Marine Magnetics Lab

We have marine magnetic profile data collected near the Tula Fracture Zone in the southern Pacific Ocean.  The data set has profiles from 64 cruises.  For three cruises you have an extract, with just the data from a straight part of the cruise.  In one case the profile has been reversed, so they will all plot west to east.  You should use these extracts for the detailed analysis.  These profiles are:

Some of the profiles in the larger database are hard to interpret because:

Deliverables.  Prepare a report on the magnetics of this region, with the following points:


Review Labs introduction.

Change program version to geology.

Go to File, Geology labs, Tula Fracture Zone magnetic anomalies.  This will download the data.  It will be in c:\mapdata\tula_fracture_zone if you want to reload, or you can just use the menu choice which will be fast once the data has downloaded.  If the file does not download, get it at, unzip and load on a bathymetry data set.

To see the profiles in profile view:

Quick Profile Exploration to cycle through all the profiles for the full file to get a sense of what they show.

Magnetic model lets you experiment with profiles to see how the various parameters affect them.

Example on the Juan de Fuca Ridge.

Magnetic anomaly profile, with the anomalies picked.  It is easiest to first find the ridge, remembering that the symmetry of anomaly 1 varies with the latitude and orientation of the profile.

The magnetic time scale

Anomaly Age Distance
3 -4.8 28
2A -2.8 72
2 -1.86 96
  -1.03 116
1 0 150
  1.03 174
2 1.86 199
2A 2.8 228
3 4.8 272
3A 6.2 365
Table created in Excel with the picks.  Age is marked as negative to the left of the ridge, which makes the next step easier, but it could be done as two separate computations (which would be required if the spreading were markedly asymmetrical).  The unnamed anomaly is the Jaramillo normal event during a reversed interval.
Graph of age versus distance in Excel.  The slope of the line will be spreading rate in km/My (or mm/yr), and in this case the value is about 26 km/My, which would make it a slow spreading ridge (indeed the case for the Juan de Fuca Ridge).  The line is very linear, suggesting a very constant spreading rate over this interval, and that spreading is symmetrical.

There are about 200 km on the profile to the east of where we stopped picking anomalies.  With the calculated spreading rate, that should be another 8 My, with the oldest crust on the right about 14 My old.  If this is the case, anomalies 4, 4A, 5, and 5A should be present, but with the slow spreading, they may not be clearly defined (note that 3 and 3A are not as distinct as we might like).

The ideal profile for looking at the magnetic anomalies will run perpendicular to the crest of the ridge.  If the profile crosses the ridge at an angle, you will have to reduce the distances by multiplying by the cosine of the angle between the track and perpendicular to the ridge.  If the profile crosses a fracture zone, you will either have anomalies missing, or repeated anomalies.

If the cruise makes multiples passes over the ridge to "mow the lawn", you want to look at the survey legs individually.

We have looked at other data sets which might help your interpretation.  You should be able to pick out ridges and transforms on the bathymetry.

Last revision 2/25/2019