Uncategorized

Magnetism

Magnetism

Harry Hess produced a paper in 1962 ‘History of Ocean Basins’ – here he proposed that magma rises at the mid ocean ridges and that the sea floor spread (moved) conveyor style to ultimately descend below the continents. In the same year the US Navy produced a report regarding its findings on seafloor magnetism. Magnetic data had been collected from World War 2 onwards (particularly in the Cold War) in order to locate submarines. As the ships travelled back and forth across the ocean distinct magnetic stripes were noted.

stripey1

Here you can see the stripes in black and white either side of the Mid Atlantic Ridge near Iceland. I have to say quite clearly that the stripes are not visible as actual black and white lines on the seabed (as thought by one of my students – Jenny you know who you are….) they are detected by a magnetometer and scientists plot their findings on to a map.

What the stripes meant and how they got there was a mystery to begin with:

  • Many magmas contain minerals that are magnetic. Magnetite being the mineral that is most commonly known.
  • When magnetite cools below a critical temperature of 580oC (Curie temperature) it acquires a magnetisation induced by the magnetic field surrounding it.

 

  • This induced magnetism is aligned with the Earth’s magnetic field.

 

  • If the rock cools slightly further (to below the BLOCKING temperature) the magnetisation becomes fixed and can last for millions of years. The alignment stays fixed unless the rock is altered again – this is known as remanent magnetism and is the basis for palaeomagnetism.

 

  • Scientists found patterns on land as well as in the oceans -with magnetism alternating through piles of lavas.

 

  • Scientists use (amongst other methods) radiometric dating to date the rocks on land. Radiocarbon dating is a type of radiometric dating BUT there is no carbon in magma and radiocarbon dating is not of any use once rocks are over 64,000 years old – more commonly potassium/argon dating is used by scientists.

 

  • These patterns gave scientists a ‘calendar’ that they could use to look back on. They could compare the data on land with that on the sea floor. The magnetism could be used to tell the age of the oceanic crust. Subsequent dredging, drilling and coring of ocean rocks has confirmed this dating.

 

Magnetic anomalies in lava piles.

stripey2

 

 

  • We call the direction of today’s magnetism normal and when the magnetism is the other way round we say it is reversed; this is produces magnetic anomalies or stripes.

 

  • These results were extremely useful as scientists found that older crust was nearer the continents and younger crust nearer the ocean ridges. This backed up Harry Hess’ idea of seafloor spreading.

 

  • Magnetic anomalies are EVIDENCE of seafloor spreading NOT A CAUSE of seafloor spreading. (A common misunderstanding by students…)

 

  • Scientists are still researching magnetic reversals and how/why they occur, this is incredibly complex. The outer core is made up of molten material (mostly iron) that convects; this convection generates the Earth’s magnetic field via a so called self-exciting dynamo. During a reversal event convection itself does not stop or change direction. Instead, a given complex pattern of convection in the outer core can support either a normal or reversed magnetic field. In practice the field flips spontaneously between the two on a time scale of several thousand years.

 

Here is some real data from the seafloor. See if you can complete the stripes:

stripey3

The traditional view as to how these magnetic anomalies formed in the ocean is as below:

Stripey4

  • New magma rises at the ridge (due to decompression melting as the plates move apart)
  • This magma cools and the magnetism aligns to the magnetic polarity of the Earth at that time.
  • The seafloor spreads and new magma fills the space left behind.

 

  • The mantle is not molten but mostly solid though it behaves in a ductile manner over geological time.

 

  • The driver of seafloor spreading is controversial – most scientists do not believe that the lithosphere dragged by a convecting mantle. Mantle drag could actually slow the plate movement down in some areas if the plate above moves at a faster rate.
  • Alternatives are:
  • Cold slab pull – the old heavy plate edges pulling at the younger thinner lithosphere
  • Ridge push – the weight of the plate from the MOR
  • All three mechanisms may work together.

 

Some ridges spread at faster rates than others:

stripey5

 

 

The plates in the Pacific are spreading at a faster rate than the plates in the Atlantic. At 130N scientists are unsure what is precisely happening.

 

The magnetic polarity changes on average every 250 thousand years but there has been around 700 thousand years since the last reversal. It is thought that each reversal takes around 2 to 6 thousand years to change completely so although in this diagram we see a clear boundary between stripes in actual fact when data is looked at on a smaller scale a gradual change can be seen. The position of volcanism on the seafloor can also affect the spatial distribution of magnetism.

 

So what is happening with the magnetism at 13 O N? Well it is very complex.

stripey6

Above: Modelled Crustal Magnetisation 13o N at the MAR

The bar at the bottom shows the expected magnetism as has been found following the reversal times at traditional spreading ridges. You can see that magnetism at 13oN deviates from this pattern somewhat.

stripey7Modelled (black dashes) and measured (red) magnetism through the Ocean Core Complex at 13o N.

The bottom diagram shows a model of the expected magnetism assuming that the spreading axis is at 0. The blue line gives the ages of the expected magnetism. The green line is the track that TOBI – a towed instrument followed in a previous expedition to this location. The Ocean Core Complex is about -10km from the zero point on the scale.

The top diagram: the black dotted line shows what the magnetic anomaly was modelled to look like taking into account the bathymetry of the seafloor (shape of the ocean bottom), the data in the diagram below. The red line is the actual data that the scientists received.

You can see that further away from the spreading centre and ocean core complex that the model and the data received are not too dissimilar. The central part of the anomaly is not what scientists expected. They are hoping that data collected on this expedition may provide them with greater clarity of what is actually happening.

Below: I include a diagram showing the magnetic ‘calendar’ back to the Upper Jurassic:

stripey8

Nigel is well:

20160210_183052-480x853[1]

 

Cheers

Angela 🙂

 

 

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s