Essentials_of_Geology

1 The Earth in Context

The geocentric model of the Universe placed the Earth at the center of the Universe. The heliocentric model placed the Sun at the center.

The Earth is one of the eight planets orbiting the Sun.This Solar System lies on the outer edge of the Milky Way galaxy. The Universe contains hundreds of billions of galaxies.

The red shift of light distant galaxies led to the expanding Universe theory. Most astronomers agree that expansion began after the Big Bang, 13.7 billion years ago.

The first atoms (hydrogen and helium) of the Universe developed within minutes of the Big Bang. These atoms formed vast gas clouds,called nebulae.

Gravity caused clumps of gas in the nebulae to coalesce into flattened disks with bulbous centers. The center of each disk became so dense and hot that fusion reactions began and they became the true stars.

The Earth, and the life forms on it, contains elements that could have been produced only during the life cycle of stars. Thus, we are all made of stardust.

Planets developed from the rings of gas and dust surrounding protostars. These condensed into planetesimals that then clumped together to form protoplanets, and finally true planets. Inner rings became the terrestrial planets. Outer rings grew into gas-giant planets.

The Moon formed from debris ejected when a protoplanet collided with the Earth in the young Solar System.

A planet assumes a near-spherical shape when it becomes so soft that gravity can smooth out irregularities.

The Earth has a magnetic field that shields it from solar wind and cosmic rays.

A layer of gas surrounds the Earth. This atmosphere(78% $N_2$,21% $O_2$, and 1% other gases) can be subdivided into layers. Air pressure decreases with increasing elevation.

The surface of the Earth can be divided into land (30%) and ocean (70%).

The Earth consists of organic chemicals, minerals, glasses, rocks, metals, melts, and volatiles, Most rocks on Earth contain silica ($SiO_2$). We distinguish among various major rock types based on the proportion of silica.

The Earth’s interior can be divided into three distinct layers: the very thin crust, the rocky mantle, and the metallic core.

Pressure and temperature both increase with depth in the Earth. The rate at which temperature increases as depth increases is the geothermal gradient.

The crust is a thin that varies in thickness from 7-10km(beneath the oceans)to 25-70km(beneath the continents), Oceanic crust is mafic in composition,whereas average upper continental crust is felsic to intermediate. The mantle is composed of ultramafic rock. The core is made of iron alloy.

Studies of earthquake waves reveal that the mantle can be subdivided into an upper mantle(including the transition zone)and a lower mantle. The core can bes subdivided into the liquid outer core and a solid inner core. Circulation of the outer core produces the Earth’s magnetic field.

The crust plus the upper part of the mantle constitute the lithosphere, a rigid shell. The lithosphere lies over the asthenosphere, mantle that can flow.

2 The Way the Earth Works: Plate Tectonics

Alfred Wegener proposed that continents had once been joined together to form a single huge supercontinent(pangaea) and had subsequently drifted apart. This idea is the continental drift hypothesis.

Wegener drew from several different sources of data to support his hypothesis:(1)the correlation of coastlines; (2)the distribution of late Paleozoic glaciers; (3)the distribution of late Paleozoic climatic belts; (4)the distribution of fossil species; and (5) correlation of distinctive rock assemblages now on opposite sides of the ocean.

Rocks retain a record of the Earth’s magnetic field that existed at the time the rocks formed. This record is called paleomagnetism. By measuring paleomagnetism in successively older rocks, geologists discovered apparent polar-wander paths.

Apparent polar-wander paths are different for different continents, because continents move with respect to each other, while the Earth’s magnetic poles remain roughly fixed.

Around 1960, Harry Hess proposed the hypothesis of sea-floor spreading. According to this hypothesis, new sea floor forms at mid-ocean ridges, then spreads symmetrically away from the ridge axis. Eventually, the ocean floor sinks back into the mantle at deep-ocean trenches.

Geologists documented that the Earth’s magnetic field reverses polarity every now and then. The record of reversals is called the magnetic-reversal chronology.

A proof of sea-floor spreading came from the interpretation of marine magnetic anomalies and from drilling studies which proved that sea floor gets progressively away from a mid-ocean ridge.

The lithosphere is broken into discrete plates that move relative to each other. Continental drift and sea-floor spreading are manifestations of plate movement.

Most earthquakes and volcanoes occur along plate boundaries, the interiors of plates remain relatively rigid and intact.

There are three type of plate boundaries-divergent,convergent,and transform-distinguished from each other by the movement the plate on one side of te boundary makes relative to the plate on the other side.

Divergent boundaries are marked by mid-ocean ridges. At divergent boundaries,sea-floor spreading produces new oceanic lithosphere.

Convergent boundaries are marked by deep-ocean trenches and volcanoes arcs. At convergent boundaries,oceanic lithosphere subducts beneath an overriding plate.

Transform boundaries are marked by large faults at which one plate slides sideways past another. No new plate forms and no old plate is consumed at a transform boundary.

Hot spots are place where volcanism occurs at an isolated volcano. As a plate moves over the hos spot, tje volcano moves off and dies, and a new volcano forms over the hot spot. Hot spots may be caused by mantle plumes.

A large continent can split into two smaller ones by the process of rifting. During rifting, continental lithosphere stretches and thins. If it finally breaks apart, a new mid-ocean ridge forms and sea-floor spreading begins.

Convergent plate boundaries cease to exist when a buoyant piece of crust(a continent or an island arc) moves into the subduction zone. When that happens,collision occurs.

Ridge-push force and slab-pull force contribute to driving plate motions, Plates move at rates of about 1 to 15 cm per year. Modern satellite measurements can detect these motions.

3 Patterns in Nature:Minerals

Minerals are naturally occurring, solid substances, formed by geologic process, with a definable chemical composition and an internal structure characterized by an orderly arrangement of atoms, ions, or molecules in a crystalline lattice. Most minerals are inorganic.

Biogenic minerals are produced by organisms. The minerals in shells are an example of biomineralization.

In the crystalline lattice of minerals, atoms occur in a specific pattern–one of nature’s finest examples of ordering.

Minerals can form by te solidification of a melt, precipitation from a water solution, diffusion through a solid, the metabolism of organisms, and precipitation from a gas.

About 4000 different type of minerals are known, each with a name and distinctive physical properties(such as color,streak, luster, hardness, specific gravity,crystal habit, cleavage, magnetism, and reactivity with acid).

The unique physical properties of a mineral reflect its chemical composition and crystal structure,By observing these physical properties,you can identify minerals.

The most convenient way to classify minerals is to group them according to their chemical composition. Mineral classes include silicates, oxides, sulfides, sulfates, halides, carbonates, and native metals.

Silicate minerals are the most common minerals on Earth. The silicon-oxygen tetrahedron, a silicon atom surrounded by four oxygen atoms, serves as the fundamental building block of silicate minerals.

Groups of silicate minerals are distinguished from each other by the ways in which the silicon-oxygen tetrahedra that constitute them are linked.

Gemstone are minerals known for their beauty and rarity. The facets on cut gems used in jewelry are made by grinding and polishing the stones with a faceting machine.

4 Up from the Inferno:Magma and Igneous Rocks

Magma is liquid rock(melt) under the Earth’s surface. Lava is melt that has erupted from a volcano at the Earth’s surface.

Magma forms when hot rock in the Earth partially melts. This process occurs only under certain circumstance—when the pressure decrease, when volatiles are added to hot rock, and when heat is transferred into the crust by magma rising from the mantle into the crust.

Magma occurs in a range of compositions:felsic(silicic), intermediate, mafic, and ultramafic. The composition of magma reflects the original composition of the rock from which the magma formed and the way the magma evolves.

Magma rises from depth because of its buoyancy and because of pressure caused by the weight of overlying rock.

Magma viscosity depends on composition. Felsic magma is more viscous thant mafic magma.

The rate at which intrusive magma cools depends on the depth at which it intrudes, the size and shape of the magma body, and whether circulating groundwater is present. The cooling time influences the texture of an igneous rock.

Extrusive igneous rocks form from lave that erupts out of a volcano. Intrusive igneous rocks develop from magma that freezes inside the Earth.

Lave may solidify to form flows, or it may explode into the air to form ash.

Intrusive igneous rocks form when magma intrudes into pre-existing rock below Earth’s surface. Blob-shaped intrusions are called plutons. Tabular intrusions that cut across layering are dikes, and those that form parallel to layering are sills. Huge intrusions, made up of plutons, are know as batholiths.

Igneous rocks are classified according to texture and composition.

The origin of igneous rocks can readily be understood in the context of plate tectonics. Magma forms at continental or island volcanic arcs along convergent margins, mostly because of the addition of volatiles to the asthenosphere above the subducting slab. Igneous rocks form at hot spots, owing to the decompression melting of a rising mantle plume. Igneous rocks form aat rifts as a result of decompression melting of the asthenosphere below the thinning lithosphere or heat transfer from mantle melts into crustal rocks, Igneous rocks form along mid-ocean ridges because of decompression melting of the rising asthenosphere.

5 The Wrath of Vulcan: Volcanic Eruptions

Volcanoes are vents at which molten rock(lava), pyroclastic debris, gas, and aerosols erupt at the Earth’s surface.

The characteristic if a lava flow depend on its viscosity, which in turn depends on its temperature and composition.

Basaltic lave can flow great distances. Pahoehoe flows have smooth, ropy surfaces, whereas a’a’ flows have rough, rubbly surfaces. Andesitic and rhyolitic lava flows tend to pile into mounds at the vent.

Pyroclastic debris includes powder-sized ash, marble-sized lapilli, and apple- to refrigerator-sized blocks and bombs. Some falls from the air, whereas some forms pyroclastic flows.

Eruptions may occur at a volcano’s summit or from fissures on its flanks. The summit of an erupting volcano may collapse to form bowl-shaped depression called a caldera.

A volcano’s shape depends on the type of eruption. Shield volcanoes are broad, gentle domes, Cinder cones are steep sided, symmetrical hills, Composite volcanoes (stratovolcanoes) can become quite large and consist of alternating layers of pyroclastic debris and lava.

The type of eruption depends on the lava’s viscosity and gas content. Effusive eruptions produce flows, whereas explosive eruptions produce pyroclastic debris.

Different kinds of volcanoes form in different geologic settings as defined by plate tectonics theory.

Volcanic eruptions pose many hazards:lava flows overrun roads and towns, ash falls blanket the landscape, pyroclastic flows incinerate towns and fields, landslides and lahars bury the land surface,earthquakes topple structures and rupture dams, tsunamis wash away coastal towns, and invisible gases suffocate people and animals.

Eruptions can be predicted by earthquake activity, changes in heat flow, changes in shape of the volcano, and the emission of gas and steam.

We can minimize the consequences of an eruption by avoiding construction in danger zones.

Immense flood basalts cover portions of te Moon, The largest known volcano, Olympus Mons, tower over the surface of Mars. Satellites have documented evidence for eruption on moons of Jupiter and Saturn.

6 Page of Earth’s Past:Sedimentary Rocks

Geologists recognize four major classes of sedimentary rocks. Clastic rocks form from cemented-together grains that were first produced by weathering, then were transported, deposited, and lithified. Biochemical rocks develop from the shells of organisms. Organic rocks consist of plant debris or of altered plankton remains. Chemical rocks precipitate directly from water.

Sedimentary structures include bedding, cross bedding, graded bedding, ripple marks, dunes, and mud cracks. They serve as clues to depositional settings.

Biochemical and organic rocks form from materials produced by living organisms.

Limestone consists dominantly of calcite, chert forms from silica, coal from carbon, shale from clay, and sandstone from quartz grains.

Evaporites consist of minerals precipitated from water.

Glaciers, streams, alluvial fans, deserts, rivers, lakes ,deltas, beaches, shallow seas, and deep seas each accumulate a different ,distinctive assemblage of sedimentary strata.

Thick piles of sedimentary rocks accumulate in sedimentary basins, regions where the lithosphere subsides.

Transgression occur when sea level rises and the coastline migrates inland. Regressions occur when sea level falls and the coastline migrates seaward.

Diagenesis involves processes leading to lithification and processes that alter sedimentary rock at temperatures lower than those that cause metamorphism.

7 Metamorphism: A Process of Change

Metamorphism refers to changes in a rock that result in the formation of a metamorphic mineral assemblage, and/or metamorphic texture, in response to change in temperature and/or pressure, to the application of differential stress, and to interaction with hydrothermal fluids(hot water solutions).

Metamorphism involves recrystallization, phase changes, metamorphic reactions(neocrystallization), pressure solution, and/or plastic deformation. If hydrothermal fluids bring in or remove elements, we say that metasomatism has occurred.

Metamorphic foliation can be defined either by preferred mineral orientation (aligned inequant crystals) or by compositional banding. Preferred mineral orientation develops where differential stress causes the compression and shearing of a rock so that its inequant grains align parallel with each other.

Geologists separate metamorphic rocks into two classes, foliated rocks and nonfoliated rocks.

The class of foliated rocks includes slate, phyllite, schist, and gneiss. The class of nonfoliated rocks includes hornfels, quartzite, and marble, Migmatite is a mixture of igneous and metamorphic rock.

Rocks formed under relatively low temperatures are known as low-grade rocks, whereas those formed under high temperatures are known hig-grade rocks. Intermediate-grade rocks develop between these two extremes. Different metamorphic mineral assemblages form at different grades.

Geologist track te distribution of different grades of rock by looking for index minerals. Isograds indicate the location at which index minerals fist appear. A metamorphic zone is the region between two isograds.

A metamorphic facies is a group of metamorphic mineral assemblages that develop under a specified range of temperature and pressure conditions.

Thermal (contact) metamorphism occurs in an aureole surrounding an igneous intrusion. Burial metamorphism occurs at depth in a sedimentary basin. Dynamically metamorphosed rocks form along faults. Dynamothermal (regional) metamorphism results when rocks undergo heating and shearing during mountain building. Hydrothermal metamorphism can take place duo to the circulation of hot water in oceanic crust at mid-ocean ridges. Shock metamorphism happens during the impact of a meteorite.

We find belts of metamorphic rock in mountain ranges. Blueschist forms in accretionary prisms. Shields expose broad areas of Precambrian metamorphic rocks.

8 A Violent Pulse:Earthquakes

Earthquakes are episodes of ground shaking. Earthquake activity is called seismicity.

Most earthquakes happen when rock slips during faulting. The place where rock breaks and earthquake energy s released is called the hypocenter, and the point on the ground directly above the focus is the epicenter.

Active faults are faults on which movement is likely. Inactive faults ceased being active long ago. Displacement on active faults that intersect the ground surface may yield a fault scarp.

During fault formation, rock elastically bends, then ruptures. When this happens,the rock vibrates, and this generates an earthquake.

Faults exhibit stick-slip behavior, in that they move in sudden increments. Most earthquakes happen when stress overcomes friction on a preexisting fault, and the fault slips again.

Earthquake energy travel in the form of seismic waves, Body waves, which pass through the interior of the Earth, include P-waves and S-waves. Surface waves, which pass along the surface of the Earth, include R-waves and L-waves.

We can detect earthquake waves by using a seismometer. Seismograms, records of earthquakes, demonstrate that different earthquake waves at different velocities. Using the different between P-wave and S-wave arrival times, seismologists can pinpoint the epicenter location.

The Mercalli Intensity scale is based primarily on documenting the damage caused by an earthquake. Magnitude scales are based on measuring the amount of ground motion, as indicated by a seismogram.

An $M_w$8 earthquake yields about 10 times as much ground motion as $M_w$7 earthquake, and 32 times as much energy.

Most Earthquakes occur in seismic belts, of which the majority lie along plate boundaries. Intraplate earthquakes happen in the interior of plates.

Earthquake damage results fro ground shaking, landslides, sediment liquefaction, fire, and tsunamis.

Seismologists predict that earthquakes are ore likely in seismic zones thant elsewhere, and can determine the recurrence interval for great earthquakes. But it may never be possible to pinpoint the exact time and place at which an earthquake will happen.

Earthquake hazards can be reduced with better construction practices and zoning, and by educating people about what to do during an earthquake.

9 Crags,Cracks, and Crumples:Crustal Deformation and Mountain Building

Mountains occur in linear ranges called orogens. An orogen forms during an orogeny, or mountain-building event.

Mountain building causes rocks to bend, break, shorten, stretch, and shear. Because of such deformation, rocks can change their location, orientation, and shape.

During brittle deformation, rocks break into pieces. During ductile deformation, rocks change shape without breaking.

Rocks undergo three kinds of stress: compression, tension, and shear. Strain refers to the way rocks change shape when subjected to a stress.

Deformation results in the development of geologic structures.

Joints are natural cracks in rocks, formed in response to tension under brittle conditions. Veins develop when minerals precipitate out of water passing through cracks.

Faults are fractures on which there has been shear. Geologists distinguish among normal, reverse, strike-slip, and oblique-slip faults.

Folds are curved layers of rocks. Anticlines are Arch-like, synclines are trough-like, monoclines resemble the shape of a carpet draped over a stair step, basins are shaped like a bowl, and domes are shaped like an overturned bowl.

Tectonic foliation forms when grains flatten, rotate, or grow so that they align parallel with one another.

Uplift in mountains, over broad regions, is controlled by the isostasy, meaning that the elevation of the Earth’s surface reflects the level at which lithosphere naturally floats.

Large collisional mountain ranges are underlain by buoyant roots and contain folds, faults, and foliations.

With modern GPS technology, it is now possible to measure the slow shortening and uplift of mountains.

Once uplifted, mountains are sculpted by erosion. When crust thickens during mountain building, the deep crust eventually becomes weak, leading to orogenic collapse.

Mountain belts formed by convergent-margin tectonism may incorporate accreted terranes.

Cratons are the old, relatively stable parts of continental crust. They include shields and platforms. Broad regional domes and basins form in platform areas.

10 Deep Time: How old is the old?

Geologic time refers to the time span since the Earth’s formation.

Relative age specifies whether one geologic feature is older than or younger than another; numerical age provides the age of a geologic rock or feature in years.

Using such principles as uniformitarianism, original horizontality, superposition, and cross-cutting relations, we can construct the geological history of a region.

The principle of fossil succession states that the assemblage of fossils in strata changes from base to top of a sequence, Once a species becomes extinct, it never reappears.

Strata are not necessarily deposited continuously at a location. An interval of nondeposition and/or erosion is called an unconformity. Geologists recognize three kinds: angular unconformity, nonconformity, and disconformity.

A stratigraphic column shows the succession of strata in a region. A given succession of strata that can be traced over a fairly broad region is called stratigraphic formation. The process of determining the relationship between strata at on location and strata at another is called correlation. A geologic map shows the distribution of formations and geologic structures.

A composite chart that represents the entirety of geologic time is called the geologic column. The column’s largest subdivisions.each of which represent a specific interval of time,are eons. Eons are subdivide into eras,eras into periods,and periods into epochs.

The numerical age of rocks can be determined by isotopic (radiometric) dating. This is because radioactive elements decay at a rate characterized by a known half-life.

The isotopic age of a mineral specifies the time at which the mineral cooled below a closure temperature. We cab use isotopic dating to determine when an igneous rock solidified and when a metamorphic rock cooled. To date sedimentary strata, we must examine cross-cutting relations with dated igneous or metamorphic rock.

Other methods for dating materials include counting growth rings in trees and seasonal layers in glaciers.

Isotopic dating indicates that the Earth is 4.57 billion years old.

11 A Biography of Earth

The Earth formed about 4.57 billion years ago. For part of the first 600 million years, the Hadean Eon, the planet was so hot that its surface was a magma ocean.

The Archean Eon began about 3.85 Ga, when the first continental crust that still remains formed. This crust assembled out of volcanic arcs and hot-spot volcanoes that were too buoyant to subduct. The atmosphere contained very little oxygen, and the first life forms–bacteria and archaea–appeared.

In the Proterozoic Eon, which began at 2.5 Ga, Archean cratons sutured together to form large cratons. Photosynthesis added oxygen to the atmosphere. By the end of the Proterozoic, soft-bodied marine invertebrates populated the planet, and continental crust had accumulated to form a supercontinent.

As the Paleozoic Era began, rifting yielded several separate continents. Sea level rose and fell, depositing sequences of strata in continental interiors. Continents coalesced again to from another supercontinent, Pangaea. Early Paleozoic evolution produced many invertebrates with shells, and jawless fish. Land plants and insects appeared in the middle Paleozoic, and by the end of the eon, there were land reptiles and gymnosperm trees.

In the Mesozoic Era, Pangaea broke apart and the Atlantic Ocean formed. Convergent-boundary tectonics dominated along the western margin of North America. Dinosaurs became prominent land animals through the Mesozoic Ear. During the Cretaceous Period, the continents flooded. Angiosperms appeared, along with modern fish. A huge mass extinction event wiped out the dinosaurs at the end of the Cretaceous Period, probably due to the impact of a large meteorite.

In the Cenozoic Era, the collision of Africa and India with Asia and Europe formed the Alpine-Himalayan orogen. Convergent tectonics persisted along the margin of South America, creating the Andes,but ceased in North America when th San Andreas Fault formed. Rifting in the western United States produced the Basin and Range Province. Various kinds of mammals filled niches left vacant by the dinosaurs, and the human genus, Homo, appeared and evolved through the Pleistocene Ice Age.

12 Riches in Rock: Energy and Mineral Resources

Energy resources come in a variety of forms: energy directly from the Sun; energy from tides, flowing water,or wind; energy from chemical reaction; energy from nuclear fission; and geothermal energy from Earth’s internal heat.

Oil and gas are hydrocarbons formed from the organic leftover of plankton, which settle out and become incorporated in black organic shale. Chemical reactions at elevated temperatures convert the organic matter to kerogen and then oil.

To form an oil reserve, oil must migrate from a source rock into a reservoir rock. The subsurface configuration of strata that hold oil is called an oil trap.

Substantial volumes of hydrocarbons also exist in tar sand, gas, shale, and oil shale.

For coal to form, abundant plant debris must be deposited in an oxygen-poor environment. Compaction creates peat that, when buried deeply and heated, transforms into coal.

Geologists rank coal based on the amount of carbon it contains. Coal occurs in beds, and can be mined by either strip mining or underground mining.

Nuclear power plants generate energy by using the heat released from the fission of uranium.

Nuclear reactors must be carefully controlled to avoid overheating or meltdown. The disposal of radioactive nuclear waste can create environmental problems.

Geothermal energy uses Earth’s internal heat to transform groundwater into steam that drives turbines; hydroelectric power uses the potential energy of water; and solar energy uses solar cells to convert sunlight ot electricity.

We now live in the Oil Age, but oil supplies may last for only another century, and use of energy resources has many environmental consequences.

Industrial societies use many types of minerals, all of which must be extracted from the upper crust.

Metals come from ore. An ore is a rock containing native metals or ore minerals in sufficient quantities to be worth mining.

Magmatic ore deposits from when ore minerals settle to the floor of a magma chamber. In hydrothermal deposits, ore minerals precipitate from hot-water solutions. Secondary enrichment deposits from groundwater carries metals away from a preexisting deposit. MVT deposits precipitate from groundwater. Sedimentary deposits settle out of water. Residual mineral deposits are the result of severe leaching. Placer deposits develop when metal grains accumulate in sediment.

Nonmetallic resources include dimension stone, crushed stone, clay, sand, and many other materials. A large proportion of materials in your home have a geological ancestry.

Mineral resources are nonrenewable. Many are now or may soon be in short supply.

13 Unsafe Ground: Landslides and Other Mass Movements

Rock and regolith on unstable slopes has the potential to move downslope under the influence of gravity. This process, called mass movement, or mass wasting, play an important role in the erosion of hills and mountains.

Slow mass movement, caused by the freezing and thawing of regolith, is called creep. In places where slopes are underlain with permafrost, solifluction causes a melted layer of regolith to flow down slopes. During slumping, a semicoherent mass of material moves down a spoon-shaped failure surface. Mudflows and debris flows occur where regolith has become saturated with water and moves downslope as a slurry.

Rock and debris slides move very rapidly down a slope; the rock and debris breaks apart and tumbles. During avalanches, snow or debris mixes with air and moves downslope as a turbulent cloud. And in a debris fall or rockfall, the material free-falls down a vertical cliff.

Large mass movement can take place on underwater slopes. Some generate tsunamis.

Intact, fresh rock in usually too strong to undergo mass movement. Thus, for mass movement to be possible, rock must be weakened by fracturing or weathering.

Unstable slopes start to move when the downslope force exceeds the resistance force that holds material in place. The steepest angle at which a slope of unconsolidated material can remain without collapsing is the angle of repose.

Downslope movement can be triggered by shocks and vibrations, changes in the steepness of a slope, removal of support from the base of the slope, changes in the strength of a slope, deforestation,weathering, or heavy rain.

Geologists can sometimes detect unstable ground before it begins to move, and they produce landslide-potential maps to identify areas susceptible to mass movement.

Engineers can help prevent mass movements by using a variety of techniques to stabilize slopes.

14 Streams and Floods: The Geology of Running Water

Streams are bodies of water that flow down channels and drain with land surface. They grow by downcutting and headward erosion. Streams carry water out of a drainage basin; a drainage divide separates two adjacent catchments. Drainage networks consist of many tributaries that flow into a trunk stream.

Permanent streams exist where the water table lies above the bed of the channel or when large amounts of water enter the channel from upstream. Where the water table lies below the channel bed, streams are ephemeral.

The discharge of a stream is the total volume of water passing a point along the bank in a second. Most streams are turbulent, meaning that their water swirls in complex patterns.

Stream erode the landscape by scouring, lifting, abrading, and dissolving. The resulting sediment provides dissolved loads, suspended loads, and bed loads. The total quantity of sediment carried by a stream is its capacity. Capacity differs from competence, the maximum particle side a stream can carry. When stream water slows, it deposits alluvium.

The longitudinal profile of a stream is concave up, Typically, a stream has a steeper gradient at its headwaters than near its mouth.

Stream cut valleys or canyons, depending on the rate of downcutting relative to the rate at which the slopes on either side of the stream undergo mass wasting. The amount of downcutting is limited by the base level. Where a stream flows down steep gradients and has a bed littered with large rocks, rapids develop, and where a stream plunges off a vertical face, a waterfall forms.

A meandering stream wanders back and forth across a floodplain. It erodes its outer bank and deposits a point bar on the inner bank. Eventually, a meander maybe cut off and turn into an oxbow lake. Natural levees from either side of the river channel.Braided streams consist of many entwined channels.

Where streams or rivers flow into standing water, they deposit deltas. Different deltas have different shapes.

Fluvial erosion can bevel landscape to nearly flat plain. If the base level drops or the land surface rises, stream rejuvenation causes the stream to start downcutting. The headward erosion of on stream may capture the flow of another.

If an increase in rainfall or spring melting causes more water to enter a stream than the channel can hold, a flood results. Some floods are seasonal and submerge floodplains or delta plains. Flash floods happen very rapidly. Officials try to prevent floods by building reservoirs abd levees.

Rivers are becoming a vanishing resource because of pollution, damming, and overuse of water.

15 Restless Realm: Oceans and Coasts

The landscape of the sea floor depends on the character of the underlying crust. Wide continental shelves form over passive-margin basins. Continental shelves are cut by submarine canyons. Abyssal plains develop on old, cool oceanic lithosphere, Seamounts form above hot spots.

The salinity, temperature, and density of seawater vary with location and depth.

Water in the ocean circulates in currents, Surface currents are driven by the wind and are deflected by the Coriolis effect. The vertical upwelling and downwelling of water create deep currents. Some of this movement is thermohaline circulation.

Tides – the daily rise and fall of sea level – are caused by a tide-generating force. The largest contribution to this force comes from the gravitational pull of the Moon.

Waves are caused by friction where the wind shears across the surface of the ocean. Water particles follow a circular motion in a vertical plane as a wave passes. Waves refract when they approach the shore because of frictional drag with the sea floor.

Sand on beaches moves with the swash and backwash of Waves. If there is a longshore current, the sand gradually moves along the beach and may build spits.

At rocky coasts, waves grind away at rocks,yielding such features as wave-cut benches and sea stacks. Some shores are wetlands, where marshes or mangrove swamps grow. Coral reefs grow along coasts in warm, clear water.

The differences in coasts reflect their tectonic setting, Whether sea level is rising of falling, sediment supply, and climate.

To protect beach property,people build groins, jetties, breakwaters, and seawalls.

Human activities have led to the pollution of coasts. Reef bleaching has become dangerously widespread.

Hurricanes produce winds of between 119 and 300 kn per hours. The force of the winds, along with surge and heavy rains, can destroy coastal areas.

16 A Hidden Reserve: Groundwater

During the hydrologic cycle, water infiltrates the ground and fills the pores and cracks in rock and sediment. This subsurface water is called groundwater. The amount of open space in rock or sediment is its porosity; and the ease with which water can flow through is its permeability.

Aquifers are relatively permeable, and aquitards are relatively impermeable.

The water table is the surface in the ground above which pores contain mostly air, and below which pores are filled with water. The shape of water table is a subdued imitation of the shape of the overlying land surface.

Groundwater flows wherever the water table has a hydraulic gradient, and moves from recharge areas to discharge areas. The velocity of flow depends on permeability and the hydraulic gradient.

Groundwater contains dissolved ions, which may come out of solution to form the cement in clastic rocks and veins.

Groundwater can be extracted in wells. An ordinary well penetrates below the water table, but in an artesian well, water rise on its own. Pumping water out of a well too fast causes drawdown, yielding a cone of depression. At a spring, groundwater exist the ground on its own.

Hot springs and geysers release hot water to the Earth’s surface. This water may have been heated by residing very deep in the crust, or by the proximity of a magma chamber.

Groundwater is a precious resource, used for municipal water supplies, industry, and agriculture. In recent years, some regions have lost their groundwater supply because of overuse or contamination.

When limestone dissolves just below the water table, underground caves develop. Soluble beds and joints determine the location and orientation of caves. If the water table drops, caves empty out. Limestone then precipitates out of water dripping from cave roofs, and creates speleothems.

Regions where abundant caves have collapsed to form sinkholes are called karst landscape. These terrains can contain sinkholes, natural bridges, and disappearing streams.

17 Dry Regions: The Geology of Deserts

Deserts generally receive less than 25cm of rain per year. Vegetation covers less than 15% of their surface.

Subtropical deserts from between latitudes of $20^\circ$ and $30^\circ$, rain-shadow deserts are found on the inland side of mountain ranges, coastal deserts are located on the land adjacent to cold ocean current, continental-interior deserts exist in landlocked regions far from the ocean, and polar deserts from at high latitudes.

Physical weathering in deserts produces rocky debris. Chemical weathering happens slowly; it produces ions that precipitate as new minerals below the surface.

Water causes significant erosion in deserts, mostly during heavy downpours. Flash floods carry large quantities of sediments down ephemeral streams. When rain stops, streams dry up, leaving steep-sided dry washes.

Wind picks up dust and silt as suspended load, and it causes sand to saltate. Where wind blows away finer sediment, a lag deposit remains. Windblown sediment abrades the ground, creating a variety of features such as ventifacts.

Desert pavements are mosaics of varnished stones armoring the surface of the ground.

Talus aprons form when rock fragments accumulate at the base of a slope. Alluvial fans form at a mountain front where water in ephemeral streams deposits sediment. When temporary desert lakes dry up, they leave playas.

In some desert landscapes, erosion causes cliff retreat,eventually resulting in the formation of mesas, buttes, and inselbergs. Pediments of nearly flat or gently sloping bedrock surround some inserlbergs.

Where sand is abundant, the wind builds it into dunes. Common types include barchan, star, transverse, parabolic, and longitudinal dunes.

Changing climates and land abuse may cause desertification, the transformation of semiarid land into deserts. Windblown dust, sometimes carrying microbes and toxins, may waft from desert across oceans.

18 Amazing Ice: Glaciers and Ice Ages

Glaciers are streams or sheets of recrystallized ice that survive for the entire year and flow in response to gravity. Mountain glaciers exist in high regions and fill cirques and valleys. Continental glaciers spread over substantial areas of the continents.

Glaciers form when snow accumulates over a long period of time. With progressive burial, the snow first turns to firn and then to ice.

Temperate glaciers melt during part of the year. Polar glaciers do not. Glaciers move by basal sliding over water or wet sediment, and by plastic deformation of ice grains. In general, glaciers move tens of meters per year.

Glaciers move because of gravitational pull; they flow in the direction of their overall surface slope.

Whether the toe of a glacier stays fixed in position, advances, or retreats depends on the balance between the rate at which snow builds up in the zone of accumulation and the rate at which glaciers melt, calve, or sublimate in the zone of ablation.

Icebergs break off glaciers that flow into the sea. Continental glaciers that flow into the sea along a coast make ice shelves. Sea ice forms where the ocean’s surface freezes.

As glacial ice flows over sediment, it incorporates clasts. The clasts embedded in glacial ice abrade the substrate.

Mountains glaciers carve numerous landforms, including cirques, aretes, horns, U-shaped valleys, hanging valleys, and truncated spurs. Fjords are glacially carved valleys that filled with water when sea level rose after an ice age.

Glaciers can transport sediment of all sizes. Glacial drift includes till, glacial marine, glacial outwash, lake-bed mub, and loess. Lateral moraines accumulate along the sides of valley glaciers, medial moraines form down the middle, and end moraines accumulate at a glacier’s toe.

Glacial depositional landforms include moraines, knob-and-kettle topography, drumlins, kames, eskers, meltwater lake, and outwash plains.

Continental crust subsides as a result of ice loading. When the glacier melts away, the crust rebounds.

When water is stored in continental glaciers, sea level drops. When glaciers melt, sea level rises.

During past ice ages, the climate in regions south of the continental glaciers was wetter, and pluvial lakes formed. Permafrost exists in periglacial environments.

During the Pleistocene Ice Age, large continental glacier covered much of North America, Europe, and Asia.

The stratigraphy of Pleistocene glacial deposits indicates that glaciers advanced and retreated many times during the ice age. The record of glaciations is more complete in oceanic sediment.

Long-term causes of ice ages include plate tectonics and changes in the concentration of $co_2$ in the atmosphere. Short-term causes include the Milankovic cycle.

19 Global Change in the Earth System

We refer to the global interconnecting web of physical and biological phenomena on Earth as the Earth System. Gloabl change involves the transformations or modifications of physical and biological components of the Earth System through time. Unidirectional change results in transformations that never repeat, whereas cyclic change involves repetition of the same steps over and over.

Examples of unidirectional change include the gradual evolution of the solid Earth from a homogeneous collection of planetesimals to layered planet, the formation of the oceans and the gradual change in the composition of the atmosphere, and the evolution of life.

Examples of physical cycles that take place on Earth include the supercontinent cycle, the sea-level cycle, and the rock cycle.

A biogeochemical cycle involves the passage of a chemical among nonliving and living reservoirs. Examples include the hydrologic cycle and the carbon cycle. Global change occurs when factors change the relative proportions of the chemical in different reservoirs.

Tools for documenting global climate change include the stratigraphic record, paleontology, oxygen-isotope ratios, bubbles in ice, growth rings in trees, and human history.

Studies of long-term climate change show that, at times in the past, the Earth experienced greenhouse periods; at other times, there were icehouse periods. Factors leading to long-term climate change include the positions of continents, volcanic activity, the uplift of land, and the addition or removal of $CO_2$, an important greenhouse gas.

Short-term climate change can be seen in the record of the last million years. In fact, during only the past 15,000 years, we see that the climate has warmed and cooled a few times. Causes of short-term climate change include fluctuations in solar radiation and cosmic rays, changes in Earth’s orbit and tilt, changes in reflectivity, and changes in ocean currents.

Mass extinction, a catastrophic change in biodiversity, may be caused by the impact of a comet or asteroid or by intense volcanic activity associated with a superplume.

During the last two centuries, humans have changed landscapes; modified ecosystems; and added pollutants to the land, air, and water at rates faster than the Earth System can process.

The addition of $CO_2$ and $CH_4$ to the atmosphere appears to be causing global warming, which could shift climate belts and lead to rise in sea level. Sources for the added $CO_2$ include fossil fuel burning, cement production, and deforestation.

In the future, in addition to climate change, the Earth will witness a continued rearrangement of continents resulting from plate tectonics and will likely suffer the impact of asteroids and comets, The end of the Earth may come in about 5 billion years, when the Sun runs out of fuel and becomes a red giant.