Extrusive Igneous Rocks: Extrusive, or volcanic, igneous rock is produced when magma exits and cools above or very near the Earth's surface. These are the rocks that form at erupting volcanoes and oozing fissures. The magma, called lava when molten rock erupts on the surface, cools and solidifies almost instantly when it is exposed to the relatively cool temperature of the atmosphere. Quick cooling means that mineral crystals don't have much time to grow, so these rocks have a very fine-grained or even glassy texture.
Hot gas bubbles are often trapped in the quenched lava, forming a bubbly, vesicular texture. Ever wondered what the difference between a rock and a mineral was?
This EarthWord should cover it As fall foliage begins to blanket New Hampshire, pleasantly diverting the attention of residents and visitors, scientists are preparing to unveil some of the geologic secrets of the famous yet not-well-known rocks that lie beneath the fiery cover. The photo was taken by Erin Todd during helicopter-supported geologic fieldwork in Lake Clark National Park this past summer.
The project is funded by the USGS Mineral Resources Program that is focused on investigating the bedrock geology of the national park and surrounding areas through geologic mapping and supporting analytical work such as geochemistry and. It was hewn from a quarry in the Texas Hill Country, where Johnson was born and raised. A specimen of gabbro, an igneous mineral that was used as a purifying agent in the iron smelting process at the Saugus Iron Works.
This photo shows the ropey texture of a recently crusted pahoehoe flow, as well as its superior strength as a natural insulator. The crack is still glowing hot but the lava is no longer moving under the crust.
This outcrop is exposed in a rock quarry in Chalfant Valley about 25 km 15 mi southwest of Long Valley Caldera. The two main units of the Bishop Tuff deposit are visible here: 1 the lower 5 m 16 f of the section consists of the pumice that fell to the ground airfall pumice downwind from the eruption; and 2 the upper m f of the section consists of the.
Skip to main content. Search Search. Apply Filter. It is smoothly rounded on three sides and a sheer vertical face on the fourth. Half Dome, which stands nearly 8, feet 2, meters above sea level, is composed of granodiorite, and is the remains of a magma chamber that cooled slowly and crystallized The most common extrusive igneous rock is basalt, a rock that is especially common below the oceans Figure 4.
The mineral compositions of igneous rocks are usually described as being felsic, intermediate, mafic, or ultramafic as examples, see Figure 4. Felsic rocks are made of light-colored, low-density minerals such as quartz and feldspar. Mafic rocks are made of dark-colored, higher-density minerals such as olivine and pyroxene. Intermediate rocks have compositions between felsic and mafic. Table 4. The rocks listed in the table above are the most common igneous rocks, but there are actually more than different types of igneous rocks.
Granite is perhaps the most useful one to humans. We use granite in many building materials and in art. As discussed in the introduction to this lesson, pumice is commonly used for abrasives. Peridotite is sometimes mined for peridot, a type of gemstone used in jewelry.
Diorite is extremely hard and is commonly used for art. Basalt is the dark, fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium Mg and iron Fe , hence basalt is called a "mafic" rock. It can be either extrusive or intrusive. Granite is the light, coarse-grained rock formed at a depth that is exposed after deep erosion.
It is rich in feldspar and quartz silica and hence is called a "felsic" rock. Therefore, granite is felsic and plutonic. Basalt and granite account for the great majority of igneous rocks. Ordinary people, even ordinary geologists, use the names freely.
Stone dealers call any plutonic rock "granite. They generally talk about basaltic and granitic or granitoid rocks among themselves and out in the field, because it takes laboratory work to determine an exact rock type according to the official classifications.
True granite and true basalt are narrow subsets of these categories. A few of the less common igneous rock types can be recognized by non-specialists. For instance, a dark-colored plutonic mafic rock, the deep version of basalt, is called gabbro. A light-colored intrusive or extrusive felsic rock, the shallow version of granite, is called felsite or rhyolite. And there is a suite of ultramafic rocks with even more dark minerals and even less silica than basalt.
Peridotite is the foremost of those. The deep seafloor the oceanic crust is made almost entirely of basaltic rocks, with peridotite underneath in the mantle. Basalts are also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents. However, continental magmas tend to be less basaltic and more granitic. The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what rocks are on the surface, you can drill down and reach granitoid eventually.
In general, granitic rocks are less dense than basaltic rocks, and thus the continents float higher than the oceanic crust on top of the ultramafic rocks of the Earth's mantle. In the first few lectures I mentioned that certain isotopes of certain elements were unstable and underwent radioactive decay. Think about a pan of pop corn on the stove.
Each kernel has the potential to pop, but they do it one at a time. You never know which particular kernel is going to pop, but you know if you wait long enough, most of them will have popped.
All else being equal i. As the number of kernels dwindles, the number of popped corn kernels increases. You could plot the number of kernels left to be popped and the number that have already popped :. The time scale is determined by the half-life or the time it takes for half the kernels to pop. From looking at the graph, it is obvious that the number remaining at any time decreases with time, and more pop per unit time than pop later, when there are fewer left to pop.
In fact, the number that pop during any interval depends on the number of unpopped kernels that were there at the beginning of the interval.
The special number is called "e" and is about 2. The time constant is some number that depends on the rate at which the pop corn pops. I used the number the natural log of 2 0.
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