Running water, ice, and gravity all transport these sediments from one place to another by erosion. During sedimentation, the sediments are laid down or deposited. In order to form a sedimentary rock, the accumulated sediment must become compacted and cemented together.
When a rock is exposed to extreme heat and pressure within the Earth but does not melt, the rock becomes metamorphosed. Metamorphism may change the mineral composition and the texture of the rock. Skip to main content. Planet Earth. Search for:. The Rock Cycle Rocks change as a result of natural processes that are taking place all the time. The minerals provide details on the chemical composition of the rock, and on the conditions in which the magma originated, cooled, and solidified.
Geologists conduct chemical analyses of minerals to determine the temperatures and pressures at which they formed and to identify the dissolved gases and chemical elements that were present in the magma. Most magmas are predominantly silicate liquids, composed largely of silica tetrahedra that have not yet bonded together to become silicate minerals.
The chemical composition of an igneous rock tells us about the origin of the magma, beginning with which type of rock melted within the earth to form the magma in the first place, and how deep in the earth the melting occurred. Once magma has formed inside the earth, its composition may be modified. Minerals can grow from the magma and separate from it, changing the chemistry of the remaining liquid.
Or, one body of magma can mix with another that has a different composition. Magmas come in a range of compositions, from rich in silica and poor and iron and magnesium felsic to moderate in silica and high in iron and magnesium mafic.
Felsic igneous rocks, as a whole rock, tend to have light colors or shades: white, pink, light brown, light gray. Mafic igneous rocks, on the whole, tend to be dark colored, commonly black or dark gray. Most mafic magma originates by melting of rocks in the mantle that are extremely rich in iron and magnesium.
Felsic magma usually originates in the crust or by the shedding of mafic minerals as magma rises through the crust. The igneous texture tells us how the magma cooled and solidified.
Magma can solidify into igneous rock in several different ways, each way resulting in a different igneous texture. Magma may stay within the earth, far below ground level, and crystallize into plutonic igneous rock also known as intrusive igneous rock.
Or, magma may flow out onto surface of the earth as a lava flow. Another way that igneous rock forms is by magma erupting explosively into the air and falling to earth in pieces known as pyroclastic material, also called tephra. Lava flows and pyroclastic material are volcanic igneous rock also known as extrusive igneous rock. The igneous texture of a rock is not how it feels in your hand, not whether it is rough or smooth.
This basics page focuses on igneous rocks and gives you the background needed to understand the terms used in the igneous rock classification table. There are two main types of igneous rocks: 1 plutonic intrusive rocks, which form by solidification of molten rock deep within the earth, and 2 volcanic extrusive rocks, which solidify from molten rock erupted to the surface.
Volcanic rocks break down into two more categories: a lava flows and b tephra pyroclastic material. Igneous rocks are classified on the basis of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemical compositions. For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. The way that magma turns into a solid rock gives it a distinctive igneous texture.
For example, magma that becomes a pluton by slowly crystallizing growing minerals within the crust will develop a very different texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash spewing across a landscape and then settling down and welding itself together into solid rock. The texture of an igneous rock results from the cooling, crystallization, and solidification history of the magma that formed it.
Once you know the texture of an igneous rock, you can usually deduce from the texture whether it was intrusive or extrusive, lava flow or pyroclastic.
Texture in this context is not whether the rock feels rough or smooth to the touch. Igneous texture terms have objective definitions that refer only to igneous rocks. Let us start with textures associated with rocks formed by lava flows. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye. This is sometimes referred to as a fine-grained igneous texture. Some lava flows, however, are not purely fine-grained.
If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix.
This texture is called porphyritic. If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy. Pumice is the name of a type of volcanic rock with a frothy texture. If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals glass by definition is not a mineral, because it does not have a crystal lattice. Such a rock is said to have a glassy texture.
Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black. Now let us briefly consider textures of tephra or pyroclastic rocks. 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. You can actually plot the curve shown above with a hand calculator. Radioactivity behaves somewhat like popcorn as describe above. There are unstable isotopes of certain elements. These " parent " elements break down into other " child" elements by shedding particles from the nucleus. The rate at which this occurs depends only on the number of atoms around, so it follows exactly the same function as that described above.
We can plot the numbers of parent and daughter atoms as we did for unpopped and popped kernels of popcorn:. The decay constant of a particular parent can be measured in the laboratory by counting the number of times particles decay per second. If the decay constant of the parent is known, the age of a particular rock sample can be determined by comparing the ratio of parent to child, assuming there was no child in the sample to begin with, and none has been lost in the mean time.
Decay rate is related to the half-life as you saw above. All radioactive elements decay in the same way, just some take a long time and some decay very rapidly. For a material to be useful to geologists, it has to have a half-life on the order of geologic processes and be around. Here is a list of commonly used isotopes and their half-lives:. Because of the requirement that no child product be incorporated in the material to begin with, the minerals that are favored separate parent from child.
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