Chapter Four - Page 1 of 8 Chapter Four: The Earth's Interior ⢠The ...
Chapter Four - Page 1 of 8 Chapter Four: The Earth’s Interior
The only rocks that can be studied are those of the Earth’s crust Mantle rocks brought to the earth’s surface in basalt flow, in diamond bearing kimberlite pipes, and also by the tectonic attachment of lower parts of the oceanic lithosphere to the continental crust Meteorites give clues about the possible composition of the core of the earth Geophysics evidence suggest that the earth is divided into 3 major layers: crust on the surface, rocky mantle beneath the crust, and the metallic core at the centre The crust and uppermost mantle can be divided into the brittle lithosphere and the plastic asthenosphere Kola peninsula - The deepest scientific well has reached 12km beneath the surface (sedimentary basins) in Russia. It penetrated ancient Precambrian basement rocks Earth has a radius of 6.370km Deep parts of the earth are studied indirectly through geophysics – the application of physical laws and principles to a study of the earth; includes the study of seismic waevs and the earth’s magnetic field, gravity and heat
Deep Drilling on Continents
Structure and composition of most of the continental crust is unknown Continents are probably largely igneous and metamorphic rock (such as granite and gneiss, overlain by a veneer of sedimentary rocks Second deepest well drilled is the KTB hole in southeastern Germany, which reached 10km
WHAT CAN WE LEARN FROM THE STUDY OF SEISMIC WAVES?
Important way of learning about earth’s interior is via study of seismic reflection Seismic reflection: the return of some of the energy of seismic waves to the earth’s surface after the waves bounce off a rock boundary If two rock layers of differing densities are separated by a fairly sharp boundary, seismic waves reflect off that boundary just as light reflects off a mirror These reflected waves are recorded on a seismogram – shows the amount of time the waves took to travel down to the boundary, reflect off it, and return to the surface allows calculation of depth of the boundary Canadian lithoprobe project – applying seismic reflection techniques to map crustal structures at the base of the crust Another method used to locate rock boundaries seismic refraction – the bending of seismic waves as they pass from one material to another. As a seismic waves strikes a rock boundary, much of the energy of the wave passes across the boundary. As the wave crosses from one rock layer to another, it changes direction. This change of direction (refraction) only occurs if the seismic waves velocity is different in each layer (happens if the rock layers differ in density/strength) Seismograph station 1 is receiving seismic waves that pass directly through the upper layer A
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Stations farther from the epicentre, such as station 2, receive seismic waves from 2 pathways: 1. A direct path through layer A 2. A refracted path through layer A to a higher-velocity layer B and back to layer A. Station 2 receives the same wave twice Seismograph stations close to station 1 receive only the direct wave or possibly two waves, the direct (upper) wave arriving before the refracted (lower) wave Stations near station 2 receive both the direct and the refracted waves Even though the refracted wave travels farther, it can arrive at a station first because most of its path is in the high velocity layer B The distance between this point of transformation and the epicentre of the earthquake is a function of the depth to the rock boundary between A and B
Sharp rock boundary isn’t necessary for the refraction of seismic waves Canadian Lithoprobe Project
Investigating the composition and structure of the Canadian shield and surrounding organic belts Aim is to develop a comprehensive understanding of the geological evolution of north America The shield is made up of distinct geological terranes that were once separate land masses but were brought together by the forces of plate tectonics Will help answer how continental configuration came to be/what tectonics were involved Will also help evaluate earthquake risk across the shield and find oil/gas reservoirs Uses seismic reflection Uses large vibroseis trucks (dancing elephants) – work together to stamp in unison
WHAT IS INSIDE THE EARTH?
Three main zones of the earth’s interior: crush, mantle and core Crust: outer layer of rock, which forms a thin skin on earth’s surface Below the crust lies the mantle – a thick shell of rock that separates the crust above from the core below The core is the central zone of earth. It is probably metallic and the source of earth’s magnetic field
The Crust
Studies of seismic waves have shown: o The crust is thinner beneath the oceans than beneath the continents o Seismic waves travel faster in oceanic crust than in continental crust It’s assumed that the two types of crust are made up of different rocks Seismic P waves travel through oceanic crust (and basalt and gabbro) at 7km/second Upper part of the oceanic crust is basalt; lower part is gabbro Oceanic crust thickness = 7km
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Seismic P waves travel slower through continental crust (and granite and gneiss) – 6km/second Continental crust is also called granitic Continental crust consists of a crystalline basement composed of granite, other plutonic rocks, gneisses, and schists, all capped by a layer of sedimentary rocks Felsic – rocks high in feldspar and silicon – continental crust Mafic – rocks high in magnesium and iron (ferric) – oceanic crust Continental crust is thicker than oceanic crust – 30 to 50 km Crust is thickest under young mountain ranges – andes and Himalayas – bulging downward as a mountain root into the mantle Continental crust is less dense than oceanic crust Mohorovicic discontinuity – the boundary that separates the crust from the mantle beneath Mantle lies closer to the earth’s surface beneath the ocean than the continents Project Mohole – use special ships to drill through the oceanic crust and obtain mantle samples Oceanic Crust – basalt underlain by gabbro; 7km Continental Crust – granite, other plutonic rocks, schist, gneiss (with sedimentary rock cover); 20-70km
The Mantle
Assumed to be mostly made of solid rock P waves travel at 8km/s in the upper mantle – suggests a different rock from oceanic or continental crust Best guess about composition of the upper mantle – ultramafic rock such as peridotite o Ultramafic rock is dense igneous rock made up of mostly ferromagnesian minerals such as olivine and pyroxene o Some contain garnet; all lack feldspar The crust and uppermost mantle form the lithosphere o Outer shell of the earth that is strong and brittle o Makes up the plate of plate tectonics theory The lithosphere averages about 70km thick under oceans and 125-250km thick beneath continents Seismic waves generally increase in speed with depth as increasing pressure alters the rock But beginning at 70-125km, seismic waves travel slower than they do at shallow layers – low velocity zone This zone extends to 200km, and is called the asthenospehere The rocks here are closer to their melting point (but not hotter) than those above/below it Melting point controlled by pressure as well as temperature This zone is important for two reasons: o It may represent a zone where magma is likely to be generated o The rocks here may have relatively little strength and therefore are likely to flow Plates of brittle lithosphere move easier over the asthenosphere120
Chapter Four - Page 4 of 8 A CAT Scan of the Mantle
Seismic tomography uses earthquake waves and powerful computers to study planar crosssections of the mantle following large earthquakes Can be used to find temperature variations in the mantle Hot rock slows down seismic waves Cold rock is dense and strong, so it speeds up seismic waves Continents have very deep roots The older the subducting rock is, the colder and denser it is Old, dense plates sink to the mantle’s base Younger plates, being less dense, stop at a depth of 670km
The Core
Seismic wave data provide the primary evidence for the core’s existence Seismic waves don’t reach certain areas on the opposite side of the earth from a large earthquake Seismic P waves spread out from a quake until, at 103 degrees of arc from the epicentre, they suddenly disappear from seismograms At more than 142 degrees from the epicentre, they reappear The region between 103 and 142 degrees, which lacks P waves, is the P-wave shadow zone It can be explained by the refraction of p waves when they encounter the core boundary P waves can travel through solids and fluids S waves can travel only through solids An S-wave shadow zone is larger than a P-wave shadow zone S waves don’t travel through the core at all – implies that the core is liquid (or acts like it) The core has two parts – a liquid outer core and a solid inner core
Diamonds – A window into the mantle
Bulk of earth’s volume lies in the mantle Fragments of mantle materials are brought to the surface via volcanoes Also found in diamond-bearing igneous rocks called kimberlites which form carrot shaped bodies called kimberlite pipes Diamonds are made of a high pressure form of crystalline carbon and pressures which can only be found at depths deep below the crust Hardest known natural material on earth The carbon that forms diamonds is from carbon-bearing rocks on oceanic plates that were subducted at collisional plate margins This carbon transformed into diamond under heat and pressure and was trapped in the mantle under continents Eruption of the kimberlite magmas through volcano-like vents of kimberlite pipes brought diamonds to the surface
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Kimberlite magmas – viscous; large amounts of dissolved gas; frothy; rises to the surface quickly which means the diamonds don’t break Diamonds are not stable at the earth’s surface and will eventually break down into graphite Kimberly diamonds – 3.3 billion years old Diamond bearing kimberlites only found on the oldest parts of continents Kimberlite pipes range in age from Precambrian to cretaceous First commercial diamond mine in north America – Ekati mine – 300km northeast of Yellowknife Ekati mine – projected lifespan of 25 years – more than 2 million carats each year Other diamond bearing kimberlite pipes – Ontario, Quebec and in the US Rocky Mountains
Composition of the Core
Made of metal – not silicate rock o Iron (along with a small amount of oxygen, silicon, sulphur, or nickel) Earth’s density = 5.5gm/cm3 Crustal rocks are low density, from 2.7 for granite to 3.0 for basalt The ultramafic rock that makes up the mantle has a density of 3.3 in the upper mantle and 5.5 at the base Choice of iron as the major component comes from study of meteorites o 10% composed for iron mixed with little nickel Existence of earth’s magnetic field suggests a metallic core
Core-Mantle Boundary
Boundary between core and mantle is marked by great changes in seismic velocity, density, and temperature The ultra-low-velocity zone that forms a border at the core-mantle boundary may be due to hot core partially melting overlying mantle rock or could be due to part of the liquid outer core reacting chemically with the adjacent mantle The mantle and core are both undergoing convection o A circulation pattern in which low-density material rises and high density material sinks o Heavy portions of the mantle sink to its base, but are unable to penetrate the denser core o Light portions of the core may rise to its top, and be incorporated into the above mantle
How Does the Elevation of Continents Change
Isostasy – balance of adjacent blocks of brittle crust floating on the upper mantle Crustal rocks can be thought of as tending to rise/sink gradually until they’re balanced by the weight of displaced mantle rocks o This concept of vertical movement to reach equilibrium is called isostatic adjustment Once crustal blocks come into isostatic balance, a tall block (mountain range) extends deep into the mantle (mountain root)
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A column of thick continental crust (mountain + root) has the same weight as a column containing thin crust and some of the upper mantle Example of isostatic adjustment: o Caused by plastic flow – the upward movement of large crust areas since the glacial ages o The weight of the thick continental ice sheets during the Quaternary depressed the crust under the ice o Depression of the crust caused local unwarping around the ice sheet margin (forebulge) o After the melting of the ice, the crust rose back upward, a process still going on in some areas o This rise of the crust after ice removal is known as crustal rebound o Unwarped areas subside to original position during rebound process
What Can Gravity Tell Us About the Earth’s Crust?
Gravity meter – measures the gravitational attraction between Earth and a mass within the instrument o Used to identify relative changes in gravity that may indicate local variations in rock density o Can be used to explore for metallic ore deposits o A cavity or a body of low-density material such as sediment causes a weaker pull on the meter’s mass o Used to discover whether regions are in isostatic equilibrium If a region is balanced, each column or rock has the same mass. If the meter were carried across the columns, it would register the same amount of gravitational attraction for each one Gravity reading higher than the normal regional gravity is called a positive gravity anomaly A region can be held down out of isostatic equilibrium o The mass deficiency produces a negative gravity anomaly – a gravity reading lower than the normal regional gravity Greatest negative gravity anomalies are found over oceanic trenches o Suggests that trenches are actively being held down and are out of isostatic balance
How Does the Earth’s Magnetic Field Change Through Time?
A region of magnetic force – a magnetic field – surrounds Earth The invisible lines of magnetic force surrounding Earth deflect magnetized objects The field has north and south magnetic poles Earth’s field is called dipolar (two poles) Strength of the field is greatest at these poles where magnetic lines of force appear to leave and enter Earth vertically Magnetic field is generated within the liquid metal of the outer core Field is created by electric currents within the liquid outer core Outer core is very hot and flows at a rate of several km/year in large convection currents, about one million times faster than mantle convection above it
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Convecting metal, in the presence of an existing magnetic field, creates electric currents, which in turn could sustain earth’s magnetic field Core is an alectrical conductor o Metals are good conductors, whereas silicate rock is poor Magnetic Reversals
Magnetic reversal – change in the polarity of the field Normal polarity – magnetic lines of force leave Earth near the south polle and re-enter near the north pole Reversed polarity – lines of force run the other way, leaving near the north and entering near the south Many rocks contain a record of the strength/direction of the magnetic field at the time the rocks formed Mineral magnetite – when crystallizing in a cooling lava flow, the atoms within the crystals respond to the field and form magnetic alignments that point toward the north magnetic pole As the lava cools slowly below the Curie point (580C for magnetite), this magnetic record is permanently trapped in the rock Unless its heated again above the Curie point, this magnetic record is retained Sedimentary rocks stained red by iron compounds also record magnetic field directions Paleomagnetism – the study of ancient magnetic fields Some lava flows have a magnetic orientation directly opposite to Earth’s present orientation
Earth’s Spinning Inner Core
Computer model of convection in the outer core that simulates a magnetic field Model utilizes circulating metallic fluids in the outer core, caused by cooling and heat loss, as the driving force of earth’s magnetic field Predicts that solid inner core spins faster than the rest of the planet Previous studies indicated that seismic waves pass through the inner core faster along a nearly north-south route
Magnetic Anomalies
Magnetometer – instrument used to measure the strength of earth’s magnetic field A deviation from average readings of the magnetic field strength is called an anomaly Very broad regional magnetic anomalies may be due to circulation patterns in the liquid outer core Smaller anomalies reflect variations in rock type Positive magnetic anomaly o Reading of magnetic field strength that is higher than the regional average Three geologic structure that can cause this: A body of magnetite ore (highly magnetic ore of iron) has been emplaced in limestone by hot solutions rising along a fracture
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The iron magnetism adds to the magnetic field, ,giving a stronger measurement at the surface A large die of gabbro has intruded into granitic basement rock o Gabbro contains more ferromagnesian minerals than granite, so gabbro is more magnetic Granitic basement high that has influenced later sediment deposits, causing a draping of the layers as the sediments on the hilltop compacted less than the thicker sediments to the sides o Can form an oil trap o Granite in the hill contains more iron than the surrounding rock Negative magnetic anomaly – reading of magnetic field strength that’s lower than the regional average o Can be produced by a down-dropped fault block (a graben) in igneous rock o The thick sedimentary fill above the graben is less magnetic than the igneous rock, so a weaker field develops Linear magnetic anomalies found in oceanic crust are caused by a variation in the direction of magnetism
Magnetotellurics: A New Tool for Investigating the Earth’s Interior
Magnetotellurics: new geophysical approach being used in remote regions of the Arctic to investigate and map structures within the underlying crust and mantle Measurement of very small variations in the earth’s electrical and magnetic fields Can measure the electromagnetic energy released from the sun as in interacts with earth’s materials Used for the search for economic ore bodies, to find geothermal energy, to look for fluids and for groundwater contamination, and to aid in the assessment of earthquake risk
How Hot is the Earth’s Core? What is the Origin of the Earth’s Heat? Geothermal Gradient
Geothermal gradient – the rate of temperature increase with depth into earth Measured by dropping probes into the mud Average temperature increase in 25C per km of depth Some regions have a higher gradient, indicating concentrations of heat at shallow depths Such regions have a potential for generating geothermal energy
Heat Flow
Small but measureable amount of heat from the earth’s interior is being loss gradually through the surface Gradual loss of heat called heat flow
The only rocks that can be studied are those of the Earth’s crust Mantle rocks brought to the earth’s surface in basalt flow, in diamond bearing kimberlite pipes, and also by the tectonic attachment of lower parts of the oceanic lithosphere to the continental crust Meteorites give clues about the possible composition of the core of the earth Geophysics evidence suggest that the earth is divided into 3 major layers: crust on the surface, rocky mantle beneath the crust, and the metallic core at the centre The crust and uppermost mantle can be divided into the brittle lithosphere and the plastic asthenosphere Kola peninsula - The deepest scientific well has reached 12km beneath the surface (sedimentary basins) in Russia. It penetrated ancient Precambrian basement rocks Earth has a radius of 6.370km Deep parts of the earth are studied indirectly through geophysics – the application of physical laws and principles to a study of the earth; includes the study of seismic waevs and the earth’s magnetic field, gravity and heat
Deep Drilling on Continents
Structure and composition of most of the continental crust is unknown Continents are probably largely igneous and metamorphic rock (such as granite and gneiss, overlain by a veneer of sedimentary rocks Second deepest well drilled is the KTB hole in southeastern Germany, which reached 10km
WHAT CAN WE LEARN FROM THE STUDY OF SEISMIC WAVES?
Important way of learning about earth’s interior is via study of seismic reflection Seismic reflection: the return of some of the energy of seismic waves to the earth’s surface after the waves bounce off a rock boundary If two rock layers of differing densities are separated by a fairly sharp boundary, seismic waves reflect off that boundary just as light reflects off a mirror These reflected waves are recorded on a seismogram – shows the amount of time the waves took to travel down to the boundary, reflect off it, and return to the surface allows calculation of depth of the boundary Canadian lithoprobe project – applying seismic reflection techniques to map crustal structures at the base of the crust Another method used to locate rock boundaries seismic refraction – the bending of seismic waves as they pass from one material to another. As a seismic waves strikes a rock boundary, much of the energy of the wave passes across the boundary. As the wave crosses from one rock layer to another, it changes direction. This change of direction (refraction) only occurs if the seismic waves velocity is different in each layer (happens if the rock layers differ in density/strength) Seismograph station 1 is receiving seismic waves that pass directly through the upper layer A
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Stations farther from the epicentre, such as station 2, receive seismic waves from 2 pathways: 1. A direct path through layer A 2. A refracted path through layer A to a higher-velocity layer B and back to layer A. Station 2 receives the same wave twice Seismograph stations close to station 1 receive only the direct wave or possibly two waves, the direct (upper) wave arriving before the refracted (lower) wave Stations near station 2 receive both the direct and the refracted waves Even though the refracted wave travels farther, it can arrive at a station first because most of its path is in the high velocity layer B The distance between this point of transformation and the epicentre of the earthquake is a function of the depth to the rock boundary between A and B
Sharp rock boundary isn’t necessary for the refraction of seismic waves Canadian Lithoprobe Project
Investigating the composition and structure of the Canadian shield and surrounding organic belts Aim is to develop a comprehensive understanding of the geological evolution of north America The shield is made up of distinct geological terranes that were once separate land masses but were brought together by the forces of plate tectonics Will help answer how continental configuration came to be/what tectonics were involved Will also help evaluate earthquake risk across the shield and find oil/gas reservoirs Uses seismic reflection Uses large vibroseis trucks (dancing elephants) – work together to stamp in unison
WHAT IS INSIDE THE EARTH?
Three main zones of the earth’s interior: crush, mantle and core Crust: outer layer of rock, which forms a thin skin on earth’s surface Below the crust lies the mantle – a thick shell of rock that separates the crust above from the core below The core is the central zone of earth. It is probably metallic and the source of earth’s magnetic field
The Crust
Studies of seismic waves have shown: o The crust is thinner beneath the oceans than beneath the continents o Seismic waves travel faster in oceanic crust than in continental crust It’s assumed that the two types of crust are made up of different rocks Seismic P waves travel through oceanic crust (and basalt and gabbro) at 7km/second Upper part of the oceanic crust is basalt; lower part is gabbro Oceanic crust thickness = 7km
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Seismic P waves travel slower through continental crust (and granite and gneiss) – 6km/second Continental crust is also called granitic Continental crust consists of a crystalline basement composed of granite, other plutonic rocks, gneisses, and schists, all capped by a layer of sedimentary rocks Felsic – rocks high in feldspar and silicon – continental crust Mafic – rocks high in magnesium and iron (ferric) – oceanic crust Continental crust is thicker than oceanic crust – 30 to 50 km Crust is thickest under young mountain ranges – andes and Himalayas – bulging downward as a mountain root into the mantle Continental crust is less dense than oceanic crust Mohorovicic discontinuity – the boundary that separates the crust from the mantle beneath Mantle lies closer to the earth’s surface beneath the ocean than the continents Project Mohole – use special ships to drill through the oceanic crust and obtain mantle samples Oceanic Crust – basalt underlain by gabbro; 7km Continental Crust – granite, other plutonic rocks, schist, gneiss (with sedimentary rock cover); 20-70km
The Mantle
Assumed to be mostly made of solid rock P waves travel at 8km/s in the upper mantle – suggests a different rock from oceanic or continental crust Best guess about composition of the upper mantle – ultramafic rock such as peridotite o Ultramafic rock is dense igneous rock made up of mostly ferromagnesian minerals such as olivine and pyroxene o Some contain garnet; all lack feldspar The crust and uppermost mantle form the lithosphere o Outer shell of the earth that is strong and brittle o Makes up the plate of plate tectonics theory The lithosphere averages about 70km thick under oceans and 125-250km thick beneath continents Seismic waves generally increase in speed with depth as increasing pressure alters the rock But beginning at 70-125km, seismic waves travel slower than they do at shallow layers – low velocity zone This zone extends to 200km, and is called the asthenospehere The rocks here are closer to their melting point (but not hotter) than those above/below it Melting point controlled by pressure as well as temperature This zone is important for two reasons: o It may represent a zone where magma is likely to be generated o The rocks here may have relatively little strength and therefore are likely to flow Plates of brittle lithosphere move easier over the asthenosphere120
Chapter Four - Page 4 of 8 A CAT Scan of the Mantle
Seismic tomography uses earthquake waves and powerful computers to study planar crosssections of the mantle following large earthquakes Can be used to find temperature variations in the mantle Hot rock slows down seismic waves Cold rock is dense and strong, so it speeds up seismic waves Continents have very deep roots The older the subducting rock is, the colder and denser it is Old, dense plates sink to the mantle’s base Younger plates, being less dense, stop at a depth of 670km
The Core
Seismic wave data provide the primary evidence for the core’s existence Seismic waves don’t reach certain areas on the opposite side of the earth from a large earthquake Seismic P waves spread out from a quake until, at 103 degrees of arc from the epicentre, they suddenly disappear from seismograms At more than 142 degrees from the epicentre, they reappear The region between 103 and 142 degrees, which lacks P waves, is the P-wave shadow zone It can be explained by the refraction of p waves when they encounter the core boundary P waves can travel through solids and fluids S waves can travel only through solids An S-wave shadow zone is larger than a P-wave shadow zone S waves don’t travel through the core at all – implies that the core is liquid (or acts like it) The core has two parts – a liquid outer core and a solid inner core
Diamonds – A window into the mantle
Bulk of earth’s volume lies in the mantle Fragments of mantle materials are brought to the surface via volcanoes Also found in diamond-bearing igneous rocks called kimberlites which form carrot shaped bodies called kimberlite pipes Diamonds are made of a high pressure form of crystalline carbon and pressures which can only be found at depths deep below the crust Hardest known natural material on earth The carbon that forms diamonds is from carbon-bearing rocks on oceanic plates that were subducted at collisional plate margins This carbon transformed into diamond under heat and pressure and was trapped in the mantle under continents Eruption of the kimberlite magmas through volcano-like vents of kimberlite pipes brought diamonds to the surface
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Kimberlite magmas – viscous; large amounts of dissolved gas; frothy; rises to the surface quickly which means the diamonds don’t break Diamonds are not stable at the earth’s surface and will eventually break down into graphite Kimberly diamonds – 3.3 billion years old Diamond bearing kimberlites only found on the oldest parts of continents Kimberlite pipes range in age from Precambrian to cretaceous First commercial diamond mine in north America – Ekati mine – 300km northeast of Yellowknife Ekati mine – projected lifespan of 25 years – more than 2 million carats each year Other diamond bearing kimberlite pipes – Ontario, Quebec and in the US Rocky Mountains
Composition of the Core
Made of metal – not silicate rock o Iron (along with a small amount of oxygen, silicon, sulphur, or nickel) Earth’s density = 5.5gm/cm3 Crustal rocks are low density, from 2.7 for granite to 3.0 for basalt The ultramafic rock that makes up the mantle has a density of 3.3 in the upper mantle and 5.5 at the base Choice of iron as the major component comes from study of meteorites o 10% composed for iron mixed with little nickel Existence of earth’s magnetic field suggests a metallic core
Core-Mantle Boundary
Boundary between core and mantle is marked by great changes in seismic velocity, density, and temperature The ultra-low-velocity zone that forms a border at the core-mantle boundary may be due to hot core partially melting overlying mantle rock or could be due to part of the liquid outer core reacting chemically with the adjacent mantle The mantle and core are both undergoing convection o A circulation pattern in which low-density material rises and high density material sinks o Heavy portions of the mantle sink to its base, but are unable to penetrate the denser core o Light portions of the core may rise to its top, and be incorporated into the above mantle
How Does the Elevation of Continents Change
Isostasy – balance of adjacent blocks of brittle crust floating on the upper mantle Crustal rocks can be thought of as tending to rise/sink gradually until they’re balanced by the weight of displaced mantle rocks o This concept of vertical movement to reach equilibrium is called isostatic adjustment Once crustal blocks come into isostatic balance, a tall block (mountain range) extends deep into the mantle (mountain root)
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A column of thick continental crust (mountain + root) has the same weight as a column containing thin crust and some of the upper mantle Example of isostatic adjustment: o Caused by plastic flow – the upward movement of large crust areas since the glacial ages o The weight of the thick continental ice sheets during the Quaternary depressed the crust under the ice o Depression of the crust caused local unwarping around the ice sheet margin (forebulge) o After the melting of the ice, the crust rose back upward, a process still going on in some areas o This rise of the crust after ice removal is known as crustal rebound o Unwarped areas subside to original position during rebound process
What Can Gravity Tell Us About the Earth’s Crust?
Gravity meter – measures the gravitational attraction between Earth and a mass within the instrument o Used to identify relative changes in gravity that may indicate local variations in rock density o Can be used to explore for metallic ore deposits o A cavity or a body of low-density material such as sediment causes a weaker pull on the meter’s mass o Used to discover whether regions are in isostatic equilibrium If a region is balanced, each column or rock has the same mass. If the meter were carried across the columns, it would register the same amount of gravitational attraction for each one Gravity reading higher than the normal regional gravity is called a positive gravity anomaly A region can be held down out of isostatic equilibrium o The mass deficiency produces a negative gravity anomaly – a gravity reading lower than the normal regional gravity Greatest negative gravity anomalies are found over oceanic trenches o Suggests that trenches are actively being held down and are out of isostatic balance
How Does the Earth’s Magnetic Field Change Through Time?
A region of magnetic force – a magnetic field – surrounds Earth The invisible lines of magnetic force surrounding Earth deflect magnetized objects The field has north and south magnetic poles Earth’s field is called dipolar (two poles) Strength of the field is greatest at these poles where magnetic lines of force appear to leave and enter Earth vertically Magnetic field is generated within the liquid metal of the outer core Field is created by electric currents within the liquid outer core Outer core is very hot and flows at a rate of several km/year in large convection currents, about one million times faster than mantle convection above it
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Convecting metal, in the presence of an existing magnetic field, creates electric currents, which in turn could sustain earth’s magnetic field Core is an alectrical conductor o Metals are good conductors, whereas silicate rock is poor Magnetic Reversals
Magnetic reversal – change in the polarity of the field Normal polarity – magnetic lines of force leave Earth near the south polle and re-enter near the north pole Reversed polarity – lines of force run the other way, leaving near the north and entering near the south Many rocks contain a record of the strength/direction of the magnetic field at the time the rocks formed Mineral magnetite – when crystallizing in a cooling lava flow, the atoms within the crystals respond to the field and form magnetic alignments that point toward the north magnetic pole As the lava cools slowly below the Curie point (580C for magnetite), this magnetic record is permanently trapped in the rock Unless its heated again above the Curie point, this magnetic record is retained Sedimentary rocks stained red by iron compounds also record magnetic field directions Paleomagnetism – the study of ancient magnetic fields Some lava flows have a magnetic orientation directly opposite to Earth’s present orientation
Earth’s Spinning Inner Core
Computer model of convection in the outer core that simulates a magnetic field Model utilizes circulating metallic fluids in the outer core, caused by cooling and heat loss, as the driving force of earth’s magnetic field Predicts that solid inner core spins faster than the rest of the planet Previous studies indicated that seismic waves pass through the inner core faster along a nearly north-south route
Magnetic Anomalies
Magnetometer – instrument used to measure the strength of earth’s magnetic field A deviation from average readings of the magnetic field strength is called an anomaly Very broad regional magnetic anomalies may be due to circulation patterns in the liquid outer core Smaller anomalies reflect variations in rock type Positive magnetic anomaly o Reading of magnetic field strength that is higher than the regional average Three geologic structure that can cause this: A body of magnetite ore (highly magnetic ore of iron) has been emplaced in limestone by hot solutions rising along a fracture
Chapter Four - Page 8 of 8 o
The iron magnetism adds to the magnetic field, ,giving a stronger measurement at the surface A large die of gabbro has intruded into granitic basement rock o Gabbro contains more ferromagnesian minerals than granite, so gabbro is more magnetic Granitic basement high that has influenced later sediment deposits, causing a draping of the layers as the sediments on the hilltop compacted less than the thicker sediments to the sides o Can form an oil trap o Granite in the hill contains more iron than the surrounding rock Negative magnetic anomaly – reading of magnetic field strength that’s lower than the regional average o Can be produced by a down-dropped fault block (a graben) in igneous rock o The thick sedimentary fill above the graben is less magnetic than the igneous rock, so a weaker field develops Linear magnetic anomalies found in oceanic crust are caused by a variation in the direction of magnetism
Magnetotellurics: A New Tool for Investigating the Earth’s Interior
Magnetotellurics: new geophysical approach being used in remote regions of the Arctic to investigate and map structures within the underlying crust and mantle Measurement of very small variations in the earth’s electrical and magnetic fields Can measure the electromagnetic energy released from the sun as in interacts with earth’s materials Used for the search for economic ore bodies, to find geothermal energy, to look for fluids and for groundwater contamination, and to aid in the assessment of earthquake risk
How Hot is the Earth’s Core? What is the Origin of the Earth’s Heat? Geothermal Gradient
Geothermal gradient – the rate of temperature increase with depth into earth Measured by dropping probes into the mud Average temperature increase in 25C per km of depth Some regions have a higher gradient, indicating concentrations of heat at shallow depths Such regions have a potential for generating geothermal energy
Heat Flow
Small but measureable amount of heat from the earth’s interior is being loss gradually through the surface Gradual loss of heat called heat flow
