Lecture #13 notes, Geology 3950 Spring 2006: CR Stern Magnetic reversals (text pages th edition and in the 5 th edition)

Transcription

1 Lecture #13 notes, Geology 3950 Spring 2006: CR Stern Magnetic reversals (text pages th edition and in the 5 th edition) The earth has a magnetic field generated by circulation of charged particles (electrons) in the liquid iron outer core. Due to the earth s rotation, which constrains how these charged particles move, the earth s magnetic field has many of the characteristics, most significantly the geometry of the magnetic field lines, as if a simple dipole bar magnet was inside the earth s core (figure 1 below), but it is not since the temperature of the core of the earth is too hot to support the presence of a stable permanent magnet. Earth s dipole magnetic field is oriented approximately north-south, and the south magnetic pole currently is aligned with the geographic north rotational pole, deviating by 11.5 degrees The earth s magnetic field is an important shield against solar winds and radiation and diverts much of this radiation around the magnetic field, or magnetosphere, into space (figure 2), except in rare solar flares when this solar radiation enters the earth s atmosphere along the magnetic field lines near the poles and creates the aurora borealis.

2 The magnetic field also captures charged particles in the Van Allen belts (figure 3 below), which encircle the earth in the upper atmosphere around the equator, and these particles further shield the earth for nasty cosmic and solar radiation. The sun also has a magnetic field that encircles the solar system (figure 4) and shields the solar system from cosmic rays. Neither the Moon nor Mars have magnetic fields because they do not have liquid iron cores. The earth s magnetic field is not a stable feature, but varies in its orientation relative to the earth s geographic rotational orientation (called polar wandering see figure 5 below), direction of the north-south dipole (in either a normal or reversed direction) and strength (called the secular variation of the field strength). The changing orientation of the earth s magnetic field with respect to the geographic rotational axis has been measured very carefully over the last few hundred years or so by navigators (figure 5) and shows smaller to greater deviations from the rotational north.

3 Information of the behavior of the magnetic field in the pre-instrumental, pre-historic and geologic past is obtained from what is called remnant magnetism contained in rocks. When a hot magma cools from >1000 C to form a solid rocks, tiny magnetic minerals -- iron oxides -- in the rock line up like little bar magnets along the direction of the earth s magnetic field and preserve information about the orientation of the magnetic field lines and strength of the field at the time the rock cooled. Similar minerals in sedimentary rocks will align with the earth s magnetic field as these grains are incorporated in a sediment. So both igneous and sedimentary rocks preserved information concerning the earth s magnetic field at the moment they formed. This information can be retrieved in the laboratory. One of the surprises discovered by the first studies of remnant magnetism in rocks was that some rocks were magnetized as if the earth s magnetic field was oriented in the opposite direction from today when these rocks formed in the past, suggesting either that the earth had tipped over 180 degrees on its rotational axis, which is very unlikely, or that the magnetic field inside the earth s had reversed its direction, perhaps because the charged particles in the core began moving in the opposite direction. The possible causes of this previously unknown process were debated for some time. It was at first thought that the rocks themselves might have reversed their remnant magnetism as a result of cooling. However, the more measurements made the more it became clear that igneous and sedimentary rocks of specific ages from all over the world preserved evidence of reversed magnetic orientation of the earth s magnetic poles. It is now accepted that this orientation has reversed numerous times in the past, and this is just one of the expressions of the instability of the magnetic field. The timing of the reversals over the last few million years as determined by measurements of remnant magnetism in rocks of different ages is shown in figure 6 below.

4 The current Bruhnes epoch of normal orientation had been in existence since 690,000, which is when the last reversal occurred, but prior to that the Matuyama epoch of reversed magnetic polarity occurred as far back as 2.43 million years ago. This epoch was interrupted by numerous short reversals such as the Jaramillo, Gilsa and Olduvai events when the magnetic field was oriented as it is today for relatively short periods of time. No such short-term reversal occurred during the 690,000 years of the Bruhnes normal period. What causes reversals remains a mystery, but the changes that occur in the magnetic field during a reversal have been documented in number of ways. One way was a study of the remnant magnetism of the Steens basalts, which are dated as 15.9 million yrs and are part of the Columbia plateau basalts in southeast Oregon and northern Nevada (figure 7) On Steens mountain, one basalt flow occurs on top of the next in a sequence over 1000 meters thick (figure 8), all dated as the same 15.9 million yrs and with the lower ones having reversed magnetization and the younger ones have normal magnetization (fig 9) Figure 8 basalt flows on Steens mountain

5 A magnetic reversal took place during the eruption of the Steens basalts that changed the earths magnetic orientation from reversed to normal. This may have taken as little as 5,000 years. The remnant magnetism of the Steen s basalts preserves the record of the changing magnetic field during this reversal, as shown in figure 10 below These two diagrams show the changing direction of the magnetic field during the reversal. It starts in the reversed direction (the open circle in the southern hemisphere of this projection) and migrates randomly about in the southern hemisphere (dashed lines) until it crosses the equator into the northern hemisphere (solid lines), then crosses back into the southern hemisphere) and finally returns to the northern hemisphere and ultimately points north (solid dot). During this random wandering of the magnetic pole direction the intensity of the field also drops to zero and then grows again as the field regains its new orientation close to geographic north. Similar results are obtained from looking at the magnetic field strength in oceanic sediments (figure 11). The curve on the right indicated the intensity of the magnetic field compared to today as it undergoes the last reversal at 690,000 years. Note how the intensity goes to zero and then recovers after the reversal in <5,000 years.

6 Thus the conclusion is that during a reversal the magnetic field strength decreases to zero and the magnetic field essentially disappears, and with it the magnetosphere, the van Allan belts and any protection they provide from the solar wind and cosmic rays. In the last few million years the average rate of reversals of the magnetic field has been once every 200,000 years, but no reversal has occurred since 690,000 so we are overdue for one. In the last 100 years the magnetic field strength has decreased 10% and in the last 1000 years by 50% (figure 12), so if you are feeling disoriented perhaps this is why. We may already be in the decreasing intensity phase of a reversal, or perhaps this is just a secular excursion of the field strength to lower values and it will recover before becoming very weak and zero as has occurred in the past (see the right side of figure 11 above) Figure 12 plots intensity of the magnetic field compared to today over the last 3000 years. Reversals of the magnetic field have produced linear magnetic anomalies on the floor of the ocean. These were first measured across the mid-atlantic ridge south of Iceland in the 1960s (see figure 13 below)

7 The linear magnetic anomaly pattern illustrated in the bottom part of the figure above indicated positive anomalies above the ridge crest and alternating negative and positive anomalies symmetrically displaced progressively off the flanks of the ridge. It was understood that these could be produced by sea-floor spreading -- the growth of the seafloor at the ridge crests by a combination of normal faulting and the intrusion of new basaltic oceanic crust if as new batches of basalt were erupted on the ocean floor they acquired the magnetic signature of the field at the time they were erupted. Basalts erupted when the field had a normal orientation, such as over the last 690,000 would add their magnetism to the earths field producing a positive anomaly, while those erupted during periods of reversed magnetism would subtract their magnetism from the normal field of today, resulting in negative anomaly. In this way the growth of the ocean floor would progressively record the reversals of the field through time as depicted in fig 14 below The linear magnetic anomalies across the mid-atlantic spreading ridge were shown to correlate with the known stratigraphy of magnetic reversals assuming the Atlantic was opening at a rate of 2.5 cm/year, enough to open the ocean 5,000 km in the last 200 million years. In the Pacific ocean the same anomalies occur across the east pacific rise, but they are further apart because the east pacific rise rifts apart at cm/yr. This information was vital in providing information about the age and spreading rates in different ocean basins and creating the theory of sea-floor spreading. This provided a mechanism for the theory of continental drift, that was based on the similarity of the coast lines of the continents across the Atlantic. Sea-floor spreading was later incorporated into the theory of plate tectonics once it was clear where all the seismic activity was concentrated (see lecture #5). The progressive increase in age from young to old of the ocean crust from the ridge crests to the continental margins has been proven by direct dating of basalts from the ocean

8 floor collected by deep sea drilling, and the age of sediments in different parts of the oceans. The oldest parts of the north Atlantic ocean, which is still growing, are approximately million years old. The oldest parts of the pacific ocean, which is currently getting smaller since it is being subducted below both the Americas and Asia as the Atlantic ocean gets bigger, is also only 180 million years old. Oceans are relatively young features that grow at oceranic spreading centers and disappear into subdcution zones. Continents, in contrast, preserve rocks as old a 4 billion years.