Term
Magnetics utilises the contrast in
magnetic ___________ of rocks |
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Definition
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Term
Earth’s magnetic field does not account
for the total magnetic field |
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Definition
Temporal and spatial deviations in
magnetic intensity and field orientation occur, resulting in discrepancies between the observable magnetic field and the theoretical dipole field |
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Term
Earth’s magnetic field does not account
for the total magnetic field |
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Definition
Variations occur at continental, regional
and local scales and are the result of
terrestrial and extraterrestrial processes |
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Term
The largest contributor to the
geomagnetic field is generated by the
Earth’s ____ |
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Definition
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Term
The main field strength ranges in
magnitude between _____nT at the
magnetic equator to ~65 000nT at the
poles |
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Definition
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Term
| The earth’s magnetic field is maintained by a self exciting dynamo process, generated in response to convection of the _________ outer core material within a weak EM field |
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Definition
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Term
| Heat ___________ within the inner and outer core could provide the driving mechanism for the dynamo |
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Definition
convection
Rotation within the inner core may give rise to changes in the orientation of the magnetic poles |
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Term
Variations in the geomagnetic field
• Geomagnetic field is typically quiet, with only short-wave variations |
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Definition
• May be disturbed by large variations due to the time of day, part of the month and season caused by ionisation in the ionosphere, dependent upon the sun
• Ionisation acts as a magnetic conductor inducing currents produced by flux changes within this region |
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Term
Quiet diurnal variations:
– Observable when there is relatively little variation in output from the sun, eg variations due to the daily _____ data |
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Definition
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Term
| Quiet diurnal variations: |
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Definition
| Daily variations are associated with solar output, monthly variations due to lunar period |
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Term
• Quiet diurnal variations
- Two thirds of the diurnal effects are due to external effects (__________) and the remainder are due to the interaction of the magnetic field and terrestrial conductors |
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Definition
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Term
| Disturbed diurnal variations and transient fields |
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Definition
Periods of high disturbance are associated with
sun and solar activity, eg sunspots |
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Term
| Disturbed diurnal variations and transient fields |
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Definition
| Highly disturbed ionosphere corresponds to magnetic storms |
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Term
| Disturbed diurnal variations and transient fields |
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Definition
| Disturbed diurnal variations describe short term deviations in the field orientation and strength as a result of atmospheric induced magnetic fields generated by the interaction of cosmic rays and ions in the upper atmosphere (response to solar flares and sunspot activity |
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Term
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Definition
Relate to long term (> years) deviations as a result of regular modification or deviations in field direction and strength as a result of changes within the physical model described by the hypothetical model
• Therefore of terrestrial origin |
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Term
Components of the geomagnetic field
– The Earth’s magnetic field is defined
anywhere on the earth’s surface by: |
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Definition
The angle of declination (the angle
between magnetic north and geographic
north) |
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Term
Components of the geomagnetic field
– The Earth’s magnetic field is defined
anywhere on the earth’s surface by: |
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Definition
The angle of inclination (the angle
between the magnetic field and a
horizontal plane at the surface) |
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Term
| The angle of declination is |
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Definition
| the angle between magnetic north and geographic north |
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Term
| The angle of inclination is |
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Definition
| the angle between the magnetic field and a horizontal plane at the surface |
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Term
| Declination (D) is _______ in the east and _______ in the west of geographic north |
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Definition
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Term
| Inclination (I) is _______ in the southern hemisphere and _______ in the northern hemisphere |
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Definition
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Term
| Total intensity (F) is the vector addition of vertical horizontal _______ in the geomagnetic field |
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Definition
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Term
| The geomagnetic field is constantly changing due to ________ and ________ variations, therefore needs to be continually checked and updated |
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Definition
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Term
Geomagnetism and palaeomagnetism
• Induced magnetism: – Within a magnetic field, crystallisation of a
magnetic mineral through the Curie temperature allows for the development of ______ magnetism |
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Definition
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Term
Geomagnetism and palaeomagnetism
• Induced magnetism: Any magnetic material placed within the
_______ field will have poles induced upon its surface in the direction of the primary field |
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Definition
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Term
Geomagnetism and palaeomagnetism
• Induced magnetism: The strength and direction of the primary field may
be distorted or overprinted by secondary magnetism. The resultant field is called the _______, or remnant magnetism |
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Definition
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Term
Types of magnetism are associated with
particular magnetic elements and compounds: Diamagnetism |
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Definition
all materials display weak diamagnetic
properties, weak reverse field aquired in response to the imposition of an external field (eg gold, quartz, feldspar) |
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Term
Types of magnetism are associated with
particular magnetic elements and compounds: Paramagnetism |
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Definition
weak and positive, causes the dipole
moments to be altered in orientation, not strength (eg Al, Mn, amphiboles, pyroxene, garnet) |
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Term
Types of magnetism are associated with
particular magnetic elements and compounds: Ferromagnetism |
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Definition
| spontaneous magnetism or alignment of all atoms within part of an element. This is due to interaction of adjacent atoms with strong dipole moments (eg Fe and Ni) |
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Term
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Definition
| results from the interaction among neighbouring atoms so that the dipole moments of all atoms in a domain are aligned in the same direction. Neighboring domains may or may not have the same orientation |
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Term
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Definition
| as above, only neighboring domains always have the equivalent field strengths in opposite directions. The net result is zero magnetism(eg haematite) |
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Term
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Definition
| similar to antiferromagnetism except neighbouring ferrimagnetic domains are not necessarily of equal strength. This leads to a positive or negative magnetic moment (eg magnetite and ilmenite) |
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Term
| Magnetic minerals and rocks, eg. |
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Definition
– magnetite (Fe3O4)
– maghemite (Fe2O3)
– ilmenite-hematite FeTiO3 - Fe2O3
– pyrrhotite (Fe1-XS) [where 0 < X < 1/7] and
– jacobsite (MnFe2O4). |
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Term
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Definition
Accessory in mafic igneous rocks,
sedimentary rocks, metamorphic
rocks and in mineralised rocks
such as skarns, altered mafic and
felsic rocks
The most magnetic mineral in
nature |
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Term
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Definition
Maghemite is a product of weathering of
magnetite and other iron oxides.
It is produced in lightning strikes
It only forms in arid environments
on the surface of the Earth
It is common in drainage channels. |
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Term
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Definition
Ilmenite is a common accessory
mineral in mafic and igneous rocks.
It is also a common mineral in mineral sands
It’s magnetism (magnetic susceptibility) is lower than magnetite
and maghemite) |
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Term
| Pyrrhotite (Fe1-XS) [where 0 < X < 1/7] |
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Definition
Pyrrhotite is an iron sulfide
formed in hydrothermal alteration
of any rock.
It usually forms in temperatures
above 400oC and with low fO2.
Pyrrhotite is responsible for the magnetisation of more metal deposits. |
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Term
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Definition
Jacobsite is a secondary manganese spinel that occur in Mn-bearing minerals in some metamorphosed Mn deposits.
It is rare. |
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Term
| Magnetic susceptibility is |
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Definition
the degree of magnetisation of a
mineral or rock in response to an
applied (external) magnetic field.
• Magnetic susceptibility is measured in Nanoteslas (nT). |
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Term
The most __________ rocks will have
higher concentrations of some or all of
the above minerals. Magnetite,
ilmenite, maghemite and pyrrhotite are
common minerals and are the most
magnetic |
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Definition
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Term
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Definition
– Mafic igneous rocks will have a higher magnetic susceptibility than felsic rocks
– Felsic igneous rocks will have a lower mag. Sus., however there is a wide variation |
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Term
| The more oxidised I-type granitoids contain __________ , whereas S-type granitoids are more reduced and don’t contain magnetite |
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Definition
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Term
| ____________ rocks have a wide variety of mag. sus. due the provenance of the rocks |
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Definition
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Term
Metamorphic rocks
Magnetic susceptibility variable depending on original rock type, however __________ may alter the mag. sus. |
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Definition
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Term
Metamorphic rocks
Serpentinisation: addition of water to _____________ rocks, producing Mg-rich hydrosilicates and magnetite. Fe is liberated during the breakdown of olivine |
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Definition
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Term
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Definition
| along the contact of granitoids metamorphic reactions of limestone and calcareous rocks commonly leads to the formation of magnetite |
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Term
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Definition
– (serpentinisation and skarn development)
– Alteration of rocks by the addition of fluids out of equilibrium with their mineral assemblages
– Magnetite and pyrrhotite are common products of hydrothermal alteration, but may also be consumed |
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Term
| The more oxidised I-type granitoids contain __________ , whereas S-type granitoids are more reduced and don’t contain magnetite |
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Definition
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Term
| Changes in magnetic fields |
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Definition
| Within an external magnetic field, crystallisation of a ferromagnetic mineral allows for the development of a permanent and remnant magnetism |
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Term
Changes in magnetic fields
The strength and direction of the primary magnetism may be distorted by |
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Definition
| younger magnetic fields with differing orientations and intensity (natural remanent magnetism) |
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Term
Changes in magnetic fields
Remanent magnetisation is due to the ability of rocks and minerals to |
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Definition
retain magnetisation in the
absence of a field or in the presence of a different magnetic field. |
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Term
Types of remanent magnetism:
TRM-Thermo remanent magnetism |
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Definition
is acquired by
igneous rocks during cooling of molten rock through the Curie Point (the temperature above which a mineral loses its characteristic ferromagnetic ability). |
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Term
Types of remanent magnetism:
DRM-Detrital remanent magnetism |
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Definition
is acquired in sedimentary rocks where a magnetic field may be acquired during consolidation and compaction with
realignment of magnetic material |
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Term
Types of remanent magnetism:
CRM-Chemical remanent magnetism |
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Definition
results from chemical changes and precipitation of new minerals during diagenesis, including mineralising
and metamorphic fluids. Alignment within the penecontemporaneous geomagnetic field results. |
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Term
Magnetic overprinting is a common
phenomenon and may result for example when primary magnetism (TRM) is overprinted by secondary magnetisation, eg |
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Definition
| eg magnetism due to lightning or alteration associated with hydrothermal fluids. DRM and CRM are the result of secondary magnetism |
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Term
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Definition
– Geological mapping, contrasting rock
types, structural synthesis of an area
– Mineral exploration, determining magnetic anomalies to detect areas of magnetisation or demagnetisation associated with hydrothermal alteration |
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Term
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Definition
– Cheap, aerial surveys are quick and
inexpensive to run
– Penetration to 5 km depth, therefore
seeing below regolith cover (very important in Australia)
– Radiometric surveys are performed at the same time
– Invaluable mapping tool |
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