JC24: Dating volcanoes on the Mid-Atlantic Ridge


Cruise diary

Sunday 15 June 2008

Everything has run smoothly today an we are continuing with the sampling dive we launched yesterday.

As we are studying the evolution of the AVR, it is very important that we know how old the lavas we are sampling are so we know what order they were erupted in. This means we have to date them and this is done in a number of ways.

In the first instance we used sediment cover, assuming that lavas that have the least sediment cover are the youngest. Isobel has been mapping out sediment cover along the dive tracks to give the group an idea of which parts of the AVR are the youngest. This method is not very accurate however as it can be affected by different rates of sedimentation in different areas and currents.

Once we have the samples on the surface we date them again from their appearance, assuming that rocks that have been more weathered by the water and have less or less fresh glass attached to them are likely to be older. Nicole then takes this data and uses it to produce another map, which can be compared to the one for sediment cover. This method is slightly more accurate than sediment dating, but still not perfect.

The rocks may also be dated by their magnetism. This is where Maurice Tivey of Woods Hole Oceanographic Institution come in.  Magnetic dating is one of his areas of expertise and today he explains how it works…

Maurice writes:
Earth's magnetic field has changed both in direction (i.e. the magnetic poles have flipped) and in strength through geologic history and these changes have been dated so that we can use this information as a timescale to measure among other things the age of the seafloor.  The rocks that cover the seafloor make up ocean crust are highly magnetic as they contain the mineral magnetite.  At a midocean ridge like the Mid Atlantic Ridge at 45N the ocean crust is forming today (or at least over that few thousand years) and then is rafted away to the sides.  The spreading rate is slow at only about 1 cm per year.  The volcanic rocks are not magnetic when they erupt but as they cool they lock in and preserve the direction and strength of Earth’s magnetic field at that time.  Thus the crust acts as a giant tape recorder.  My interest and job on this cruise is to measure the magnetic field of the recent lava flows and from their signals determine their age.  The last major polarity reversal (i.e. when the Earth’s magnetic poles flipped position with the north magnetic pole switching to the south geographic pole and the south magnetic pole switching to the north geographic pole) is well know to be about 780,000 years ago.  For our study site where we are at the youngest crust of the MAR at 45N we have much younger rocks perhaps only 1,000 to 200,000 years old, so we will likely not find any reversals in the survey area.  We therefore are using the variation in Earth’s field strength as our measuring stick.  Presently Earths field strength is of moderate strength but it has decreased for the past 2000 years.  The field was about a third stronger approximately 2000 years ago.  Before 2000 years ago Earth’s field was progressively weaker with increasing age until about 37,000 years ago when it was only perhaps one third of what we are today.  This dramatic change is recorded in the lavas of the seafloor.  Lavas that are 2000 years old will be strongly magnetic, those that are 37,000 years old will be much weaker.  This overall change in strength with age makes a magnetic anomaly peak that overlies the volcanic ridge that we can measure with our magnetic sensors.

In terms of sensors, we are using a magnetometer mounted on the towed TOBI sidescan sled and another one mounted to the Remotely-Operated Vehicle (ROV) ISIS.  The TOBI magnetic measurements give us a broad view of the axial ridge magnetism because TOBI is flown at 400 meters above the seafloor.  The ROV ISIS magnetometer gives a much more detailed signal as we are flying within visual range of the bottom at only 1 or 2 meters.  We must calibrate the magnetic sensors on ROV ISIS and TOBI and correct for the magnetic effects of these vehicles.

We hope to match the shape and magnitude of the magnetic anomalies with the volcanic features we map with the TOBI sidescan and types of the lavas we find with the ROV.  In this way we hope to put some constraints on the age of the lavas and their spatial distribution with the overall goal of understanding the volcanic history of the Axial Volcanic Ridge (AVR).  We have an additional magnetic tool at our disposal as well.  We are collecting volcanic glass samples from the various lava flows we observe with ROV ISIS.  Back in the laboratory my colleague Milene Cormier will measure the magnetic properties of this glass with very sensitive magnetic sensors and determine the past magnetic field strength of these glass samples, also known as magnetic paleointensity.  These results give us an absolute value of Earth’s magnetic field strength for each sample.  Those samples that show very high values must be about 2000 years old while those with very low values are probably 37,000 years old.  These magnetic measurements will be combined with our other dating techniques such as the Uranium decay dating, the rock sample observations and visual geologic observations to arrive at a consistent age pattern for the AVR.

The magnetics sensor attached to ISIS.

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May 2008