Chapters 10 & 11 (Magnetism)

Paleomagnetism and Mineral Magnetism

Earth’s magnetic field, present and past

Unit of Measure: Magnetic field is measured in nano Testlas (nT)

Earth’s magnetic field: The earth acts like a large magnetic dipole

Ø Magnetic north is measured by the tilt, which is 11 ½ °.

Ø Magnetic inclination: the angle at which the magnetic pole points downward (-90 in the south and +90 in the north). This changes with latitude.

where: I = inclination and l = latitude

Ø Magnetic declination: the difference between true north and magnetic north.

The Earths magnetic field in the past:

Ø Secular variations: slow changes in magnetic north over time. Historically we know when this has occurred and by how much. The prehistoric record is in rocks in terms of paleomagnetics. It appears that the field takes a couple of thousand years to complete a loop.

Ø Polarity reversals: reversals occur at irregular intervals over time. The current sense of polarity is called normal and the opposite is a reversal.

Paleomagnetism

Paleomagnetics: the ability for rocks to store the magnetic field in which they were exposed to and/or cooled in.

Measuring a paleomagnetic direction:

Ø Cores are sampled and their position and relative directions to the north are noted. They are taken back to the lab and their magnetism is measured. Several samples from one area are typically used to give the average for the area of interest. The values are plotted on a stereonet. A mean direction and error are typically the end result. The error is plotted as a circle whereby there is 95% probability that the true direction is within this “circle of confidence”.

Paleopoles and paleolatitudes: Because rocks that are magnetized record the position in which they were emplaced, paleolatitudes and paleopoles can be calculated to see if the rocks are in their original position on earth’s surface. The inclination and declination of the rocks are calculated and provide a paleopath of of the rocks position. This information enables us to calculate the apparent pole of the system.

Apparent pole wander (APW) paths: This provides a map of where rocks have traveled in the past and to map where the continents or other landmasses have been in the past.

Magnetism of Rocks

Remanent magnetism: This is the ability of the rock to hold its magnetism. To retain magnetism in a rock it is necessary for ferromagnesian minerals to be present. These minerals will have magnetism associated with them. When these minerals are exposed to a large magnetic field they will rotate to be inclined with the field.

Magnetic domains:

Ø When magnetic mineral grains reach a certain size (0.001 to 1 mm) they divide themselves into domains along their easy directions and are separated by walls.

Ø Subdivided grains are multidomain grains (in contrast to single domain grains)

Ø Remanence magnetism is less once the field is removed

Ø Magnetism that relies on the field is called induced magnetism

Curie and blocking temperatures:

Blocking temperatures are the range of temperatures where demagnetism will take place.

Curie temperatures are high temperatures where the minerals will no longer align themselves and demagnetism occurs.

Thermal remnant magnetism (TRM):

Ø As a rock cools through its Curie temperature magnetism is emplaced. Similarly, as the rock cools through blocking temperatures then each domain is magnetized. This enables us to measure the cooling history of rocks by plotting the vector components to obtain the path.

Magnetic minerals: Magnetite, hematite, and maghematite are the major magnetic minerals (others are listed in table 10.1 of the text).

1) Magnetite has the most remanence

2) Hematite is slightly weaker.

3) Maghematite, which is fairly high in magnetism, has the same chemical structure as hematite, but has the crystal structure of magnetite.

4) Grain size will also be a factor for the amount of remanence a rock will yield.

Mechanisms that magnetize rocks:

1) IRM: isothermal– constant temperature

2) CRM: chemical– alteration due to weathering or precipitation

3) DRM: depositional– turbulent flows or debris flows

4) VRM: viscous– partial remagnetisation through thermal fluctuations

Testing magnetism

Lab and field tests provide information on when remanence of a rock was acquired

Lab tests:

1) Thermal demagnetization– heat a sample in a zero field environment. This method takes a long time and has the potential of changing the nature of the minerals in the rock.

2) Alternating field demagnetization– uses a magnetic field of defined value.

*Both techniques require that secondary magnetization is easier to remove than the primary. This doesn’t work well for CRM samples.

Field tests:

1) Fold Test: by straightening the fold we can get the primary field (assuming that the rock was magnetized prior to folding).

2) Conglomerate test:

a. If the clasts have random field directions then magnetism pre-dates deposition

b. If they are aligned then magnetism occurred after deposition.

3) Baked Contact Test: by measuring the field of the intrusion, its contact and the country rock, it can be determined if magnetism has occurred prior to intrusion or after.

Magnetostratigraphy

Changes in Earth’s magnetic field can be used to establish stratigraphic order and to date rocks.

Reversals in Earth’s magnetic field are useful magnetostratigraphic markers because they are abrupt, easily recognizable, global events.

Magnetic polarity timescales:

Ø Ages are obtained by radiometric dating of rocks

Ø This is typically done on the ocean floor where there are continuous successions of lavas or ocean floor basalts, which preserve the best magnetic record (granitic rocks have the possibility of being tilted an unknown amount).

Ø The polarity time-scale only extends back to 160 Ma

Ø Used for long time periods

Secular variations

Ø Shorter periods

Ø Useful for rapid geologic processes (e.g. measure the ages of successive lava flows)

Mineral Magnetism

Magnetic susceptibility (c) is the ability of a rock to temporarily hold a magnetic field. This type of magnetism is called induced magnetization.

(induced magnetization = susceptibility X field)

The susceptibility (c) (“kai”) is dependent on the type of magnetic mineral and on its concentration.

Magnetic Fabrics: Anisotropy of susceptibility

Alignment of magnetic minerals caused by flow or strain. Max c is aligned along the direction of flow.

Magnetic Surveying

Magnetic anomalies are a measure of the direction of magnetization of a body relative to the surrounding rock, the magnetic latitude, and the Earth’s magnetic field; therefore you have to use sensitive instrumentation to measure the field.

Magnetometers

Two main types:

1) Proton magnetometer: measures the total strength of the magnetic field (getting the total field anomaly)

2) Fluxgate: measures a component of the field along the axis of the sensor

a. On land this is typically the vertical component.

b. In air, ships or satellites it is aligned with the field; so you obtain direction as well.

3) Both types can measure the field within 1 nT or less.

Data Acquisition

Ground Surveys:

Ø Need to be free of anything that may cause a magnetic field (belt buckles, bracelets, watches, etc.)

Ø Typically the sensor is mounted on a pole

Ø Must try to stay away from large objects that may induce a field (railways, fences, etc.)– these will show up as large spikes in the data

Aeromagnetics:

Ø Sensor is mounted on a boom extending from the plane

Ø This technique is good for areas that are inaccessible or for large areas.

Ø This is a fast and cheap technique, good for regional surveys.

Shipborne:

Ø the sensor is towed at some distance behind a ship.

Data Reduction

The corrections for magnetics are fewer than for gravity.

Types of Corrections:

Diurnal Variations: change in the field over a day. Can reach up to 100 nT. If a magnetic storm is happening it can be as much as 1000 nT.

Global Field: needed for regional surveys. The field is subtracted from the IGRI.

Anomalies:

Ø The anomaly will be different for small bodies, spheres and sheets.

Ø The depth of an anomaly can be determined by the sharpness of the anomaly.

Ø Half-slope method: will help determine the depth of the anomaly.