igneous rocks

composition of the original rock

The rock beneath the Earths surface is sometimes heated to high enough temperatures that it melts to create magma. Different magmas have different composition and contain whatever elements were in the rock or rocks that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust. Table 1.1 lists the abundance of elements found in the Earths crust and in magma. The remaining 1.5% is made up of many other elements that are present in tiny quantities. Element Symbol Percent Element Oxygen Silicon Aluminum Iron Calcium Sodium Potassium Magnesium Total Symbol O Si Al Fe Ca Na K Mg Percent 46.6% 27.7% 8.1% 5.0% 3.6% 2.8% 2.6% 2.1% 98.5%

magma composition

Different factors play into the composition of a magma and the rock it produces.

what melts and what crystallizes

As a rock heats up, the minerals that melt at the lowest temperatures melt first. Partial melting occurs when the temperature on a rock is high enough to melt only some of the minerals in the rock. The minerals that will melt will be those that melt at lower temperatures. Fractional crystallization is the opposite of partial melting. This process describes the crystallization of different minerals as magma cools. Bowens Reaction Series indicates the temperatures at which minerals melt or crystallize (Figure 1.1). An under- standing of the way atoms join together to form minerals leads to an understanding of how different igneous rocks form. Bowens Reaction Series also explains why some minerals are always found together and some are never found together. If the liquid separates from the solids at any time in partial melting or fractional crystallization, the chemical composition of the liquid and solid will be different. When that liquid crystallizes, the resulting igneous rock will have a different composition from the parent rock. Bowens Reaction Series. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL:

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how rocks melt

Whether rock melts to create magma depends on: Temperature: Temperature increases with depth, so melting is more likely to occur at greater depths. Pressure: Pressure increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures. Water: The addition of water changes the melting point of rock. As the amount of water increases, the melting point decreases. Rock composition: Minerals melt at different temperatures, so the temperature must be high enough to melt at least some minerals in the rock. The first mineral to melt from a rock will be quartz (if present) and the last will be olivine (if present). The different geologic settings that produce varying conditions under which rocks melt will be discussed in the chapter Plate Tectonics.

instructional diagrams

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questions

igneous rocks are classified by their

a) composition and density.

b) density and texture.

-->  c) texture and composition.

d) none of these.

what are the properties of mafic rock?

a) high density.

b) dark color.

c) contains olivine.

-->  d) all of the above.

ultramafic igneous rocks have the lowest amount of silica of any igneous rocks.

-->  a) true

b) false

the composition of an igneous rock is related to the rocks

a) crystal size.

-->  b) density.

c) location where it formed.

d) all of these.

the mineral quartz is likely to be found in ____ rocks.

-->  a) felsic

b) intermediate

c) mafic

d) ultramafic

granite is a type of felsic rock.

-->  a) true

b) false

which of these minerals or rocks are ultramafic?

a) komatiite

b) olivine

c) peridotite

-->  d) all of the above

if you were trying to identify a light colored igneous rock with tiny crystals, you would look at these rock types

a) mafic intrusive

b) mafic extrusive

c) felsic intrusive

-->  d) felsic extrusive

granite and andesite have the same composition but different texture.

a) true

-->  b) false

gabbro and basalt have the same composition but different texture.

-->  a) true

b) false

diagram questions

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