1 Igneous Rocks Granite Basalt
2 Formation of Igneous RocksThe inside of the earth is very hot - hot enough to melt rocks. And the deeper you go the hotter it gets. Below the surface the molten rock is called magma; at the earth's surface it becomes lava, although nothing has changed except the name. The fresh magma is white hot, brillant enough that you would have trouble looking at it. But as it cools it turns yellow, and then various shades of red. Eventually it cools enough to solidify completely and form an igneous rock, such as the granite and basalt below. Granite and basalt are the two most abundant igneous rocks at the earth's surface.
3 Formation Contiuned Magma/lava is a mixture of elements such as silica, iron, sodium, potassium, etc. As the magma/lava cools these elements chemically combine, or crystallize, in geometric patterns to form the eight rock forming minerals. For example, in the granite above the pink is orthoclase, the black biotite, and clear to gray mostly quartz . These eight minerals form the bulk of igneous rocks. They are arranged in Bowen's Reaction Series (BRS) by temperature of formation, high temperature ones at the top and low temperature ones at the bottom. Although it is useful to know these minerals they are not essential for a basic understanding of igneous rocks. Cooling is progressive in a magma/lava, some minerals becoming solid at high temperatures (top of BRS) and others at lower temperature (bottom BRS), so that part way through the cooling the magma/lava is a mixture of minerals and still molten rock. Magma/lava also contains lots of gasses such as water, sulfur dioxide, carbon dioxide, etc., and these are driven off into the atmosphere during cooling.
4 ALKALI GRANITE Typical MineralsQUARTZ > 20% ORTHOCLASE is greater than Plagioclase Mafics - < 10% A felsic igneous rock from the bottom of Bowen's Reaction Series. Typically light colored (few mafic minerals) with orthoclase the most obvious mineral (typically pink but also white or greenish), but quartz >20% is essential. This rock can be confused with a syenite which appears superficially similar. The amount of quartz is the key: <20% and the rock is "quartz syenite".
5 A mafic igneous rock from the top of Bowen's Reaction Series.Typical Minerals Minerals too small to identify (except phenocrysts). Composition will be from the top of Bowen's Reaction series (i.e. mafic) and we would expect Ca plagioclase, pyroxene, and perhaps some olivine. Note that these minerals are present in this specimen, just too small to see. A mafic igneous rock from the top of Bowen's Reaction Series. Typically dark colored, although weathered specimens can appear quite light, or reddish
6 Slow Cooling Magma If cooling is "slow" (thousands to millions of years) below ground the minerals grow large enough to see with the eye, as with the granite to the left. These are "coarse grained" (or phaneritic). Any rock in which the grains can be seen by eye are coarse grained.
7 Quick Cooling If cooling is "quick" (days to weeks) as at the earth's surface, the minerals do not have enough time to grow, and so are microscopic in size. These are fine grained (or aphanitic). For example, the rhyolite.
8 Very Quick Cooling If cooling is "very quick" (hours to days) the elements and compounds are frozen in place, no minerals form, and the result is a glass. For example the scoria to above left and the obsidian.
9 Basalt Mafic 1. Dark color 2. High specific gravity 3. Olivine/pyroxene/Calcic plagioclase rich 4. Fine grained - crystals only seen under high power 5. Forms at the surface, principally in the ocean basins, but also in isolated "hot spots" on the continents. 6. Forms high in Bowen's Reaction series
10 Granite Felsic . Light color 2. Low specific gravity 3. Quartz and orthoclase and sodium plagioclase rich 4. Coarse grained - crystals large enough to see by eye 5. Forms on the continents deep underground 6. Forms low in Bowen's Reaction series
11 Classification of Igneous Rocks Igneous rocks are classified in several different ways, but all rock classifications are a combination of texture and color/composition of the rock.
12 Texture of Igneous RocksCooling History Example Glassy Vesicular (cellular) Aphanitic (fine grained) Phaneritic (coarsegrained) Porphyritic (two grain sizes) Very fast cooling; non-crystalline. Very fast cooling with rapid gas escape forming bubbles in the non-crystalline rock. Slow cooling; microscopic crystal growth. Very slow cooling; crystals grow to visible size. Two stage cooling; one slow underground creating visible phenocrysts, the second fast at the earth's surface producing a fine grained groundmass. Obsidian Pumice, scoria Rhyolite, andesite, basalt Granite, diorite, gabbro Any aphanitic rock with the adjective
13 Classification of Igneous Rocks The color/composition of the rock is at its simplest divided into dark colored rocks (mafic), intermediate colored rocks (intermediate), and light colored rocks (felsic). If we combine texture/cooling history and color/composition in a grid we get the classification in the table below.
14 Bowen Reaction Series
15 IGNEOUS ROCK EVOLUTIONOne of the most important ideas geology has discovered is that igneous rocks evolve. That the earth began with a composition similar to that of the moon, that is, composed mostly of an mafic/ultramafic parent rock, and from that simple beginning all the other rocks have evolved through a sequence of fractionation processes. It is a core concept essential to understanding the Earth's evolution.
16 Rock Evolution
17 PLATE TECTONICS, AND IGNEOUS ROCK DISTRIBUTION
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19 Extrusive Rocks Fine-grained rocks form if molten rock cools rapidly when it is extruded onto the surface. Basalt is a common extrusive igneous rock. Some fine grained rocks forming at the edge of a lava flow have small holes or vesicles which are void spaces left by escaping gasses. Very rapid cooling can produce rocks with a glassy texture. Obsidian is such a rock where there was too little time for ions to combine into an orderly crystalline structure
20 Intrusive Igneous Rocks FormationsWhen magma intrudes into pre-existing rock it cools rather slowly because the surrounding host or country rock, as it is called, insulates the magma. As a result, crystals grow larger giving the rock mass a coarse texture. Such intrusive igneous, or plutonic rocks where the mineral grains are easily seen with the unaided eye are called porphyritic. The rock mass itself is called a pluton.
21 Batholiths Physical geographers are especially interested in rocks that form beneath the surface after they have been exposed by erosion of the overlying host rock. A batholith is a huge intrusive igneous rock mass or pluton that when uncovered creates topographic highs in mountainous regions. The Idaho Batholith for example covers some 16,000 square miles. These landforms are massive in character lacking linear ridges and valleys.
22 Sill, Laccolith, and DikeWhen magma intrudes between the layers or rock and solidifies it creates a feature called a sill. Sills form a hogback or cuesta when the layer of rock is dipping, or the tops of mesas when lying horizontal. If the magma solidifies as a pocket of igneous rock that warps the overlying rock it will form a dome or laccolith. The Black Hills of South Dakota is such a domal feature. If magma cools in near vertical fractures a dike is formed . Dikes often form on the flanks of volcanoes. When exposed by erosion they take the form of a linear ridge.
23 Sill or Hogback Steeply dipping hogback near Colorado Springs, CO.
24 Laccoliths Half Dome, a batholith in Yosemite National Park. (Courtesy of USGS)