Geology Word of the Week: E is for Eclogite

Eclogite from the Mariánské Lázně Complex in the west Czech Republic.
Keele collection. Check out those gorgeous pink garnets!
Photo courtesy of Ian Stimpson.

def. Eclogite:
A high-pressure, high-temperature, coarse-grained metamorphic rock consisting primarily of pink-red garnet (almadine-pyrope variety) and green pyroxene (omphacite, a sodium-rich variety). Eclogites may also contain small amounts of other high-pressure minerals such as kyanite, quartz, hornblende, and zoisite. Eclogites form when mafic rock (basalt or gabbro) descends deep within the Earth, generally at a subduction zone. Mafic rocks consist primarily of pyroxene and plagioclase (along with some amphibole and olivine). At high pressures and temperatures, the original minerals in mafic rock are squished into the more compact (denser) minerals garnet and omphacite, and the mafic rock becomes eclogite. Eclogites form when mafic rock encounters temperatures greater than ~400 degrees Celsius and pressures greater than ~12 kbar (or ~1.2 GPa). These temperatures and pressures mean that eclogites form at a minimum depth of ~40 km; some eclogites may form as deep as ~150 km. As a reference, ocean crust (which is comprised primarily of basalt and gabbro) is generally only 6-10 km thick. Because they are very dense and inclined to descend even deeper into Earth's mantle, eclogites are rarely brought to Earth's surface. Eclogites may be exposed in ophiolite sequences and other places where deep mantle rocks are brought to Earth's surface. Often, eclogites experience partial or full retrograde metamorphism as they are brought to Earth's surface. That is, if eclogites are brought to the surface slowly, their minerals may change back into minerals that are stable at lower temperatures and pressures. Sometimes, higher-pressure minerals will have rims of lower-pressure minerals around them.

Bad Geology: Young Earth Creationists are Not Geologists

On twitter today, @geosociety and @earthmagazine were asking if people would attend a field trip led by Young Earth Creationists-- people who believe Earth is only ~6,000 years old. The question on twitter was in response to this Earth Magazine article about Young Earth Creationists leading a field trip at a recent Geological Society of America conference. I replied that, yes, I would attend such a field trip, but only so that I could rip the Young Earth Cretationists leading the field trip to shreds.

Accretionary Wedge #35: Favorite Geology Words

As many of you know, I hosted the Accretionary Wedge Geoblog Carnival for June, and I asked What's Your Favorite Geology Word? Turns out, many of you have favorite geology words! Geologists-- like many scientists, I suppose-- are fond of their jargon. Personally, I'm so fond of jargon that I blog about a geology word every week. I love many geology words, but if I had to pick an absolute favorite, it's ophiolite.

Thanks so much to everyone who participated and shared a favorite geology word! The words are listed below, in the order in which they were posted. If I somehow missed your word, please let me know in the comments, and I'll add it.

Chondrite Town

The road to the chondritic uniform reservoir. Switzerland, June 2010.

Last June I spent ten days in Switzerland and Italy exploring the geology of The Alps as part of Woods Hole Oceanographic Institution's annual Geodynamics Program. As we were driving one day on the trip, several geochemists in the group became very excited about the above sign. Apparently, the mysterious chondritic uniform reservoir (CHUR) is located in Switzerland! We tried our best to persuade our instructors that we should visit this important Earth reservoir, but we were told that Chur is a charming little Swiss town, not a place where we could find chondrites. So, we continued our drive to the mountains, but not before I snapped a photo as we passed a sign for Chur.

***For those who are not familiar, chondrites are primitive, stony meteorites that have not undergone significant melting or differentiation. Thus, chondrites are considered the original "starting material" for planet Earth, which scientists believe formed through accretion of chondritic bodies. The Earth probably started out as roughly chondritic then underwent chemical differentiation. To put it simply, the heavy stuff sunk to the center of the Earth (the core) and the lighter stuff floated to the top of the Earth (the crust) and the medium stuff stayed in the middle (the mantle).  For long-lived chemical systems (for example, the samarium-neodymium isotope system), geochemists like to think about how these chemical systems may have evolved over the ~4.54 billion year history of the Earth. Often, geochemists like to compare the evolution of certain chemical systems to a baseline. One baseline that is sometimes used is CHUR-- the chondritic uniform reservoir. The chemical composition of CHUR is based on the average chemical composition of chondritic meteorites. Where do we obtain these chondrites, if planets differentiated? Well, not all bits and pieces of the original chondrite parent bodies were incorporated into planets, which then differentiated. Some chondrite bits and pieces survived and have been zooming around our solar system. Chondrites often fall to Earth as meteorites, where they are sometimes recognized. Since these meteorites are stony, they look like Earth rocks and are tricky to tell apart. There could be one in your backyard! Most chondrite meteorites are found in places where there are few rocks, such as in deserts and in Antarctica.***

Geology Word of the Week: D is for Delta

The Nile Delta as seen from Earth orbit. Photo courtesy of
NASA and taken from Wikipedia here.

def. Delta:
1. The fourth letter of the Greek alphabet (uppercase Δ, lowercase δ).
2. A popular US airline with questionable service (except for those delicious little snacks they serve with your drink), often-delayed flights, and a hilarious in-flight safety video. 
3. A triangle-shaped deposit of sediment that forms where a river or stream flows into an ocean, lake, or other large, standing body of water.

Sorry, Folks, but Callan was Attacked by a T-Rex

This morning I had the pleasure of meeting up with Callan Bentley of Mountain Beltway. After several months of interacting on the internet, it was great to meet up in real life! I hope to meet some more of my fellow geobloggers over the next year or so.

Unfortunately, shortly after we met up, Callan was attacked by a T-Rex in the University of Wyoming Geology Museum:

A T-Rex attacks Callan.

Fortunately, Callan managed to escape, and we were able to tour the rest of the museum and have a sandwich afterward. Safe travels to Montana, Callan!

Limestone, Wildflowers, and Big Sky: A Wyoming Trail Run

I arrived in Wyoming last Sunday, and I'll be here for two months working with one of my advisors. I haven't had too much time to explore yet, but today (Monday) I went on a beautiful trail run*. Wyoming really is beautiful. I enjoy all the quiet, wide-open spaces. Here are a few pictures from today's trail run:

Wyoming big sky 1.