding plate adjacent to the deep ocean trenches that mark convergent plate boundaries - continental volcanic arcs: grow on continental crust in locations where oceanic lithosphere subducts beneath continental lithosphere. - volcanic islands arc grow on oceanic crust where one oceanic plate subducts beneath another - beneath volcanic arcs a variety of intrusions (plutons, dikes, skills) develop to be exposed only later by erosion of the overlaying volcanoes - some localities, arc related igneous activity produces huge batholiths

Causes of Melting

1. Decompression: pressure prevents melting so a decrease in pressure can trigger melting as long as the rock remains hot; takes place where hot mantle rock slowly rises; as the rock moves up its pressure decreases due to decrease in overburden while its temp remains nearly unchanged; upward movement causes decompression melting in mantle plumes, beneath rifts, and beneath mid ocean ridges. 2. Addition of volatiles: volatiles are substances such as water and CO2 that evaporate easily; magma forms at locations where volatiles mix with hot mantle rock; volatiles mix with hot, dry rock and react with minerals and break chemical bonds so the rock begins to melt (flux melting); adding volatiles decreases.a rocks melting temp; they seep into hot asthenosphere in the region just above subducting oceanic lithosphere causing volcanism at convergent plate boundaries. 3. Heat transfer: very hot magma from the mantle rises into the crust, the heat it brings raises the temp of the surrounding crustal rock; sometimes the rise in temp may be sufficient to cause the crustal rock to begin melting (heat-transfer melting)

What did Geologists find when studying materials beneath the pre-Nile canyon?

1. Layers of shells of plankton that had settled out of the water or clay that rivers had carried to the sea. 2. 2 km thick layer of halite and gypsum.

Types of glassy Igneous rocks

1. Obsidian: a mass of solid, felsic glass; tend to be black or brown; breaks conchoidally so sharp edged pieces split off its surface when you hit a sample with a hammer; pre industrial people worldwide used such pieces for arrowheads, scrapers, and knife blades. 2. Tachylite: rare, vesicle free mafic glass 3. Pumice: felsic volcanic rock contains abundant tiny vesicles each surrounded by a thin screen or glass; can look like a sponge; some specimens can float on water; forms from quickly cooling, volatile rich frothy lava.

Turbidite

A deposit from a turbidity current.

Clast

A fragment or grain produced by the physical or chemical weathering of a pre-existing rock.

Sedimentary Structures

A geometry/arrangement/layering of material in sedimentary rock that formed during or shortly after deposition and the arrangement of grains within layers, e.g. cross beds and mud cracks.

Geologic Map

A map showing the distribution of rock units and structures across a region.

Stratigraphic Formation

A recognizable layer of a specific sedimentary rock type or set of rock types, deposited during a certain time interval, that can be traced over a broad region. E.g. a region may contain a succession of altering sandstone and shale beds deposited by rivers, overlaid by beds of marine limestone deposited later when the region was submerged by the sea.

Detritus

Accumulation of clasts.

Bedding/stratification

Arrangement of sediment into a sequence of beds. - Stratification can be subtle in some outcrops. - Successive beds have different colors, textures, and resistance to erosion, so bedding gives outcrops a striped appearance.

Physical Weathering: Root Wedging

As roots grow they apply pressure to their surroundings and can push joints open.

Strata of Sedimentary Rock

Several layers/beds together.

Physical Weathering/Mechanical Weathering

Breaks intact rock into unconnected grains or chunks (clasts).

Why does bedding form?

Due to changes in climate, water depth, current velocity, or the sediment source that control the type of sediment deposited at a location at a given time. E.g. on a normal day a slow-moving river may carry only silt which collects on the riverbed. During a flood, the river flows faster and carries sand and pebbles, so a layer of sandy gravel forms over the silt layer. When flooding stops, more silt buries the gravel. If this succession of sediments becomes lithified and later exposed for us to see, it appears as alternating beds of siltstone and sandy conglomerate. - If sea level rise and water submerges an area in which desert sand had been accumulating, layers of carbonate shells may be deposited over layers of sand. When lithified sand layers become beds of sandstone, shell layers become beds of limestone.

Bed-Surface Marking

Features that develop on the surface of a bed as a consequence of events that happen during deposition or soon after, while the sediment layer remain soft. - Mud cracks: openings between plates. Mud layer dries up after deposition and crack into roughly hexagonal plates that curl up at their edges. - Scour marks: small troughs parallel to the current flow that erode when a current flows over a sediment surface. - Fossils: the trace of an ancient living organism that has been preserved in rock or sediment. Can be shell imprints or footprints on a bedding surface. - These can be preserved through burial and lithification.

Cross Beds

Internal laminations in a bed, inclined at an angle to the main bedding, cross beds are a relict of the slip face of dunes and ripples. Developed when current of air or water moving in one direction erodes and picks up clasts from the upstream part of the bedform. They are then deposited on the downstream or leeward face of the crest. Sediment builds up until gravity causes it to slip down the leeward face. With time, the dune or ripple builds in the downstream direction. Surface of slip face establishes the shape of the cross beds. Eventually a new cross-bedded layer builds over pre-existing one. Main bedding is the boundary between 2 successive layers.

Beds of Sedimentary Rock

Layers that occur in the upper part of the crust and form a cover that buries the underlying basement of igneous and/or metamorphic rock. Caused by small changes in composition. E.g. Turbidites. Thickness varies. Gives us info about sedimentation process. Boundary between 2 beds is a bedding plane.

Regolith

Loose debris (sediment or soil).

Bedrock

Mineral crystals that precipitate out of water.

Talus

Rock rubble at the base of a slope.

Sedimentary Rock

Rock that forms at or near the surface of the Earth in one of several ways: 1. Cementing together of loose clasts (fragments/grains) that had been produced by physical or chemical weathering of pre-existing rock. 2. Growth of mounds of shells. 3. Cementing together the shells and shell fragments. 4. Accumulation and subsequent alteration of organic matter derived from living organisms. 5. Precipitation of minerals directly from surface-water solutions.

Biochemical Sedimentary Rock

Sedimentary rock formed from material produced by living organisms e.g. shells. E.g. Limestone, Coquina, chalk, Fossiliferous. - Limestone: solid shells if calcium carbonate CaCO3 made by organisms crystallizes as either calcite or aragonite (same composition, different crystal structure). Organisms in deep water/open ocean that are responsible for production of oxygen during photosynthesis die and shell remains may accumulate. Rock forms. AKA carbonate rock. Built up of microscopic shells. Variety of textures as it accumulates in different ways. Fossiliferous limestone consist of visible fossil shells/shell fragments found in high energy environments (waves), micrite consists of fine carbonate mud, and chalk consists of plankton shells. Massive light gray to dark bluish grey rock. Breaks into chunky blocks. Several processes change its texture over time after it has been buried deeply. E.g. water precipitates cement and dissolves carbonate grains causing new ones to grow. - Chert: reddish porcelain-like rock. Occurs in 3-15 cm thick layers. If hit with a hammer the rock cracks to form smooth, spoon-shaped (conchoidal) fractures. Made from cryptocrystalline quartz. Consists of quartz grains that need a microscope to be seen. Formed from shells of silicate secreting plankton that accumulated on the seafloor. After burial, shells dissolve and form silica-rich gel. Chert formed when gel solidifies.

Erosion

The breaking off/grinding away and removal of rock or sediment by moving water, air, or ice.

Volcanic Ash

Tiny glass shards formed when a fine spray of exploded lava freezes instantly upon contact with the atmosphere.

Bedforms

When clastic sediments accumulate in moving fluids (wind, rivers, waves) and sedimentary structures interfere between the sediment and fluid. Develop at a given location and reflect such factors as the velocity of the flow and the size of clasts. Growth of ripple marks and dunes produces cross bedding (special type of lamination within beds). - Ripple Marks: relatively small elongated ridges that form on a sedimentary bed surface at right angles to the direction of current flow. Found on modern beaches, streambeds, or bedding planes of ancient rocks. - Dunes: a pile of sand generally formed by the deposition from the wind. Looks like a ripple but much larger.

What do lava flows, dikes, and sills tend to be composed of?

fine-grained rocks because of the relationship between cooling rate and texture.