Geology Reference
In-Depth Information
Precession of the orbital perihelion
or
apsidal precession
The slow
clockwise precession of the Earth's orbital perihelion in space.
Pseudo single domain (PSD)
A small, multidomain ferromagnetic grain
that behaves like a single domain grain. It is usually envisioned as con-
taining just a few domains (regions of uniform magnetization), but PSD
behavior could also be due to nonuniform magnetization configurations
(vortex or flower) as indicated by micro-magnetic modeling.
Red noise
A noise model for climate and geological time series in which
low frequency noise has more power than higher frequency noise. The
red noise spectrum results from the system having inertia or “memory.”
Remanence
Permanent, rather than induced, magnetization of a rock.
Remanence is carried by ferromagnetic minerals. The important ferromag-
netic minerals in rocks are magnetite, hematite, and some iron sulfides,
such as greigite.
Saturation magnetization (J
sat
)
The ferromagnetic magnetization deter-
mined by a hysteresis loop measurement that is the greatest magnetiza-
tion acquired by a sample in the presence of the magnetic field causing it.
Saturation isothermal remanent magnetization (SIRM)
The isothermal
remanence acquired by a sample after it has been exposed to a saturating
DC field. See J
rs
above.
Secular variation
The natural directional and intensity variations of the
geomagnetic field during a polarity interval. These changes occur over
10
3
-10
4
year time periods.
Single domain (SD)
Magnetic mineral grains that are small enough to have
only one magnetic domain. The entire magnetic grain is uniformly
magnetized.
Slepian tapers
A specific kind of data taper used in an MTM spectral
estimation. Also known as a discrete prolate spheroidal sequence or DPSS.
Smoothing
Time-wise averaging of a time series to “smooth” out high-
frequency variations.
S-ratio
A magnetic mineral parameter that quantifies the ratio of low coer-
civity magnetic minerals (e.g., magnetite) to high coercivity magnetic min-
erals (e.g., hematite). The S ratio is calculated from S = −IRM
−300 mT
/SIRM.
Susceptibility
The susceptibility is the proportionality constant between
the induced magnetization and the field that is inducing it. J=χ H.
For AMS (anisotropy of magnetic susceptibility), the susceptibility is
described by a second rank tensor (3 × 3 matrix).
Tapers
A function that decreases to zero at each end gradually and
smoothly. It is multiplied by a time series to remove the effects of sharp
truncation at the ends of a finite length time series.
Virtual geomagnetic pole (VGP)
The position of the north magnetic pole
on the surface of the Earth that would generate the paleomagnetic direction
observed for a rock sample assuming that the Earth's field is caused by a
dipole at the center of the Earth. Since PSV (paleosecular variation) has
not been time averaged, VGPs do not typically lie on the N geographic pole
(spin axis of the Earth), but cluster around the N geographic pole during
times of normal polarity.