Eruptions Word of the Day: Dacite
Introducing the Eruptions Word of the Day - and we'll start with a favorite of mine: dacite.
I write the Eruptions blog on Big Think. I've been mesmerized with volcanoes (and geology) all my life. It helps that part of my family comes from the shadow of Nevado del Ruiz in Colombia, where I could see first hand the deadly effects of volcanic eruptions. Since then, I've taken a bit of a winding path to become a volcanologist. I started as a history major at Williams College, almost went into radio, but ended up migrating to geology, including an undergraduate thesis on Vinalhaven Island, Maine. I followed this up by changing coast to get my Ph.D. from Oregon State University. Then I ran a MC-ICP-MS lab at University of Washington for a spell (and wrote for an indie rock website). I spent three years as a postdoctoral scholar at University of California - Davis studying the inner workings of magmatic systems. I am now an assistant professor at Denison University and have projects in New Zealand, Chile and Oregon.
I am fascinated by volcanoes, their eruptions and how those eruptions interact with the people who live around the volcanoes. I started this blog after getting frustrated with the news reports of volcanic eruptions. Most of them get the information wrong and/or are just sensationalistic. I will try to summarize eruptions as they occur, translate some of the volcanic processes that are happening and comment on the reports themselves.
And no matter what people tell you, I definitely do not have a cat named Tephra. (OK, I do).
You can find out more about my research by visiting my website. If you have any comments, questions or information, feel free to contact me at eruptionsblog at gmail dot com.
So, I've had requests on the blog to help to do some defining of volcanologic terms on the blog, so I thought I'd try a new column called Eruptions Word of the Day. I'm not sure how often it will run, but let's give it a try.
Eruptions Word of the Day for July 5, 2010: Dacite
Dacite is a magma type defined by silica (SiO2) content between 63-68 (or 69) weight percent. That is the textbook definition, but some other typical characteristics of dacite lavas (or magmas) is the presence of certain minerals: plagioclase feldspar and hydrous minerals (containing water in their mineral structure) such as amphibole (typically hornblende) or biotite mica. If the dacite is hot and dry (lacking in water), you would expect to find pyroxene in the lava as well, although it is common in almost all dacites that even have hydrous minerals. Typically, dacite erupt anywhere from 800 to 1000 degrees Celsius. The intrusive equivalent for a dacite is granodiorite.
Dacite lava hand sample, with abundant amphibole and plagioclase feldspar.
Dacite is found in a wide variety of tectonic settings but is most common in continental subduction zone/arc settings, such as the Andes or the Cascades. They tend to be products of magma mixing or crustal assimilation (by another magma) to form the dacite. The types of eruptions that dacite magma produces can vary from lava flows and domes (effusive, passive eruptions) to explosive, plinian-style eruptions - but they are most famous for the explosive eruptions like Mount St. Helens, Lassen Peak and Unzen in Japan (see below).
Pyroclastic flows from Mt. Unzen in Japan. A flow like this killed the Kraffts in 1991.
However, there are impressive effusive dacite lavas flows, such as the Chao Dacite and Volcan Aucanquilcha (see below) in Chile and Llao Rock at Crater Lake, where minor explosive deposits preceded the lava flows that stretch upwards of 5-10 km from the vent. These lava flows are usually steep-sided with well-formed levees on the sides and pressure ridges along the flow tops (see below). It is thought that dacite lava flows or domes form when the magma is allowed to degas before erupting, allowing for a passive rather than explosive eruption.
Dacite lava flows on Volcan Aucanquilcha, Chile. Note the steep sides of the flows coming from from the main summit. Click on the image to see a larger version. Image by Erik Klemetti.
Dacite tend to be a "garbage bag" of minerals - they have collected a variety of crystals from different sources. These sources include the active magma of the eruption in question (phenocrysts), crystals from previous magmatism at the volcano (antecrysts) and wholly unrelated crystals (xenocrysts). This mixing creates disequilibrium, where minerals only stable in one condition find themselves in another, creating impressive reaction textures (see below). This variety of crystals supports the ideas that many dacites are the product of magma mixing.
Amphibole crystal breaking down in a mixed dacite from Volcan Aucanquilcha, Chile. The interior is cored with biotite mica, iron-titanium oxide (such as magnetite), and quartz, while the outer roughly hexagonal shape is amphibole. The clear crystals along the edge of the grain are formed from the breakdown of the larger amphibole crystal when it is in disequilibrium. Click on the image to see a larger version. Image by Erik Klemetti.
Explore how alcohol affects your brain, from the first sip at the bar to life-long drinking habits.
- Alcohol is the world's most popular drug and has been a part of human culture for at least 9,000 years.
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If you want to know what makes a Canadian lynx a Canadian lynx a team of DNA sequencers has figured that out.
- A team at UMass Amherst recently sequenced the genome of the Canadian lynx.
- It's part of a project intending to sequence the genome of every vertebrate in the world.
- Conservationists interested in the Canadian lynx have a new tool to work with.
If you want to know what makes a Canadian lynx a Canadian lynx, I can now—as of this month—point you directly to the DNA of a Canadian lynx, and say, "That's what makes a lynx a lynx." The genome was sequenced by a team at UMass Amherst, and it's one of 15 animals whose genomes have been sequenced by the Vertebrate Genomes Project, whose stated goal is to sequence the genome of all 66,000 vertebrate species in the world.
Sequencing the genome of a particular species of an animal is important in terms of preserving genetic diversity. Future generations don't necessarily have to worry about our memory of the Canadian Lynx warping the way hearsay warped perception a long time ago.
Artwork: Guillaume le Clerc / Wikimedia Commons
13th-century fantastical depiction of an elephant.
It is easy to see how one can look at 66,000 genomic sequences stored away as being the analogous equivalent of the Svalbard Global Seed Vault. It is a potential tool for future conservationists.
But what are the practicalities of sequencing the genome of a lynx beyond engaging with broad bioethical questions? As the animal's habitat shrinks and Earth warms, the Canadian lynx is demonstrating less genetic diversity. Cross-breeding with bobcats in some portions of the lynx's habitat also represents a challenge to the lynx's genetic makeup. The two themselves are also linked: warming climates could drive Canadian lynxes to cross-breed with bobcats.
John Organ, chief of the U.S. Geological Survey's Cooperative Fish and Wildlife units, said to MassLive that the results of the sequencing "can help us look at land conservation strategies to help maintain lynx on the landscape."
What does DNA have to do with land conservation strategies? Consider the fact that the food found in a landscape, the toxins found in a landscape, or the exposure to drugs can have an impact on genetic activity. That potential change can be transmitted down the generative line. If you know exactly how a lynx's DNA is impacted by something, then the environment they occupy can be fine-tuned to meet the needs of the lynx and any other creature that happens to inhabit that particular portion of the earth.
Given that the Trump administration is considering withdrawing protection for the Canadian lynx, a move that caught scientists by surprise, it is worth having as much information on hand as possible for those who have an interest in preserving the health of this creature—all the way down to the building blocks of a lynx's life.
The exploding popularity of the keto diet puts a less used veggie into the spotlight.
- The cauliflower is a vegetable of choice if you're on the keto diet.
- The plant is low in carbs and can replace potatoes, rice and pasta.
- It can be eaten both raw and cooked for different benefits.
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