Paleomagnetism Laboratory [1969 to 2019]
PALEOMAGNETISM LABORATORY [1969 to 2019]
By Professor Emeritus Henry Halls; Professor UTM 1970- 2010.
On May 22nd 2019, the Paleomagnetism Laboratory, variously known as the “Rock Lab” or “Lunar lab” closed its doors for the last time, after 50 years of operation, ironically in the same year as the 50th anniversary of the lunar landing! The lab was housed in a long, low building beside Principal’s Road that leads to Lislehurst , the Principal’s residence. The building was specifically designed to house a laboratory specializing in Paleomagnetism and the Physics of Rock Magnetism1 and was therefore made of non-magnetic materials including all furniture.
The idea for a magnetism laboratory at UTM was originally spearheaded by a group of geophysicists from the St. George Campus (Gordon West, George Garland, David Strangway and J. Tuzo Wilson), who needed an environment where the earth’s magnetic field could be virtually eliminated, thus providing a suitable environment for rock magnetic experiments and for testing electromagnetic survey equipment. The group was spearheaded by Gordon West who supervised graduate students on the St. George and Scarborough campuses where magnetic noise due to traffic and especially street cars and the University subway line in Toronto, presented severe problems to experiments. A quiet magnetically noise-free area was sought. In 1967 a proposal was made to the then fledgling Erindale College in Mississauga (now UTM) for the construction of a suitable building in a rural setting, to be financed by the Department of Physics. Tuzo Wilson, earlier the leader of the St. George geophysics group and by that time Erindale’s de facto first principal, was a keen supporter of the project. Dave Strangway, a future President of the University of Toronto and an Associate Professor in the Physics department, was at that time the head of the Lunar sampling mission at NASA. Lunar samples were scheduled to be returned to Earth in 1969, and those destined for Canada, personally transported across the border by Strangway, required a laboratory for their study. The building of the laboratory was duly completed in 1969, with a “shielded” room subsequently added from the Physics Department on the St. George Campus, in which experiments could be conducted in a magnetically field free space. The laboratory did indeed receive the first lunar samples to arrive in Canada. To commemorate this spectacular event, Principal Wilson had a light in the shape of a crescent moon erected outside the lab, which still stands today, although it has been moved from its original location.
1970 saw the arrival at Erindale of two scientists interested in carrying out research in Rock and paleo-magnetism; David Dunlop (UTM 1970-2006) and Henry Halls (now professors emeriti in the Department of Chemical and Physical Sciences). During their careers these two scientists produced close to 300 journal publications,(~200 for DD and ~ 100 for HH), much of which were based on research carried out in the Paleomagnetism Laboratory. Altogether about 18 M.Sc., 14 Ph.D and 11 PDFs were supervised and over 100 summer students were employed. Professor Dunlop transferred his lab to the Willian G. Davis building in 2000 and Henry Halls remained behind to look after the laboratory until 2019.
During the lifetime of the lab, many discoveries were made:
(1) The discovery that Lunar rocks carried a magnetization, perhaps caused by a weak, long-extinct internally-generated magnetic field;
(2) The identification of a 30 km-diameter meteorite impact crater forming the Slate Islands in northern Lake Superior and the discovery that it contained a remanent magnetization caused by the high temperatures and/or shock pressure of the impact itself;
(3) Early analyses of how to identify, recognize and separate superimposed magnetizations of different ages present in a rock;
(4) The first successful recovery of coherent cooling magnetizations from deformed and metamorphosed rocks belonging to the Grenville Province (a one billion year old, deeply eroded mountain belt, similar to the Himalayas, which extends for thousands of kilometres along the eastern coast regions of North America, passing under Toronto on its way to the southwest).
(5) Systematic paleomagnetic studies on Precambrian (0.6 to 2.5 billion years old) dyke swarms worldwide, in an effort to provide primary magnetizations and geomagnetic poles for early continental reconstructions.
(6) Some of the earliest studies of magnetic properties of rock cores from up to 1 km-deep drill holes on the Mid-Atlantic Ridge.
(7) The first determination of the intensity of Earth's magnetic field at ~3 billion years old, from volcanic rocks in southern Africa.
(8) A landmark experiment demonstrating that magnetic oxides mimicking those in Earth's oceanic crust do not appreciably change the magnetic field direction they record during long-term chemical alteration. This demonstration was vital in validating the hypothesis of seafloor spreading and continental drift. It was made possible by carrying out the experiment inside the magnetically shielded room.
Before the erection of the adjacent Grounds Department building in 2010, the lab was physically rather isolated, but was scientifically so active that diversions were needed to provide light relief to post- graduates and undergraduates during lunch breaks! One of these was a badminton court erected on the grass lawn in front of the lab and on special occasions, a mini golf course where holes were set up in safe locations along Principal’s Road and into the grounds of the nearby Artists Cottage (now the Crime Scene House) where Professor Halls then lived. Three photos of the lab’s interior, were taken just one month prior to closure.
Final decommissioning of the lab began in early April 2019, in response to the space being allotted to a new, developing program in Artificial Intelligence. Most of the paleomagnetic equipment was moved to the Universities of Brock and Western to form a new paleomagnetic facility under the management of Dr. Phil McCausland of Western University. A particular challenge to clearing the lab space was the eight-foot square shielded room made of more than 2000 pounds of mu-metal, (a special metal that blocks an external magnetic field) and put together with hundreds of bolts and screws using a wooden framework. Initially a professional team of shielded-room dismantlers was contacted who estimated that they could do the job for US$12,000. On learning that the shielded room was not built of a NASA design with which they were familiar, they finally declined to offer their services. At the 11th hour, just when scrapping the shielded room was the only option, a knight in shining armor appeared in the form of Dr. Chris Charles, an alumnus of the Earth Sciences Department on the St. George Campus. He had just taken a job at the TRIUMF facility in Vancouver where rare isotopes for medical applications and research were being produced, using a powerful linear accelerator. It turned out that a new accelerator was being built and the Earth’s external magnetic field needed to be locally excluded along the path of the ion beam in the planned accelerator. So Chris came to UTM and supervised the dismantling and shipping of the mu metal to TRIUMF! In the end, despite the age of some of the paleomagnetic equipment, nothing was scrapped, and the precious mu metal found a new home!
Acknowlegments: Dr. Phil McCausland for all photographs, for help in packing unused sample cores, and for helping to make the decommissioning of the lab a more cheerful experience; Professors Emeriti Gordon West and David Dunlop for comments and corrections on an earlier version of the article; Gordon provided a file from 1967 in which the plans for the Lunar lab were first proposed.
1Paleomagnetism is the science of measuring the remanent or “fossil’ magnetization that nearly all rocks possess. If the rock is first oriented precisely in the field, it is possible to measure, using a spinner or similar magnetometer, the direction of the natural remanent magnetization. By subjecting the sample to progressively higher temperatures and alternating magnetic fields, with intervening spinner measurements prior to each increase, it is possible to define and remove magnetizations that may have formed during the later geological history of the rock, leaving only that magnetization which was originally formed when an igneous rock, like a lava flow for example, originally cooled. By repeating the procedure on many samples from several lava flows (for example) it is possible to obtain an average direction which represents the time averaged direction freed of short term changes like secular variation in the Earth’s magnetic field. A geomagnetic pole, closely coincident with the geographic pole, can then be calculated. If similar experiments are performed for other lava flows of different ages from the same continent, a polar wander path can be constructed which reflects the motion or “drift” of the continent through time. Repeating the entire experiment for different continents produces other apparent polar wander paths. Two continents can then be assembled, (fitted together) for a given time interval if their apparent polar wander paths during this time can be superimposed. When this procedure is extended to all continents it is possible to re-assemble all the continents to discover if super-continents (like Pangea in the late Paleozoic) existed in earlier times.
In order to correctly interpret the paleomagnetic signal, it is necessary to separate different remanence components, and to correctly identify their age and origin. To be successful it is necessary to obtain K-Ar and U-Pb radiometric ages for the rock and to understand the physics of remanence acquisition in both magnetite and hematite, the two principal minerals which carry the ancient magnetic signature. The procedure becomes more challenging the older the rock is, because it has been subject to a longer geological history and witnessed more heat-producing geological events capable of partly resetting the earlier magnetic signature.
Professor Emeritus Henry Halls; Professor UTM 1970- 2010.
