Geology Question

[joesixpack]

I live just north of the mouth of the Columbia River, on the Long Beach Peninsula. The whole town is built on a big sand spit. The other day, I noticed something that really amazed me, and when I showed it to some other locals, they were amazed too. I had dropped a magnet on the ground, and when I picked it up, it was covered with sand. I mean, the sand was sticking to it. I cleaned it off, got a handful of sand and held the magnet over it, and to my amazement, witnessed ordinary sand being picked up by a magnet. Apparently, the sand must have a very high content of iron.

I'm assuming that there is some source for iron that is coming down the Columbia as silt and being deposited on the beach here. Anyone here know enough about Geology to tell me where it's coming from?

 

[aggle-rithm]

I remember reading as a kid that any little bits of sand picked up by a magnet were micro-meteorites, which are constantly falling to earth.

Have you tried dropping a non-magnetic piece of metal and seeing if sand sticks to it as well?

 

[joesixpack]

I remember reading as a kid that any little bits of sand picked up by a magnet were micro-meteorites, which are constantly falling to earth.

Well, there's quite a large portion of the sand here seems to want to stick to a magnet. I can't believe that so much of our local sand is of non-terrestial origin.

Have you tried dropping a non-magnetic piece of metal and seeing if sand sticks to it as well?

Yes, I've dropped lots of things in the sand here, change, keys, tools, &c. I never noticed the sand clinging like that to anything untill I dropped something magnetic into it.

 

[Correa Neto]

It all depends on the rocks at the source-area of the sediments carried by the river.

Common rocks such as granites may have small magnetite contents (~0.5-1%). Water flow tends to concentrate heavy minerals (such as magnetite) at some places (channels, bars, etc.).

Other common rocks that may have magnetite are basalts, andesites and their metamorphic equivalents. Notr that if the river passes through an area with banded iron formations (iron ore), these rocks may have a very high magnetite content, since they are basically composed by interlayered bands of quartz and iron oxides (magnetite and/or hematite).

 

[Soapy Sam]

The Columbia River flows across the aptly named Columbia River plateau (what a coincidence, eh? Isn't Nature amazing?) which is largely basalt.
See-http://vulcan.wr.usgs.gov/Volcanoes/ColumbiaPlateau/framework.html

Now an average basalt is about 12% iron oxides of one sort or another, so it's perfectly possible you would find some in sand at the mouth of the Columbia.
Because of its density it will be concentrated by hydraulic action in certain areas,- just as gold is concentrated in placers- so you can easily find local concentrations which would coat magnets- magnetite grains may even stick to a lump of steel which is not magnetised.

Or it may be you are finding grains of iron from a wrecked ship of course.

 

[Madalch]

As Correa Neto implied, there are rocks that are magnetic without being metallic iron (meteoric or otherwise). Magnetite (Fe3O4, or FeFe2O4 as was discussed earlier) is the most common one. Wikipedia assures me that hematite (Fe2O3) can also be magnetic.

 

[Correa Neto]

You are probably talking about mushketovite.
It is a magnetite pseudomorph after hematite. Hematite is reduced (looses oxigen), becoming magnetite, but the external crystal form is maintained.

High intensity magnetic fields may also make hematite magnetic, IIRC.

 

[Madalch]

You are probably talking about mushketovite.
It is a magnetite pseudomorph after hematite. Hematite is reduced (looses oxigen), becoming magnetite, but the external crystal form is maintained.

I didn't think hematite was magnetic, but that's what I get for trusting wikipedia.

As an apology, I will allow you to send me some Antarctica Guarana.....

 

[opqdan]

The Columbia River flows across the aptly named Columbia River plateau (what a coincidence, eh? Isn't Nature amazing?) which is largely basalt.
See-http://vulcan.wr.usgs.gov/Volcanoes/ColumbiaPlateau/framework.html

Now an average basalt is about 12% iron oxides of one sort or another, so it's perfectly possible you would find some in sand at the mouth of the Columbia.
Because of its density it will be concentrated by hydraulic action in certain areas,- just as gold is concentrated in placers- so you can easily find local concentrations which would coat magnets- magnetite grains may even stick to a lump of steel which is not magnetised.

Or it may be you are finding grains of iron from a wrecked ship of course.

Timely topic as I just spent the long weekend exploring the Grand Coulee and surroungings with my wife. The area is the most interesting (geologically speaking) I have ever been too. Just about the only rock there is basalt, and it makes sense that the millions of tons that were washed away during the catastrophic flooding, would have to end up somewhere.

I picked up a piece while we were there (I promise it wasn't from a state park), so I could feasibly check out what happens with regards to magnets.