INTRUSIONS PART 1: INTRUSIVE IGNEOUS ROCKS
Magma is intruded somewhere deep down, it stays down there, it cools down there, never to be seen for the next couple of million years – why should anybody care about it? Well, as we’ll see, in many places it DID come up. In fact, a good percentage of Earth’s surface is made up of intrusive rocks. They give volcanologists and geologist invaluable insights and answers to many questions like: what is the magma like before it hits the surface/cold air/water, what is the original composition of the deep magmas? how does magma work its way up? how does hot magma interact with other rocks within the crust? In this part we will deal with the rock types of intrusions. I have planned a second part on the plutonic features we can see in the landscape.
INTRUDERS FROM THE ABYSS
Intrusive (or plutonic) igneous rocks form when cooling, crystallization and loss of gases cause a magma to solidify underground. Actually, most igneous rocks cool within the earth, rather than being extruded to the surface by volcanoes. An intrusive body is called a “pluton”. Plutonic rocks that form deep within the earth are called abyssal. Those that form near the surface are called hypabyssal, or subvolcanic. Intrusions vary widely, from mountain-range-sized batholiths to thin fracture fillings of aplite or pegmatite veins. Plutonic rocks form 7% of the Earth’s current land surface.
Magma usually starts its way upwards from Earth’s upper mantle region. It penetrates into faults, crevices or other cavities in the solid earth crust. This process is called intrusion. Unlike in an eruption, at one point the conditions are no longer right for the magma to travel further up. It cools in situ. Through its great heat, the new magma often changes the chemistry of the surrounding country rock (contact metamorphism). Sometimes, fragments of the host rock into which the magma has penetrated are not completely melted and remain as foreign inclusions, called xenoliths*, in the rock.
Later in geological time those plutonic masses may come to the surface by upheavals and folding in the earth’s crust. Others appear after their covering “ceiling” has been removed by erosion. The intruded rock is generally harder than its host rock. Therefore, most weathered plutons stand out in the landscape as solitary mountains.
Intrusive bodies are often the source for rich and rare ore deposits. During the long time a magma body stays in place, heavier minerals sink down by gravity. They accumulate in layers and pockets which are much sought after by mining companies.
*Xenoliths (from Ancient Greek: foreign rocks) are fragments of various rocks entrained in (and, in some cases, picked up and transported by) magma. Reaction of xenoliths with their host magma causes changes on both sides of the magma/rock interface. Xenoliths in mantle-derived rocks (such as kimberlites) are an invaluable (and, in some cases, the only available) source of information about the makeup of the Earth’s upper mantle and lower crust. The studies of xenoliths also help us understand how magmas are emplaced and crystallize.
GRAIN SIZE MATTERS
Erupting (extrusive) magmas cool relatively fast in air or in water. In contrast, a magma body that stays way down in the crust takes centuries to millions of years to cool down. Enough time for the crystals in it to grow in size to anything between visible (mm) to huge (meters!). The latter are often quartz crystals in pegmatites.
The outer margin of an intrusive body is usually finer grained because it is here where the most rapid cooling took place. This area of the intrusion is called the chill zone. Grain size increases away from the chill zone toward the center, where it remained the hottest for the longest time. Also mostly fine-grained are the subvolcanic rocks. They occur nearer to the surface and therefore crystallize faster.
Crystals stop growing when either all available space is filled up or when the temperature has dropped low enough to solidify all molecules of that mineral. At this time, other minerals might still remain liquid and keep rising until they also crystallize at a lower temperature. Very slowly a solid mass of rock is formed.
The classification for plutonic rock types is charted in the so-called QAPF diagram. The acronym stands for “Quartz, Alkali feldspar, Plagioclase, Feldspathoid”, it is also called “Streckeisen Diagram”. They are basically the same as their extrusive counterparts – classified according to the relative amounts of feldspars, quartz, and ferromagnesian minerals.
In fact, many of the subvolcanic intrusions are petrologically indistinguishable from lavas of similar composition. The important factor is the time they had for cooling. Thus, the same magma that would produce rhyolite lava at the surface would form a granite pluton kilometers below the surface. Granite is the most common intrusive rock on the continents. Gabbro is the most common intrusive rock in oceanic crust – comparable to basaltic lava.
THE PLUTONIC ROCK TYPES
Video: Nice one on intrusive rocks, by CVshorey (9 min.)
In the following I’ve assembled a selection of intrusive (plutonic) rock types. This is by no means complete, as geologists have made many more distinctions and sub-classifications by varying mineral contents of a given rock type. It would be possible to create a separate blog post for each rock type to describe its various sub-types (but this would go far beyond my intentions). Where available I have also stated the extrusive rock (lava) equivalent to each of the plutonic rocks.
Felsic to Intermediate Composition:
Granite is classified as a felsic rock – high in silica content. It is the intrusive equivalent of the extrusive rhyolite. Granites are light-colored, with coarse grains. They are primarily formed from continental crust. The main minerals found in granites include plagioclase feldspars, quartz and biotite. Potassium-plagioclase feldspar is what gives some granites their pink color. Other minerals that may be present in a piece of granite include amphibole and muscovite.
Pegmatites often form from granitic magmas in voids and cracks at the edges of a pluton, during the final stage of crystallization. They are the last parts to solidify from a melt and they use up any still available space. Minerals can develop very large crystals in pegmatites. Rule by thumb is, if the grains are larger than half an inch (or if they are larger than your thumb is wide) it is a pegmatite. While similar in overall composition to granite, pegmatite rocks often contain rare minerals that are not found in the rest of the pluton.
Granodiorite is felsic to intermediate in composition. It is the intrusive equivalent of the extrusive dacite. It contains a large amount of plagioclase, potassium feldspar and quartz. Minor components are the lighter colored muscovite mica and the darker biotite and amphiboles. The latter give it a distinct two-toned or overall darker appearance. Mica may be present in well-formed hexagonal crystals, and hornblende may appear as needle-like crystals.
Diorite rocks are classified as intermediate igneous rock between felsic and mafic. It is the intrusive equivalent of the extrusive andesite. Diorite is a relatively rare rock that is gray or dark-gray in color, with coarse grains. The mineral composition is primarily composed of plagioclase feldspars and amphibole. Smaller amounts of pyroxene, biotite and quartz may also be found in diorite.
Syenite has a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (<5%). The volcanic equivalent of syenite is trachyte. Syenite generally forms in thick continental crustal areas, or in Cordilleran subduction zones. Nepheline syenite is another plutonic rock that consists largely of nepheline and alkali feldspar. It is the intrusive equivalent of the extrusive fine-grained rock phonolite.
Gabbro is classified as a mafic rock type. It is the intrusive equivalent of the extrusive basalt. Gabbros are primarily formed from oceanic crust and are dark in color, with coarse grains. In addition to a high content of iron silicates and magnesium, gabbro primary mineral content includes calcium-plagioclase feldspar and pyroxene. Smaller amounts of olivine and amphibole may also be found within gabbro.
Dolerite (or diabase or microgabbro) is a mafic subvolcanic rock. It is an intrusive equivalent to extrusive basalt or plutonic gabbro. It occurs in shallow intrusive bodies and often exhibits fine-grained to aphanitic chilled margins which may contain tachylite (dark mafic glass). Diabase dikes occur in regions of crustal extension and often form dike swarms of hundreds of individual dikes or sills radiating from a single volcanic center.
Diabase is the preferred name in North America, yet dolerite is the preferred name in most of the rest of the world, where sometimes the name diabase is applied to altered dolerites and basalts. Many petrologists prefer the name microgabbro to avoid this confusion.
Ultramafic Composition and Foids:
Peridotite is an ultramafic rock. It is an intrusive equivalent to the extrusive komatiite. It is almost completely composed of ferromagnesian minerals. These rocks are dark in color, with coarse grains. Peridotite is believed to be a major component of the Earth’s mantle due to an extremely high melting point. As a result, peridotite is rarely found on the planet’s surface. A sub-type is dunite which is almost exclusively composed of olivine (>90%). This mineral gives the rock its olive-green color.
Kimberlite, also an ultramafic rock, occurs in the Earth’s crust in vertical structures known as kimberlite pipes (diatremes) as well as igneous dykes. It also occurs in horizontal sills. Kimberlite pipes are the most important source of mined diamonds today. The consensus on kimberlites is that they are formed deep within the mantle. Formation occurs at depths between 150 and 450 kilometres (93 and 280 mi), potentially from anomalously enriched exotic mantle compositions. They are erupted rapidly and violently, often with considerable carbon dioxide and other volatile components. It is this depth of melting and generation which makes kimberlites prone to hosting diamond xenocrysts.
The Foidolite rock is a special case in the rock family. It is the intrusive equivalent to the extrusive foidite. It is a rare coarse-grained intrusive igneous rock. There was enough potassium, calcium and sodium available in the magma to form feldspars (>60%), but foidolites are free of silica. Crystals of alkali feldspar, plagioclase, biotite, amphibole, pyroxene, and/or olivine may be present within the rock. The varieties of foidolites are named according to their main content of feldspathoid** minerals.
**Feldspathoids (or foids) are aluminosilicates containing essential alkali metals (usually Na and/or K) or Ca. Like structurally similar feldspars, feldspathoids are typically colorless (but may be “tinted” green, gray or red by tiny inclusions of other minerals), have moderate hardness values (5-7, with the exception of some zeolite-group minerals), and are fairly susceptible to alteration. The two most common foids are nepheline (Na3KAl4Si4O16) and leucite (KAlSi2O6).
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– Using the QAPF diagram
– Methods of pluton emplacement (Wikipedia)
– James St. John’s geology pages
– Flow Chart for the Classification of Igneous Rocks
– Alkaline Rocks by A.R. Chakhmouradian
– Discovery of a LCT pegmatite in […] Southern Alps, 2000 (PDF)
– Pegmatites: Crystallization Dynamics (2007, PDF)