Geology Reference
In-Depth Information
polarity chron numbering system is based on the numbering of seafloor
magnetic anomalies up to ~157 Ma. The numbering system uses “C” as
the designator for normal polarity anomalies back to the end of the
Cretaceous Long Normal superchron at 83 Ma. Before that the designator
is “M” for the Mesozoic chrons back until 157 Ma when the geomagnetic
field reversed rapidly during the Late Jurassic. The subchron numbering
system is complicated. See Tauxe (2010) for details.
Cyclostratigraphy
The study of periodic behavior of the properties of a
sequence of sedimentary rocks.
Demagnetization
The procedure used to isolate the most ancient rema-
nence in a rock sample. Less-magnetically stable components of magneti-
zation are removed by a stepwise procedure using either alternating
magnetic fields or temperatures below a magnetic mineral's Curie temper-
ature, at which point all remanence is removed.
Diamagnetism
The induced magnetization acquired antiparallel to the
applied field by non-ferromagnetic particles.
Eccentricity
The deviation of the Earth's elliptical orbit around the Sun
from circularity. An eccentricity of 0 is a circular orbit, an eccentricity of
1 is a parabola. Earth's orbital eccentricity has ranged from 0.0034 to 0.058
over the past several hundred thousand years.
Evolutionary spectrogram
A power spectrum that depicts the changes in
the spectral peaks through the time series. It is calculated from a spectral
estimator applied to a window moving through the time series.
Fast Fourier Transform (FFT)
An algorithm that computes a discrete
Fourier transform on a digital computer.
Ferromagnetism
The permanent magnetism acquired by iron oxide and
iron sulfide particles whose behavior is described by a hysteresis loop.
First-order reversal curve (FORC)
A curve that results from contouring
the second derivative of the surface fit to the magnetizations measured
from multiple hysteresis loops with decreasing peak fields, and chang-
ing the coordinate system from that used for the generation of the
hysteresis loops. The FORC diagram that results can be used to measure
the coercivity of the collection of magnetic grains in a sample as well as
the importance of magnetic interactions between the magnetic sample
grains. FORC diagrams can characterize particle magnetization
behavior.
Geomagnetic polarity time scale (GPTS)
The record of the polarity inter-
vals, reversed and normal, of the geomagnetic field as a function of age,
through geological time. The GPTS is divided into chrons and subchrons
and is based on the record of seafloor magnetic anomalies back to
~170 Ma.
Hard Isothermal Remanent Magnetization (HIRM)
A rock magnetic
parameter that quantifies the amount of “hard” or high coercivity magnetic
minerals in a sample. It is calculated from: HIRM = (SIRM + IRM
−300 mT
)/2,
where SIRM is the saturation isothermal remanent magnetization and
IRM
−300 mT
is the IRM acquired in a backfield of 300 mT after a sample has
been saturated.
