A basic introduction to rocks part I - The igneous rocks

Saturday, August 24, 2002
By: Alan V. Morgan 


The new grant from NSERC's "Promo Science" initiative has allowed us to strike out in a new direction. Because it enables us to use colour it finally allows us the last stage in what we have been trying to do in black and white over the last 15 years. It allows us to explain things in still more detail, and in a way that was impossible before. To make the most of this we intend to embark on a series of small vignettes that hopefully will assist teachers and students in understanding some of the fundamentals in geology and the other "Earth sciences". Perhaps the most traumatic experience for many is in understanding rocks.
Our world is made of rocks. We use them on a daily basis, usually by walking or driving on them (or on some crushed and reconstituted version thereof). Our homes and office buildings are often made of rock materials;- bricks as refired clays, walls as reworked gypsum, dining utensils such as plates, cups and saucers (pottery, ceramic and china), counter tops and floors as slabs of rocks of various origins. Rocks are aggregates of different mineral grains and can be divided into three major families or rock groupings.
First are the Igneous (or "fire-formed") Rocks, usually created by outpourings from various volcanoes or by cooling deep under the crust. Ultimately, even deeply buried rocks are exposed to surface weathering and break down into their constituent minerals. These mineral grains are removed as sediment and are transported by gravity, wind, ice and water to a place of deposition where they accumulate, normally as marine sediments. The sediments, whether marine or terrestrial, become compressed and are often invaded by cementing agencies carried by percolating water. They are then lithified - turned from loose grains back to solid rock - for example limestones and sandstones, forming the second great rock group of the Sedimentary Rocks.
Sedimentary rocks suffer one of two fates. They can be weathered, broken down again into constituent grains and recycled as sediments, or they can be still more deeply buried, heated and involved in different types of tectonic movement. The associated heat and pressure together with circulating fluids modify and change these former sedimentary rocks into a third group known as the Metamorphic Rocks. Incidentally igneous rocks and earlier metamorphic rocks can also be modified or re-modified in the same fashion if they are involved in similar Earth movements. Schists, gneisses and marbles are examples of metamorphic rocks. These aspects of formation, weathering, erosion, deposition, lithification, and modification were covered in a description of "The Rock Cycle" in an earlier issue of WAT ON EARTH (Volume 13, No. 1. November 1999).
In this issue I would like to first describe the Igneous Rocks, with later issues of "What on Earth" covering the other rock families. All aspects of these topics are covered in as much detail (sometimes more) in our companion website; www.whatonearth.org.

The Igneous Rocks

Igneous rocks are typified by the "interlocking" nature of the crystal grains in the rock types where these are easily visible. It is important to understand the classification of these rocks since they are very common in most of the Canadian Shield and pieces have often been transported into areas further south, west and east, by the glaciers of the recent past. Igneous rock classifications vary from being relatively simple (rocks can be divided into dark and light and coarse to fine) to extremely sophisticated, with categories depending on the chemical and optical classification of the constituent minerals. We will only deal with the simple classification of igneous rocks, although it is important to understand that this does rely on the fundamental chemical makeup of the rock, which in turn is a product of the way in which these rocks originated. For example, the presence and percentage of silica in the rock is extremely important as well as the makeup of feldspar minerals and the presence of accessory minerals. In its simplest form (centrefold) the classification is based on colour (horizontal) and grain size or texture (vertical).


The left hand side of the chart is dominated by silica-rich and hence, light coloured rock types. Moving to the right, the rocks become progressively darker as the percentage of iron- and magnesium-rich minerals increases. Remember that there truly is a gradation from light to dark, and while it might be moderately easy to recognise the end members, the rocks in between are far more difficult to differentiate. These "in between" rocks fall into an "intermediate" category between the light coloured, silica-rich left side (termed acid or [acidic]) and the much darker right side (basic) rocks.


Igneous rocks commonly exposed at the surface originate in magmatic masses deep beneath the earth, usually at depths of anywhere from 10 to 50+ km and occasionally, and far more unusually, at much greater depths (~150+ km). Rocks that cool below the crust are termed intrusive. When these magma masses cool over long periods of time inside the crust the crystals within the rocks can be quite coarse, although large crystals are also a function of abundant elements that allow the crystals to grow. We call these rocks "plutonic" and the coarse-grained texture is described as "phaneritic". Crystals can range from millimetres to larger than one metre in size. These giant crystals form pegmatites (see WAT ON EARTH Fall Issue 2001, volume 15, number 1.)
Generally as the magma moves towards the surface the grain size decreases to one millimetre or less. This is a medium-grain size. Rocks in this higher crustal position often have two distinctly different crystal sizes with quite coarse crystals sitting in a far finer medium-grained matrix. These rocks are termed "hypabyssal" and the two-grain size texture is termed "porphyritic". In a porphyry the larger crystals set into the medium-grained matrix are known as "phenocrysts". We presume that these crystal "two-sized" rocks were created when magma moved more rapidly into near-surface conditions allowing the initial large crystals that had already formed to be enveloped in a finer crystal mass.
Eventually the magma reaches the surface where it is extruded as lavas of various types. Rocks that are poured out on the surface are extrusive. A lava is magma that has lost most of its volatiles which "boil off" as gases and liquids, including water. This rapid quenching in the much cooler surface environment means that crystals have little time to form. As such they have a fine-grained texture. This textural category is termed "apahanitic" and the rocks are said to be volcanic in nature. The crystals within the aphanitic texture usually cannot be discerned with the eye and even are difficult to see with a hand lens. In some cases the lava cools so rapidly that volcanic glasses are formed, as in the case of obsidian.
The chemical makeup is important in not only determining the colour of the rock and its constituent minerals, but also the shape of the volcano and the nature of the volcanic eruption. I will discuss this in a later article.
If we return to our igneous rock classification we can see in the "acidic" category that granite (light coloured, coarse-grained intrusive), give rise to microgranite (medium-grained intrusive) and eventually to the light-coloured extrusive lava type known as rhyolite. Typical colours represented in these categories are predominantly white, light-grey to buff and pink. Very rapidly cooled rhyolitic lavas often have glass crusts that appear black in colour. In fact when these are looked at in thin flakes, the rock colour is quite light grey. These glasses are known as obsidian, and although typically black when massive, they can also be greenish, purple, brown, yellow and even red in colour. The volcanoes that produce rhyolitic lavas are very gaseous. Extremely violent eruptions can produce a highly frothed lava, typically silver or buff-brown in colour, that is known as pumice. Such explosive eruptions will allow thick deposits of ash to accumulate around, and downwind from, the volcano. These ash deposits will lithify to form a consolidated rock known as tuff.
The "intermediate rocks" can be sub-divided into "lighter" and "darker" categories. At the lighter end the phaneritic rock syenite, gives way to medium-grained microsyenite and then to the lava trachyte. In the darker section, phaneritic diorite gives way to the hypabyssal rock type, microdiorite, and then to the aphanitic lava, andesite.
In the basic category the dark-coloured phaneritic rock type, gabbro, is replaced higher in the crust by medium-grained diabase (also known as dolerite in Europe) and then by the aphanitic lava type, basalt. Fast moving lava flows cool very rapidly and tachylyte (thin glassy films) are created on the surface. This is particularly true if the lavas are produced under water, or under ice. Although eruptions by basic volcanoes are not that explosive, initially there are a lot of lava fountains. Lava clots and ash get ejected near the vent, and the gas content creates voids in the lava ejecta forming scoria, the basic equivalent of pumice.
In the classification the final category involves the ultra-basic rocks. These are rich in iron and magnesium, generally formed deep within the crust or even at the top of the underlying mantle. They are exposed at the Earth's surface by tectonic movements that have brought mantle rocks to the surface in plate tectonic collisions of various sorts. The rocks are dark coloured, and frequently greenish, because of the presence of the mineral olivine. Typical examples are dunite (named from Mt. Dun in New Zealand and made exclusively from the glassy green mineral, olivine), and peridotite, that contains olivine, and other dark minerals. In Canada peridotite and a companion rock sepentinite, are found in Gros Morne Park in Newfoundland, as well as the in the area south of Quebec City. They are also present elsewhere in northern Quebec, northern Ontario, in parts of Nunavut and in British Columbia.
In later features we will deal with the other two families of rocks as well as products that are associated with volcanoes and the near-crust environment.
Alan V. Morgan

How many different types of rocks are used around you? A good student exercise would be to identify natural and "artificial" (human-modified) rocks.