In the crystals in which they are bound, minute grains of magnetic min-erals act like tiny bar magnets, and these mineral grains can record the direction of the Earth’s magnetic field..
Trang 1to plot the magnetic-polarity history of the Earth
back to the beginning of the Late Jurassic (Figure 2)
This polarity history is referred to as the
geomagnetic-polarity time-scale (GPTS) It provides a globally
consistent pattern of normal- and reversed-polarity
intervals that can be used to estimate the ages of
rocks and the events that they record during the last
160 Ma of Earth history
In the history of the Earth’s magnetic field, a
super-chron is an interval of tens of millions of years during
which the polarity remains constant There are two
well-established superchrons: the Cretaceous normal
superchron, from about 118 Ma to 83 Ma ago, and
the Permo-Carboniferous (also called Kiaman, after a
place in Australia) reversed superchron, from about
316 Ma to 262 Ma ago (Figure 1)
Early studies of magnetostratigraphy named the
magnetic-polarity intervals (then called ‘epochs’) after
scientists and mathematicians (Brunhes, Matuyama,
Gauss, Gilbert) and the polarity events after the places
where they were first identified (Jaramillo, Mammoth,
Olduvai) However, it was subsequently realized that there are so many intervals (now called ‘chrons’) and events (now called ‘subchrons’) that numbering them is simpler than assigning them names Chron numbers are followed by an ‘n’ or an ‘r’ to indicate whether they are normal or reversed
Remnant Magnetization Most rocks contain minerals that are naturally mag-netic, such as the iron oxide minerals haematite (Fe2O3) and magnetite (Fe3O4) In the crystals in which they are bound, minute grains of magnetic min-erals act like tiny bar magnets, and these mineral grains can record the direction of the Earth’s magnetic field Thus, when lava flows cool, these magnetic minerals align themselves with the Earth’s magnetic field Mag-netic mineral grains also align themselves with the magnetic field when they are deposited in sediments These processes provide a record of the state of the magnetic field (normal or reversed polarity) when a rock is formed This record is called the remnant magnetization
Heat destroys the magnetization of a rock Indeed, magnetic minerals lose their magnetization at a cer-tain temperature (usually above 500C), called the Curie point (after the chemist Pierre Curie) The nat-ural remnant magnetization is locked into a rock when it cools below its Curie point, which is approxi-mately 650C for haematite and 580C for pure magnetite
There are three kinds of remnant magnetization Thermal remnant magnetization is the result of a molten rock cooling below its Curie point, at which magnetic minerals align with the current magnetic field and become locked into the crystal with that alignment
Detrital remnant magnetization occurs when an igneous rock erodes, and its magnetic minerals become loose sedimentary particles These tiny magnetic grains (which are only a few micrometres in diameter) act as bar magnets and align with the magnetic field as they settle through the water column and are deposited as sedimentary particles (detritus)
Chemical remnant magnetization takes place when iron weathers out of a rock, moves through ground-water, and precipitates elsewhere Usually it pre-cipitates as some form of haematite During the precipitation, the magnetic minerals, which were initially aligned when the original rock was formed, realign themselves with the magnetic field at the time
of precipitation Thus, the new alignment is a younger magnetization than the original magnetization that was acquired when the rock formed
Figure 2 The global polarity time scale for the last 160 Ma is a
well established and widely accepted record of magnetic polarity
reversals since the beginning of the Late Jurassic.