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douglas.r.j

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@vigovideo.brasil

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Latent Heat and the Lower Crust

When continents collide, sedimentary rocks are shoved deep into the lower crust, where they are squeezed and heated and turned into metamorphic rocks. This rock shows basically the upper limit of metamorphism; the rock is no longer solid and I don’t know whether to call this metamorphic or igneous. This rock type is called a migmatite: it has crossed its melting temperature and part of the rock has melted to form light colored bands that separated from the still-solid dark colored bands. This rock type is common in metamorphic terranes and it occurs when you take some of the most common minerals on Earth, including Quartz, Plagioclase, and K-Feldspar, up to a temperature of about 650℃, a temperature we’d consider part of the Amphibolite Facies. Those minerals are found all the time in sedimentary rocks, including shales and sandstones, so when rocks are thrust to the bottom of continents in mountain ranges, migmatites should be common. In fact, new research suggests that this rock type imposes a fundamental constraint on metamorphism, limiting the upper temperature of the crust.

If you had a high school chemistry class, you probably heard of the concept of Latent Heat. If you take a phase, such as water or ice and add heat to it until it undergoes a phase transition, that phase stops heating up and stays at a constant temperature until the phase transition has finished. Ice water (at 1 atmosphere), for example, is fixed at 0℃ until the ice melts, while boiling water is fixed at 100℃. Rocks are more complicated because they’re not just a single mineral, but the rule is the same; when the rocks start melting, they get stuck very close to a single temperature, because any extra heat that comes into the rock goes into latent heat rather than raising the temperature.

On the other hand, granitic rocks may not melt at the same temperature as a sedimentary rock, because the melting temperature of a dry granite is higher than the melting temperature of fine-grained sedimentary rocks. Even though the melting temperature of these rocks is a fairly narrow range, they are common globally, because once the rocks start melting, generating that melt locks the temperature where it starts until the whole rock has melted away.

If Facebook doesn’t reduce the image size too much, you can zoom in on this photo and see that there are some nice garnets forming in the darker layers. If it’s too reduced, click through to the original image at the link below.

-JBB

Image credit: https://flic.kr/p/8Rm94D

Supercontinental breakup in the Jurassic

When continents break apart and new oceans form and grow between them, they tend to follow a certain sequence. No one knows for sure what kick starts the process, but convection currents in the mantle building up heat under the blanketing effect of the continental crust and plumes of hot mantle rising from the depths are the main contenders. The crust starts to pull apart and thin, and molten rock starts to flow into and through the continental rocks. The same process is happening right now in the Great African Rift with its copious magmatism (such as the massed basalts of the Ethiopian highlands) accompanied by the widening of Earth's youngest Sea, growing between Africa and Arabia and pushing the latter into Eurasia, resulting in the Zagros mountains of Iran.

After the separation of the supercontinent Pangaea, two main land masses known as Laurussia and Gondwana drifted apart. As the era proceeded, forces stirring deep below started to prepare the scene for the rifting apart of Gondwana, which took up much of the Cretaceous. Vast outpourings of lava breached the crust, amongst others in what is now Antarctica some 180 million years ago. While subsequent erosion has removed much of the rock erupted above ground, the plumbing systems and large underground intrusions of lava into the country rocks that accompanied the event are some of the best exposed on the planet, and are unfortunately found in one of the globe's remoter and harsher spots: the Dry Valleys. A sequence 4km thick is open for exploration to those geologists willing to endure the somewhat rough working conditions.

These rocks are called the Ferrar dolerite complex (after the geologist on Scott's 1901-4 expedition), and it comprises four massive horizontal sills extending for tens of kilometres and ranging from 100 to 350 metres thick, interconnected by a complex system of vertical dykes. The photo shows a split sill of black dolerite (a medium grained frozen magma of the same chemical composition as basalt) that intruded into the Beacon sandstone (an arkose, made of quartz and feldspar crystals deposited in shallow marine conditions between 400 and 250 million years ago) , where it stalled in the crust, intruding through and between the layers of sandstone.

Loz

Image credit: Michael Hambrey

Okay, so where was I? 

Extrusive rocks. Those are the volcanic rocks. They are very fine-grained, because they cooled quickly, reason why? They are a result of cooling lava. The difference between lava and magma?

Magma is the stuff underground flowing beneath the Earth’s crust while lava is the stuff that spews out of volcanoes.