What do basaltic and mafic have in common

Fractional Crystallization - When magma crystallizes it does so over a range of temperature. Each mineral begins to crystallize at a different temperature, and if these minerals are somehow removed from the liquid, the liquid composition will change.

The processes is called magmatic differentiation by Fractional Crystallization. Because mafic minerals like olivine and pyroxene crystallize first, the process results in removing Mg, Fe, and Ca, and enriching the liquid in silica. Thus crystal fractionation can change a mafic magma into a felsic magma. Crystals can be removed by a variety of processes. If the crystals are more dense than the liquid, they may sink. If they are less dense than the liquid they will float. If liquid is squeezed out by pressure, then crystals will be left behind.

Removal of crystals can thus change the composition of the liquid portion of the magma. Let me illustrate this using a very simple case. Imagine a liquid containing 5 molecules of MgO and 5 molecules of SiO 2. If we continue the process one more time by removing one more MgO molecule. Bowen's Reaction Series Bowen found by experiment that the order in which minerals crystallize from a basaltic magma depends on temperature.

As a basaltic magma is cooled Olivine and Ca-rich plagioclase crystallize first. Upon further cooling, Olivine reacts with the liquid to produce pyroxene and Ca-rich plagioclase react with the liquid to produce less Ca-rich plagioclase.

But, if the olivine and Ca-rich plagioclase are removed from the liquid by crystal fractionation, then the remaining liquid will be more SiO 2 rich. If the process continues, an original basaltic magma can change to first an andesite magma then a rhyolite magma with falling temperature.

The intrusive or plutonic environment is below the surface of the earth. This environment is characterized by higher temperatures which result in slow cooling of the magma. Intrusive or plutonic igneous rocks form here. Where magma erupts on the surface of the earth, temperatures are lower and cooling of the magma takes place much more rapidly. This is the extrusive or volcanic environment and results in extrusive or volcanic igneous rocks. When magmas reach the surface of the Earth they erupt from a vent called a volcano.

They may erupt explosively or non-explosively. Non-explosive eruptions are favored by low gas content and low viscosity magmas basaltic to andesitic magmas and sometimes rhyolitic magma. Usually begin with fire fountains due to release of dissolved gases Produce lava flows on surface Produce Pillow lavas if erupted beneath water. Tephra that falls from the eruption column produces a tephra fall deposit. If eruption column collapses a pyroclastic flow may occur, wherein gas and tephra rush down the flanks of the volcano at high speed.

This is the most dangerous type of volcanic eruption. The deposits that are produced are called ignimbrites. Intrusive Environments. Magma that cools at depth form bodies of rocks called intrusive bodies or plutonic bodies called plutons, from Greek god of the underworld - Pluto. When magma intrudes it usually affects the surrounding rock and is also affected by the surrounding rock.

It may metamorphose the surrounding rocks or cause hydrothermal alteration. The magma itself may also cool rapidly near the contact with the surrounding rock and thus show a chilled margin next to the contact. It may also incorporate pieces of the surrounding rocks without melting them. These incorporated pieces are called xenoliths foreign rocks. Magma intrudes by injection into fractures in the rock and expanding the fractures.

The may also move by a process called stoping, wherein bocks are loosened by magma at the top of the magma body with these blocks then sinking through the magma to accumulate on the floor of the magma body. In relatively shallow environments intrusions are usually tabular bodies like dikes and sills or domed roof bodies called laccoliths.

Deeper in the earth intrusion of magma can form bulbous bodies called plutons and the coalescence of many plutons can form much larger bodies called batholiths. During a magmatic event there is usually a close relationship between intrusive activity and extrusive activity, but one cannot directly observe the intrusive activity.

Only after erosion of the extrusive rocks and other rock above the intrusions has exposed the intrusions do they become visible at the earth's surface see figure 6. The rate of cooling of magma depends largely on the environment in which the magma cools. Cooling time for material erupted into air and water can be as short as several seconds.

For lava flows cooling times are on the order of days to weeks. Shallow intrusions cool in months to years and large deep intrusions may take millions of years to cool. Because cooling of the magma takes place at a different rate, the crystals that form and their interrelationship texture exhibit different properties. Igneous rocks are classified on the basis of texture and chemical composition, usually as reflected in the minerals that from due to crystallization.

You will explore the classification of igneous rocks in the laboratory portion of this course. Basalts, Andesites, and Rhyolites are all types of volcanic rock distinguished on the basis of their mineral assemblage and chemical compostion see figure 6. These rocks tend to be fine grained to glassy or porphyritic. Depending on conditions present during eruption and cooling, any of these rock types may form one of the following types of volcanic rocks.

Obsidian - dark colored volcanic glass showing concoidal fracture and few to no crystals. Usually rhyolitic. Pumice - light colored and light weight rock consisting of mostly holes vesicles that were once occupied by gas, Usually rhyolitic or andesitic.

Vesicular rock - rock filled with holes like Swiss cheese or vesicles that were once occupied by gas. Usually basaltic and andesitic. If vesicles in a vesicular basalt are later filled by precipitation of calcite or quartz, the fillings are termed amygdules and the basalt is termed an amygdularl basalt.

Result from explosively ripping apart of magma. Loose assemblages of pyroclasts called tephra. Depending on size, tephra can be classified as bombs. Rock formed by accumulation and cementation of tephra called a pyroclastic rock or tuff. Welding, compactioncause tephra loose material to be converted in pyroclastic rock. Shallow intrusions like dikes and sills are usually fine grained and sometimes porphritic because cooling rates are similar to those of extrusive rocks.

Coarse grained rocks, formed at deeper levels in the earth include gabbros, diorites, and granites. Note that these are chemically equivalent to basalts, andesites, and rhyolites, but may have different minerals or different proportions of mineral because their crystallization history is not interrupted as it might be for extrusive rocks see figure 6. Pegmatites are very coarse grained igneous rocks consisting mostly of quartz and feldspar as well as some more exotic minerals like tourmaline, lepidolite, muscovite.

These usually form dikes related to granitic plutons. Igneous activity is currently taking place as it has in the past in various tectonic settings. These include diverging and converging plate boundaries, hot spots, and rift valleys.

Divergent Plate Boundaries. At oceanic ridges, igneous activity involves eruption of basaltic lava flows that form pillow lavas at the oceanic ridges and intrusion of dikes and plutons beneath the ridges.

The lava flows and dikes are basaltic and the plutons mainly gabbros. Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars.

Mafic magmas are usually produced at spreading centers, and represent material which is newly differentiated from the upper mantle.

Common mafic rocks include basalt and gabbro. Please note that some geologists with questionable motives switch the order of the magnesium and iron and come up with the term "femag.

Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silicon and oxygen , aluminum, and potassium. As we just learned, there are two main types of igneous rocks: intrusive rocks also known as plutonic rocks and extrusive rocks also known as volcanic rocks.

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.

Igneous textures include the rock textures occurring in igneous rocks. Igneous textures are used by geologists in determining the mode of origin of igneous rocks and are used in rock classification. There are six main types of textures; phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic. Because extrusive rocks make contact with the atmosphere they cool quickly, so the minerals do not have time to form large crystals.

The individual crystals in an aphanitic igneous rock are not distinguishable to the naked eye. Examples of aphanitic igneous rock include basalt, andesite and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian and pumice. Pegmatitic texture occurs during magma cooling when some minerals may grow so large that they become massive the size ranges from a few centimetres to several metres.

This is typical of pegmatites. As magma cools slowly the minerals have time to grow and form large crystals. The minerals in a phaneritic igneous rock are sufficiently large to see each individual crystal with the naked eye. Examples of phaneritic igneous rocks are gabbro, diorite and granite. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly.

The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained aphanitic matrix. The result is an aphanitic rock with some larger crystals phenocrysts imbedded within its matrix.

Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals. Figure 1. Different cooling rate and gas content resulted in these different textures.

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.