About.com Geology

The Mohs Scale

Thu, 02 Jul 2009 20:53:34 +0000

It's never out of style to go over the basics, and one of the most basic basics is the Mohs scale of mineral hardness. What catches some people by surprise is how low-tech it is. A fellow in the Forum found a strange rock and took it into his shop, applying grinding bits with his Dremel tool to check its hardness. That's not what geologists do—they pull out their nail clippers or pocket change and apply elementary scratch tests. More than nine times out of ten, that's all you need. The remaining times, a bit of sandpaper and maybe a piece of fluorite can characterize the hardness of nearly any mineral. The only difficult part might be memorizing the ten standard minerals of the full Mohs scale. This page lists them all, along with links to more information on each one.

Diamond is Mohs hardness 10 — Lars Plougmann, flickr.com

USGS Opens Its Map Toolbox

Wed, 01 Jul 2009 12:43:33 +0000

The U.S. Geological Survey's "Digital Map Beta" is a start toward a grand vision: anyone can build their own map, one as good as a printed USGS quadrangle map, by assembling current databases on the fly. The example shown combines aerial photography with road and placename databases into a quickie quad map PDF. And it's suitable for mashups with Google Maps. And it's all free. (Thanks to Kyle House, with his nose for the new, for tipping me off to this.)

Return to Ring Mountain

Tue, 30 Jun 2009 17:49:53 +0000

Ideally, you should visit field localities more than once. They never show you the same things. That's certainly true of Ring Mountain Preserve, a classic locality for high-pressure metamorphic rocks in Tiburon, California. I took time to revisit there this morning, and came back with a new photo for the Serpentinite Gallery.

Ring Mtn serpentinite — Geology Guide photo

Have We Got Concretions

Mon, 29 Jun 2009 17:46:45 +0000

It seems that by far the most popular part of the Geology Forum is people asking for help identifying their rocks, and the most popular of those, in turn, seem to be people with concretions. It has always been that way, even before forums and computers. The earliest geological thinkers had to figure out what to do with the shapes found within rocks. If you know nothing about rocks, how do you know what's a crystal, what's a fossil, what's a concretion, what's a vug or a geode or an amygdule? How do you know which came first? The first geologist, Nicholas Steno, made his name with an essay on "solids contained within solids," and it was a great advance. Concretions are solids that are younger than the rocks they inhabit. Come see a bunch of them in this picture gallery.

Concretion photo courtesy "bueuwe" in the Geology Forum

And Now a Message from...

Sat, 27 Jun 2009 22:13:54 +0000

Every Monday morning at 1 o'clock, my weekly email newsletter goes out to thousands of subscribers. What's it like, you may wonder, and is it worth subscribing to given all the spam that will surely result? Well, I'm here to tell you today that subscribing doesn't bring you ANY spam, and your address stays thoroughly confidential. To sign up, just use that link next to my picture on the home page. The first click is the important one—we offer you more options afterward, but your subscription starts right away. To see last week's newsletter, go here.

Italy's Dolomites a New World Heritage Site

Fri, 26 Jun 2009 13:49:49 +0000

UNESCO's World Heritage Committee has named some new localities to the list of sites, one of which is dear to geologists: the Dolomite range in northern Italy. The announcement noted the beauty of the mountains, but also the diversity of landforms, dynamic processes (i.e. landslides) and fossil record. This is a great day for the geoheritage movement.

The Dolomites are named for the rock dolomite, which in turn was named for a geologist, D�odat de Dolomieu.

Pity the Inselberg

Thu, 25 Jun 2009 20:46:19 +0000

A common landform in deserts is the inselberg, a remnant rock standing alone after all around it has eroded away. You pronounce it like a German would, "INN-sel-BAIRG," which means that you can never make it rhyme in a poem (except in German). But that's only a slight detraction from its coolness. See the inselberg with other erosional landforms in my latest picture gallery.

Inselberg — Geology Guide photo

New Deep Observatory Dedicated

Wed, 24 Jun 2009 01:01:54 +0000

It was a big day for science Tuesday as the Sanford Underground Laboratory was formally dedicated in the former Homestake Mine in the Black Hills of South Dakota. The Rapid City Journal reported on the occasion, noting the number of local jobs for former miners as well as the local opportunities for education. For America, the Sanford Lab will put us back among the leaders in large-scale physics projects. And for the world, the lab will help push back the frontiers of knowledge, using the quiet depths of former mines as listening posts for signals from the deep cosmos.

The Karrens of Columbia, California

Tue, 23 Jun 2009 23:22:02 +0000

Once a band of gold prospectors discovered a peculiar area of spiky limestone protuberances high above the Stanislaus River. Between those spikes lay fist-sized gold nuggets, and soon an army of diggers had plucked the world's richest placer deposit clean. Today Columbia, California, lives for its Gold Rush history, but that odd landscape—a genuine karrenfeld—remains to instruct the student of geomorphology. (That would be you and me.) See what I saw during my recent visit.

Marble karst — Geology Guide photo

Sarychev from Space

Tue, 23 Jun 2009 14:05:43 +0000

Usually I try to help you understand the big picture, but every now and then a big photo is just the thing. This is Sarychev volcano, in the Kuril Islands volcanic arc, in eruption on June 12, photographed from the International Space Station. See a much bigger version at NASA's Earth Observatory, where it's the Image of the Day today.

The eruption plume consists of brown ash. The white cloud cap formed in the air pushed upward by the rising plume in the cold stratosphere. It is a pileus cloud (named for an ancient Greek hat), now being penetrated by the eruption plume. The big ring of clear air around the island formed as the air around the plume moved downward in response. On the ground, three ashflows are moving down the volcano's slopes. The one on the bottom appears to be white with steam.

About volcanoes
About volcanism
Volcanic arcs
Kudriavy, another Kuril volcano