Ch 11: Exploring Earth's Interior
What support is there for the hypothesis that Earth's core is made of iron and nickel?
1. These metals are abundant in the universe 2. Iron & nickel are dense enough to explain the mass of the core (1/3 of Earth's total mass) 3. this is consistent with the theory that the core formed by gravitational differentiation (proposed by Emil Wiechert and supported by the discovery of meteorites made almost entirely by iron and nickel)
What is a geotherm?
A geotherm is a curve that describes the geothermal gradient in Earth's interior.
A reasonable hypothesis is that regions where seismic waves speed up are composed of relatively ________ ________ rock, (such as subducted plates), whereas regions where seismic waves slow down are composed of relatively _________, ________ rock (for example, rising plumes).
A reasonable hypothesis is that regions where seismic waves speed up are composed of relatively cool, dense rock, (such as subducted plates), whereas regions where seismic waves slow down are composed of relatively hot, bouyant rock (for example, rising plumes).
Why is the lowest boundary layer of the mantle sometimes described as a "graveyard"?
A region about 300 km thick, it is an area of dense, iron-rich parts of the oceanic crust. It may be an "upside-down version" of tectonics at the Earth's surface, with accumulations of heavy, iron-rich material from "anticontinents" constantly pushed to and from across the core-mantle boundary by convection currents.
Please explain magnetic reversals of the geodynamo
A remarkable behavior of the geodynamo is spontaneous reversals of the magnetic field. The magnetic field reverses its direction at irregular intervals (ranging from tens of thousands to millions of years), exchanging the north and south magnetic poles, as if the magnet were flipped 180 degrees. Earth can reverse its magnetic field spontaneously, purely through internal interactions!
How is the ability to perform spontaneous reversals by the geodynamo an illustration of a fundamental difference between the geodynamo and the dynamos used in power plants?
A steam-powered dynamo is an artificial system engineered by humans to do a particular job. The geodynamo, in contrast, exemplifies a self-organized natural system. It is NOT one whose behavior it determined by external constraints. It emerges from interactions within the system.
Explain in simple terms how the magnetic field varies over time.
As was recorded by British navy navigators, magnetic north was not consistently the same as "true north"; these corrections showed that the north magnetic pole was moving at rates of 5 to 10 degrees per century. We now know that these changes were caused by convective movements deep in Earth's core.
At what depths do the S-wave velocity start to increase?
At depths of about 200 to 400 km, the S-wave velocity again increases with depth. At these depths, pressure increases with depth, but temperature does not rise as rapidly as at the surface.
What can happen when waves pass through more than one type of material? Please use the terms reflected and refracted in your response.
At the boundary between two different materials, some of the waves bounce off—they are reflected—and others are transmitted into the second material. The waves that cross the boundary between two materials are bent—refracted—as their velocity changes from that in the first material to that in the second.
What happens to the S-wave velocity at the core-mantle boundary, and why?
Because of the complete loss of rigidity, the S-wave velocity drops from about 7.5 km/s to ZERO. This is because S-waves cannot be transmitted through liquid.
Why do geologists theorize that ocean depth depends primarily on the age of the seafloor? And does this theory match what has been discovered in seafloor topography?
Because the cooling depth should increase as the square-root of seafloor age, because seafloor that is 40 million years old should have subsided twice as much as seafloor that is only 10 million years old. This theory has matched very well with seafloor topography near the mid-ocean floor ridge crests.
Below 410 km, mantle properties change ( slowly / quickly ) as depth increases, but at a depth of about 660 km, the S-wave velocity abruptly ( decreases / increases again, as a second major phase change in olivine happens to an even more closely packed crystal structure.
Below 410 km, mantle properties change slowly as depth increases, but at a depth of about 660 km, the S-wave velocity abruptly increases again, as a second major phase change in olivine happens to an even more closely packed crystal structure.
Describe the zone below the transition zone.
Below the transition zone (below 660 km), the seismic wave velocities increase gradually. They do not show any more unusual features. This is the lower mantle, and is a relatively homogenous region. It is convecting, and exchanges mass with the upper mantle.
How did Beno Gutenberg (1914) use reflected waves, including PcP and ScS, to expand our understanding of the core-mantle boundary?
Beno Gutenberg used these reflected waves to calculate the travel times from the core-mantle boundary and estimate the depth of the core-mantle boundary, which modern estimates now put at about 2890 km.
What happens to the P waves at about 11,600 km?
British seismologist R.D. Oldham provided the first evidence of a liquid outer core. He showed that P waves encounter the core-mantle at about 11,600 km: their velocity drops by almost a factor of two, and the waves are refracted downward into the core, and emerge at greater angular distances forming a P-wave shadow zone between 105-142 degrees.
What are compressional waves?
Compressional waves (like sound waves) are seismic waves that travel with a push-pull motion, and travel through solids faster than shear waves. They are the first to arrive at a seismographic station: the P waves
How does conduction of heat occur?
Conduction of heat occurs when thermally agitated atoms and molecules jostle one another, mechanically transferring kinetic energy from a hot region to a cool one. Heat is transferred from regions of high temperature to regions of low temperature by this process.
What else does conductive cooling of the lithosphere account for, besides the fact that ocean depth depends primarily on age of the seafloor?
Conductive cooling also accounts for: 1. subsidence of passive continental margins and thermal subsidence basins 2. Why the amount of heat flowing out of oceanic lithosphere is high near spreading centers and decreases as the oceanic lithosphere gets older 3. why the average thickness of the oceanic lithosphere is about 100 km
Define convection
Convection is the mechanical transfer of heat energy that occurs as a heated material expands, rises, and displaces cooler material, which is itself heated and rises to continue the cycle
Which process is more efficient in conducting heat, convection or conduction? Why?
Convection transfers heat more efficiently than conduction, because the heated material itself moves, carrying its heat with it.
What did Danish seismologist Inge Lehmann discover (1936), and why were these new waves labeles PKIKP and PKiKP?
Danish seismologist Inge Lehmann discovered Earth's inner core by observing compressional waves refracted by its outer boundary. She determined it to be about 5150 km deep. Ray paths through the inner core are labeled I: PKIKP: waves Lehmann observed (refraction) PKiKP: waves reflected from the top side of the inner core-outer core boundary (reflection)
What happens to density at the core-mantle zone? And what does this do to the core-mantle boundary?
Density increases by about 4 g/cm3. This large density difference keeps the core-mantle boundary VERY FLAT, and prevents large-scale mixing of the mantle and core.
What is depositional remanent magnetization, and what kinds of rocks tend to take on this form of remanent magnetization?
Depositional remanent magnetization is a weak magnetization of sedimentary rock created by the parallel alignment of magnetic sediment particles in the direction of Earth's magnetic field as they settle and preserved when the sediments are lithified. It is as if these tiny magnets within rocks are compasses pointing in the direction of the magnetic field prevailing at the time of deposition.
Define dipole
Dipole refers to two opposite polarized magnetic poles
What are two ways that earth cools?
Earth cools in two ways: 1. Through the slow transport of heat by conduction, which dominates in the lithosphere 2. Through the more rapid transport of heat by convection, which is more important through most of Earth's interior
What are some of the sources of Earth's internal heat engine?
Earth's internal heat engine is powered by: * kinetic energy released by impacts with planetesimals heated by its outer regions * gravitational energy released by differentiation of the core heating its deep interior * decay of radioactive isotopes in Earth's interior * cooling as heat flows from the hot interior to the cool surface
How is the magnetic field essential for sustaining the planet's biosphere?
Even though the geodynamo operates deep within the core, its magnetic lines of force reach far into outer space, forming a barrier that shields Earth's surface from the damaging radiation of the solar wind. Without the protection of a strong magnetic field, the intense stream of high-energy, electrically charged particles would be lethal to many organisms, including US!
Why is the speed of shear waves in gases and liquids zero?
Gases and liquids have no resistance to shearing. Shear waves cannot propagate through any fluid—air, water, or the liquid iron in Earth's outer core.
How has paleomagnetism been useful to geologists?
Geologists have used paleomagnetism in conjunction with isotopic dating to figure out the time sequence of magnetic reversals over the last 170 million years. They have also used this information to date new rock formations. Paleomagnetic stratigraphy of continental sediments has been used to date sediments containing the remains of predecessors of our species.
Describe heat conduction from the core.
Heat conducted out of the core increases the temperatures as the base of the mantle by as much as 1000 degrees C! The paths of seismic waves passing near the base of the mantle demonstrate a region of exceptional geologic activity. There is decreased seismic wave velocity, showing partial molten region at the mantle boundary, at least in some places.
If the geodynamo stopped producing a magnetic field, what would happen?
If the geodynamo stopped producing a magnetic field, Earth would be bombarded by the solar wind, which would gradually strip away Earth's atmosphere, further degrading the terrestrial environment. This seems to be what happened to Mars. Paleomagnetism in the ancient Martian crust has been detected there, so we know that at one point Mars had an active geodynamo with a strong magnetic field.
What is the reference to a "graveyard" in seismology used by seismic tomographers?
In the seismic tomographic mapping of the oceanic lithosphere (the "Ring of Fire"), there is a red region of relatively low S-wave velocities beneath the the central Pacific Ocean, surrounded by a broad blue ring of higher S-wave velocities that seismologists speculated represent a "graveyard" of cold oceanic lithosphere subducted beneath the Pacific's volcanic island arcs and mountain belts during the last 100 million years or so.
Water resists compression much more than air, so what happens to the speed of sound waves in water? Do they speed up or go down? What about when sound travels through solid material?
In water, the speed of sound waves in water is higher, about 1.5 km/s, because of increased compression. Solid materials are even more resistant to compression, so sound waves travel through them at even higher speeds. For example, sound travels through granit at about 6 km/s ==> 13,500 miles/hour!
What evidence is there to support the hypothesis that the core below the mantle is liquid, but the inner core is solid?
Lehmann first discovered that P waves penetrate to the depths of 5150 km suddenly speed up, indicating the presence of an inner core, a metallic sphere two-thirds the size of the Moon. Seismologists have shown this inner core transmits shear waves; therefore it is solid ==> "a planet within a planet".
What is the nondipole field?
Measurements at Earth's surface have revealed that only 90% of Earth's magnetic field can be described as the simple dipole field. The remaining 10% are referred to as the nondipole field, with a much more complex structure: the size of the nondipole field actually increases relative to the size of the dipole field.
What kinds of materials conduct heat best? worst?
Metal conducts better than plastic, and rock and soil are very poor conductors. Rock conducts heat so poorly that a lava flow takes a long, long time to cool from 1000 degrees C to ground surface temps.
What happens to S-wave velocity near the base of the of the lithosphere? And from there what happens to its velocity?
Near the base of the lithosphere, the S-wave velocity abruptly decreases, beginning a low-velocity zone. This is due to the beginning of the melting temperature of peridotite at 100 km. Even though the amount of melting is small (in some places less than 1%), it is sufficient to decrease the rigidity of rock and slow S-waves.
Is the outer core purely made of iron and nickel, according to the latest hypotheses?
No, a pure iron-nickel alloy is about 10% too dense to match the data for the outer core. The hypothesis is that the core includes minor amounts of some lighter element(s): Oxygen, sulfur, and/or other silicates.
What is the difference in the way P waves travel on the surface of earth versus deep within Earth?
P waves travel much faster through rock deep within Earth than through rock at Earth's surface. This makes sense, because rock subjected to the great pressures in Earth's interior are squeezed into tighter crystal structures. The atoms in these tighter structures would therefore be more resistant to further compression, which would cause P waves to travel through them more quickly.
What happens to the P-wave velocity at the core-mantle boundary? What are these zones called?
P-wave velocity drops from more than 13 km/s to about 8 km/s, resulting in the core shadow zones.
Seismologists have developed a simple labeling system for the various ray paths taken by seismic waves. Please define P, S, PcP, ScS, PP and SS waves.
P: Compressional wave refracted/reflected from Earth's surface S: Shear wave refracted/reflected from Earth's surface PP: Compressional wave reflected once at Earth's surface SS: Shear wave reflected once at Earth's surface PcP waves: compressional waves reflected from the top of the outer core ScS waves: shear waves reflected from the top of the outer core
Please define PKP. And why do we use "K" instead of "C"?
PKP: a compressional wave that propagates through the crust and mantle, into the outer core, and back through the mantle and crust to a seismograph at Earth's surface. We use "K" instead of "C" because the German word for core begins with a "K" :).
Define paleomagnetism. How has paleomagnetism helped us in understanding Earth's history?
Paleomagnetism—the geologic record of ancient magnetization—has provided crucial information for understanding Earth's history by confirming the existence of seafloor spreading, and the best data for tracking plate movements since the breakup of Pangaea 200 million years ago. Paleomagnetic data from old continental rocks have been essential for establishing the existence of earlier supercontinents, including Rodinia.
REVIEW (thickness of crust): Earth's crust is _________ under the oceans, ___________ under the stable, flat-lying continents, and ___________ under the high mountains.
REVIEW (thickness of crust): Earth's crust is thin (@ 7 km) under the oceans, thicker under the stable, flat-lying continents, and thickest (up to 70 km!) under the high mountains.
What happens to the velocity of P-waves at the Moho (the Mohorovičić discontinuity)? What does this indicate about the mantle below the Moho?
Recall that P-wave velocities at the upper part of the continental crost is made mostly of low-density granitic rocks (@ 6 km/s). The crust on the deep seafloor consists entirely of basalt and gabrro (@ 7 km/s) ==> the velocity of P waves increases abruptly to 8 km/s at the Moho which marks the base of the crust. That velocity indicates that the mantle below the Moho is made primarily of dense peridotite!
Besides finding deeply buried oil and gas reservoirs, what else has seismic exploration been helpful for?
Reflected seismic waves are also used to measure the depth of water tables and the thickness of glaciers. Additionally, oceanographic ships routinely use the underwater sound produced by compressional waves to measure the depth of the ocean and the thickness of sediments on the seafloor.
Rough values for P-wave velocities in igneous rocks are as follows: Felsic rocks (typ. of upper crust: granite): _____ km/s Mafic rocks (typ. of oceanic crust/lower cont. crust: gabbro): _____ km/s Ultramafic rocks (typ. of upper mantle: peridotite): _____ km/s
Rough values for P-wave velocities in igneous rocks are as follows: Felsic rocks (typ. of upper crust: granite): 6 km/s Mafic rocks (typ. of oceanic crust/lower cont. crust: gabbro): 7 km/s Ultramafic rocks (typ. of upper mantle: peridotite): 8 km/s
What happens to the S-wave velocity at about 400 km below the surface? Why?
S-wave velocity increases about 10% at about 400 km below the surface. This jump in S-wave is explained by a phase change in olivine, the major mineral constituent of the upper mantle, whose crystal structure is transformed into a denser, more closely packed structure at high pressures ==> more compact arrangement at temps and pressures at depths of about 410 km.
What type of energy has been harnessed to illuminate Earth's deepest regions, allowing us to construct three-dimensional images of geologic features in the lower crust, the rising and falling of convection currents in the mantle, and the workings of the outer and inner core?
SEISMIC ENERGY!
What is seismic profiling?
Seismic profiling is using seismic waves to probe the shallow parts of earth's crust for a variety of practical purposes, including finding deeply buried oil and gas reservoirs.
Do seismic ray paths travel in straight lines?
Seismic ray paths do NOT travel straight lines. Rather, they are refracted and reflected by Earth's layered structure.
What is seismic tomography?
Seismic tomography is the use of seismic waves to sweep Earth's interior in many different directions and construct three-dimensional images of what's inside
Why do seismic wave velocities vary through different rock types?
Seismic wave velocities vary through different rock types because they depend on a rock's density and its resistance to compression and shear, which depend on chemical composition and crystal structure. In general, higher densities correspond to higher P-wave velocities.
How are seismic waves generated for seismic profiling?
Seismic waves are generated by artificial sources, including dynamite exlosions on land and compressed-air explosions at sea, and are reflected by geologic structures at shallow depths in the crust.
About what temperature do geologists estimate is the temperature of the outer liquid core? How about the solid inner core?
Seismology tells us the outer liquid core is high enough to melt the iron alloy, and therefore likely greater than 3000 degrees C. The solid inner core, also iron-nickel, is hypothesized to be slightly less than 5000 degrees C. However, some geologists believe it may be as high as 6000 to 7000 degrees C
What are shear waves?
Shear waves travel in a side-to-side motion, displacing material at right angles to their path of travel and arrive second at seismographic stations: the S waves.
What are some evidences of Earth's internal heat that we can see?
Some evidences of Earth's internal heat that we can see include: volcanoes, hot springs, and the elevated temperatures measured in mines and boreholes.
Why do sound waves travel faster through water than through air, and not at all through a vacuum?
Sound waves need something to compress, such as air or water. Without something to compress, they cannot exist. The more force it takes to compress a material, the faster sound will travel through it.
What are three characteristics about matter that both temperature and pressure determine?
Temperature and pressure determine: 1. The state of matter: solid or molten 2. Viscosity: resistance to flow 3. How its atoms are packed together in crystals
What kind of temperature range can be expected near the base of the lithosphere? How is this consistent with partial melting theory?
Temps near the base of the lithosphere range from 1300 degrees C to 1400 degrees C. At these temps, the geotherm rises above the melting point of mantle rock. The geotherm intersects the melting curve about 100 km beneath most oceanic crust, and a bit deeper (150-200 km) beneath most continental crust ==> consistent with the existence of a shear-wave low velocity zone, and widespread evidence suggesting that basaltic magmas are produced by partial melting in the upper part of the asthenosphere
What important discovery in terms of the Earth's magnetism did an Australian graduate student make in the 1960s in an ancient Aborigine campsite?
The Australian graduate student found stones whose magnetism was exactly the reverse of Earth's present magnetic field. He proposed to his professor that, as recently as 30,000 years ago, when the campsite was occupied, the direction of the magnetic field was the reverse of the present one—that a compass needle would have pointed south rather than north.
While the P waves reappear at 142 degrees, what happens to the S waves at the core-mantle boundary?
The S waves never reappear. This is because S waves cannot travel through the outer core, because it is liquid, and liquids have no resistance to shearing. Thus, there is an S-wave shadow zone beyond 105 degrees from the earthquake focus, where S-wave ray paths encounter the core-mantle boundary and do not move beyond.
What forms the geodynamo? And how is the geodynamo different from the simple dipole field?
The geodynamo is formed by rapid convective movements in the liquid, iron-rich, electrically conducting outer core. The magnetic field produced by the geodynamo is more complex than a simple dipole field, as it is constantly changing over time due to these fluid movements.
Why does the geothermal gradient drop with depth in the lower mantle to about .5 degrees C per kilometer?
The geothermal gradient drops to about .5 degrees C per kilometer, as expected in a convecting mantle, because convection mixes cooler material near the top of the mantle with warmer material at greater depths, averaging out the temperature differences (similar to what happens when we stir our bathwater)
What have geologists found is the geothermal gradient per kilometer in most continental crust?
The geothermal gradient is 20 degrees C to 30 degrees C per kilometer in most continental crust
Define the geothermal gradient
The geothermal gradient is the increase in temperature with depth in the Earth's interior
How are both seafloor spreading and plate movements direct evidence of the solid-state convection of mantle rock below the lithosphere behaving as ductile material, flowing like a very viscous fluid?
The hot mantle material that rises under mid-ocean ridges builds new lithosphere, which cools as it spreads away. In time, it sinks back into the mantle at subducton zones, where it is eventually resorbed and reheated. Through this process, heat is carried from Earth's interior to its surface.
The iron-rich material in the outer core has a low viscosity and therefore convects ( with difficulty / very easily ). Convective movement in the outer core distributes heat throughout the core ( inefficiently / efficiently ) and generates Earth's __________ field.
The iron-rich material in the outer core has a low viscosity and therefore convects very easily. Convective movement in the outer core distributes heat throughout the core efficiently and generates Earth's magnetic field.
What is the layer between 410 km and 670 km in depth called, and why?
The layer between 410 km and 670 km in depth is called the transition zone, because there are at least two major phase changes. Phase changes are transitions in a rock's minerology, but not in its chemical composition.
REVIEW: What is the lithosphere? What is the thickness of the lithosphere?
The lithosphere is the rigid layer that forms the plate, and includes both the crust and the upper part of the mantle, including the Moho. While the average thickness of the lithosphere is 100 km, it is highly variable, ranging from almost no thickness in new oceanic spreading centers to over 200 km beneath cold, stable continental cratons.
What does the lower case 'c' in PcP and ScS mean?
The lower case 'c' in PcP and ScS refers to a core reflection
What is the most basic instrument used for sensing Earth's magnetic field?
The most basic instrument used for sensing Earth's magnetic field is the magnetic compass, invented by the Chinese more than 22 centuries ago! In 1600, William Gilbert provided a scientific explanation: He proposed that "the whole Earth is itself a great magnet" whose field acts on a small magnet of a compass needle, aligning it in the direction of the north magnetic pole.
Where does the most extreme change in properties occur anywhere in Earth's interior? What changes here?
The most extreme change in properties occurs at the core-mantle boundary! It is a very sharp interface, where material abruptly changes from solid silicate rock to a liquid iron alloy.
How have scientists use paleomagnetic data to reconstruct the history of Earth's magnetic field?
The oldest magnetized rocks found so far (3.5 billion years old, Australia) show that Earth had a magnetic field similar to the present one at that time. This is consistent with the ideas about Earth's differentiation, showing that a convecting liquid core must have been established very early in Earth's 4.5 billion-year history
Describe the pressure at the center of the planet.
The pressures at Earth's core are immense: over 4 million times the atmospheric pressure at Earth's surface.
What is the primary cause of layering in the upper mantle?
The primary cause of layering in the upper mantle is increased temperature and pressure on peridotite. Olivine and pyroxene undergo partial melting, and at greater depths, increasing pressure forces the atoms of these minerals closer together into more compact crystal structures.
What is the principle of isostasy?
The principle of isostasy (from the Greek "equal standing") states that Earth's crust is floating on the denser, underlying mantle, and that changes in thickness or density in the earth's crust must be compensated by corresponding changes in the earth's mantle. So when a mountain range is formed by the collision of tectonic plates, the added weight of the mountains causes the crust to sink into the mantle; and when material erodes from that mountain range and the weight is reduced, the crust will rise back up to maintain isostatic equilibrium.
What is the speed of sound in air?
The speed of sound in air is Mach 1 (the jargon of jet pilots): about .34 km/s or 760 miles per hour
What does the steep geothermal gradient near Earth's surface tell us about heat transport throught the lithosphere? and what about below the lithosphere, where the temperature does not rise as rapidly?
The steep geothermal gradient near Earth's surface tells us that heat is transported THROUGH the lithosphere by conduction; On the other hand, below the lithosphere, the temperature does not rise rapidly ==> if it did, the temps would be so high that the lower mantle would be entirely molten, which is not consistent with seismological observations.
Where in the earth's layers is the upper mantle? And what is it primarily made of?
The upper mantle extends from the Moho to 410 km, and is made primarily of peridotite, a dense ultramafic rock composed primarily of olivine and pyroxine, minerals that contain less silica and more magnesium and iron that crustal rocks.
What do light, sound, and seismic waves have in common?
The velocity at which light, sound, and seismic waves travel depends on the material through which they are passing.
Reminder: what are magnetic chrons? And what are subchrons?
There are periods of "normal" and reversed magnetic field orientation, which are called "magnetic chrons"—the time interval between polarity reversals of Earth's magnetic field. While these have irregular lengths, on average they last about a half-million years. Superimposed on the chrons are transient, short-lived reversals known as subchrons, lasting anywhere from several thousand years to tens of millions of years
What causes thermoremanent magnetization? And what major theory was this key toward understanding?
Thermoremanent magnetization is caused by heating and cooling, and is "remembered" by the rock long after the magnetizing field has disappeared. It is the same process that magnetizes lava flows and newly formed oceanic crust. This discovery of thermoremanent magnetization in these igneous rock types was a key ingedient in formulating the theory of plate tectonics.
Define thermoremanent magnetization:
Thermoremanent magnetization: Permanent magnetization of magnetizable materials in igneous rocks when groups of atoms of the material align themselves in the direction of the magnetic field that exists when the material is hot and are then locked into place when the material cools below about 500 degrees C.
What did seismologists discover happens after the P and S waves had traveled beyond about 11,600 km?
To their surprise, seismologists discovered that at about 11,600 km, the P and S waves seemed to suddenly disappear!
What is a method that allows us to investigate convection in the outer core?
We can investigate convection in the outer core through observations of Earth's magnetic field.
Why has it been so difficult for seismologists to be able to discover information about convection in the earth's outer core?
We know that like the mantle, Earth's outer core transports most of its heat by convection. But we don't know much else. The outer core has a very low viscosity: its molten material can flow as easily as water or liquid mercury. Even small density variations caused by convection are quickly smoothed out by the rapid flow under the force of gravity. Any lateral variations in seismic wave velocity caused by convection are much too small for us to see using seismic tomography.
What do we now understand about Earth's magnetic field? And what is the geodynamo?
We now understand that the source of the magnetic field is a geodynamo, powered by core convection, not a permanent magnet at Earth's center. The geodynamo: The global geosystem that produces Earth's magnetic field, driven by convection in the outer core
Let's hope our geodynamo keeps on geodynamo-ing!
We thank you, wonderful geodynamo, for faithfully magnetizing and generating for us a protecting field and for giving animals such a fun way to navigate!
Why do we think that many types of organisms—pigeons, sea turtles, whales, butterflies, honey bees, salmon—use the magnetic field for navigation?
We think that because life evolved within a strong magnetic field, the consequences are that many types of organisms evolved sensory systems that use the magnetic field for navigation. Their basic sensors are small crystals of magnetite that become magnetized by Earth's magnetic field as they are biologically precipitated within the organism: these crystals act as tiny compasses to orient the organism within the magnetic field!
What happens to a fluid (liquid OR gas) when it is heated?
When a fluid (liquid OR gas) is heated, it expands and rises, because it has become less dense than the surrounding material.
While both the dipole and nondipole components of Earth's magnetic field are changing over time, which is changing faster?
While both the dipole and nondipole components of Earth's magnetic field are changing over time, the time-related variation for nondipole is fastest; changes in field strength occur on time scales of decades and indicate that fluid movements within the geodynamo are measured in millimeters per second.
