Sep 302009
 

Southern Sumatra Earthquake Magnitude 7.6, September 30, 3009 Recorded at Keele University, UK.
20090930 Sumatra Waveform
20090930 Sumatra and 20090929 Samoa Earthquakes
20090930 Sumatra Travel time
20090930 Sumatra to Keele path

So, large earthquakes are like buses. You wait months for one to come along and then two arrive within hours of each other. I used to think that one event couldn’t trigger another at this sort of range but it does look a bit more than coincidence.

Note that this earthquake was deeper than the Samoan one (about 80km down) so the surface waves are much less pronounced. Also, as this event is a bit closer to Keele so we are not in the core shadow and can see the P, PP, S & SS arrivals.

The Keele seismic station is part of the UK Seismology for Schools programme.

Sep 302009
 

Samoa Earthquake September 29, 2009, M 8.0 recorded at Keele University, UK.
20090929 Samoa
20090929 samoa earthquake
20090929 samoa earthquake travel time curve
20090929 samoa earthquake path to keele

This earthquake caused a tsunami which hit the local islands causing at least 65 deaths in Samoa and at least 20 deaths in American Samoa according to the BBC.

It is unusual in that although it occurs close to the trench where the Pacific Plate is subducted beneath the Australian Plate, the faulting mechanism has been determined as being a normal fault rather than a thrust.

Note that as Keele is 15665km as the earthquake travels from the epicentre the first arrival reaching Keele is actually PP, rather than the direct P-wave which is intercepted by the Earth’s core.

The Keele seismic station is part of the UK Seismology for Schools programme.

Sep 242009
 

In a previous post I looked at my trip down the Tara zinc and lead mine near Navan in Ireland. After a very nice lunch we were then taken on a tour of the processing plant. The ore is first crushed underground and brought by conveyor to the surface. The whole process is monitored via computers from a central control room.
Tara Control Room

The material is screened and the larger fraction passed to a cone crusher to reduce the particle size to about 16mm. The rock is further crushed in a rod mill (right) and three successive ball mills (centre and left). This gets the ore down to about 75 microns size. This part of the process is imminently being replaced with a new autogenous grinding mill which actually uses large blocks of ore rather than rods or balls do do the grinding.
Tara Rod and Ball Mills

Mineral separation is undertaken by froth floatation. First lead and later zinc particles are made hydrophobic by adding a surfactant or collector chemical to a slurry of water and powdered rock. This slurry (or pulp) is aerated, creating bubbles and the hydrophobic ore particles attach to the air bubbles, rising to the surface forming a froth. The froth is then skimmed removing the ore. The slurry passes through several banks of ‘roughers’ removing 75% of the lead and 93% of the zinc.
Tara Froth Floatation

The ore froth then passes to large settling tanks where the mineral concentrates then settle out.
Tara Settling Tank

The concentrates are then de-watered and the dry concentrate is stored awaiting shipment. Normally the concentrate is taken by rail to Dublin for shipping to smelters in Europe but a recent rail bridge collapse has meant that it has to travel by road until the rail bridge is repaired.
Tara zinc concentrate awaiting transport

Annual production is approximately 360,000 tonnes of zinc concentrates grading 56% zinc and 70,000 tonnes of lead concentrates grading 65% lead.

Thanks to the operators of New Boliden Tara Mine for their fascinating visit.

Note: I’m just about to embark on a very heavy teaching semester – blogging will be light

Sep 222009
 

David Bressan at Cryology and Co is hosting Accretionary Wedge #20 and I’m late with this and don’t really have much time what with the start of teaching next week – sorry.

He asks …

What remains to be discovered for future earth scientists what we (still) don’t know about earth? What are the geological riddles that still lack answer – all questions are allowed – it could be a local anomaly, or a global phenomena, or something strange

Nasa blue marble 2005 photoshopped

I have always used the analogy that geology is like having just a few pieces of a thousand piece jigsaw and being asked what the whole picture is depicting. Each piece has to be examined in minute detail for the smallest clues to where it fits in the main image. It could be an edge or a corner which helps, but most of the time it is a middle part which doesn’t provide that much information. Do you have a piece of the sea or the sky to suggest where it might approximately fit in the bigger picture? Are you looking at the piece upside down? Occasionally you will be given a new piece to look at. If you are really lucky you might find that two pieces actually fit together. The thing is, as geologists, we are never going to get to be given all the pieces. Some have been lost down the back of the sofa for ever, others are unobtainable, never being in the box in the first place.

So what are we left to discover as geologists? Well I think pretty much everything. We have an incredibly young subject. Plate tectonics as a concept is less than fifty years old and it is just a good working hypothesis. It is total arrogance that we know pretty much everything about geology. We might be standing on the shoulders of giants but there will be even bigger titans to come to stand on ours. When they look back to our time they will think we knew virtually nothing.