Thar’s “gold” In Them Thar Martian Hills!

A small agate in volcanic rock near Gooseberry Falls north of Duluth, Minn. Water, swirling through a small pocket in the rock, deposited layers of silica eroded from other rocks into characteristic bands. The red color is from iron oxide also carried by the water. Photo: Bob King

I’ve been reading a book about Lake Superior geology this month to help me better understand the origins of Duluth’s rocky landscape. Walls of pink and black volcanic rock line every creek and river in the area. These were once sheets of lava that oozed from cracks in Earth’s crust 1.1 billion years ago and spread across the landscape over the next 25 million years piling up 12 miles deep in some places.

Water, carrying minerals like quartz and calcite, flowed through fractures in the cooling rock and deposited veins of milky white crystals that stand in beautiful contrast to the dark volcanics. Agates formed in hollows created by bubbles of hot gas rising through the once-molten layers.

Water was the agent in ferrying all these materials into the rocks, because, let’s face it, water can travel anywhere. It finds the tiniest of cracks and goes there. And when it carries dissolved minerals or metal, those materials slowly settle out and grow into veins of quartz, gold, copper, calcite, you name it.

This color view of a mineral vein called "Homestake" comes from Opportunity's panoramic camera. Opportunity examined it in November 2011 and found it to be rich in calcium and sulfur, possibly the calcium-sulfate mineral gypsum. Click to read more. Credit: NASA/JPL-Caltech/Cornell/ASU

That’s why scientists are so excited about a vein of white rock sticking up like a row of bad teeth in the Martian soil near the rim of the crater Endeavour. It’s only the width of a thumb and about 18 inches long, but it sure stands apart from the dark, rusty-red Mars rocks. The Mars Opportunity Rover analyzed the Homestake vein – the first of its kind seen since the rovers’ mission began in 2004 – with its Alpha particle X-ray spectrometer last month. The instrument determines a rock’s composition by measuring what’s scattered back after bombarding it with X-rays and alpha particles. Scientists practically jumped from their seats when the analysis showed the vein was composed of calcium sulfate or gypsum – the same stuff you find in drywall and plaster of Paris.

The vein likely formed when water dissolved calcium from Martian rocks, picked up sulfur from other sources and deposited calcium sulfate in an underground fracture that later became exposed on Mars’ surface. Talk about uncovering evidence for ancient water flow on the planet – this is it! And it’s brings a special delight to me – to anyone – knowing that a similar process has shaped all those agates and calcite veins in our local rivers. Nature works from the same toolbox be it here or light years away.

The moon, Betelgeuse, Orion's Belt and Rigel all lie in a straight line this evening. The map shows the sky facing southeast around 9 p.m. local time. Created with Stellarium

Take a look up at the moon tonight. No eclipses are in store of course, but if you like casual alignments, we’ve got a nice one this evening. You can shoot a line straight from the moon through Betelgeuse, Orion’s Belt and Rigel in the constellation Orion. Watch as Orion becomes brighter and easier to see in the coming nights as the moon both wanes and rises later.

2 Responses

  1. Nikki Stresemann

    I stumbled on your site a couple months ago and Love It! Im from Mankato so its great to have a fairly close person updating all the time. Anyway, my husband and I love to look for agates and was excited to see the above pic, we’ve been to Gooseberry lots and would love to see this, can you tell me where in the park it is? Have you seen anymore in other Parks? Ive never thought to look for them there.

    1. astrobob

      Hi Nikki,
      Great to hear from another fellow agate hunter. Can you believe it – while I’ve seen lots of other materials in rock cavities like zeolite, spherulite, calcite, etc., this is the only agate I think I’ve come across embedded in a rock matrix. That’s why I stopped to photograph it. The agate-in-matrix is on the Superior Hiking Trail between Split Rock and the Gooseberry River. You would find it underfoot while hiking along Breaf Loaf Hill. Park in the lot at Split Rock and take the west side trail about 1.7-1.9 miles west of the Split Rock River toward Gooseberry. It’s midway along a high ridge that goes for a long way.

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