2.4: Rocks

Rocks Defined

Rocks are aggregates of the crystals of one or more minerals. A rock can be made of quartz, feldspar, and mica, or a rock can just be made of many grains of quartz. There are three main types: igneous, sedimentary and metamorphic. They form in different ways, and they have specific properties that help us to distinguish them. Through geological processes the rocks on Earth can be transformed from one type to another, and this concept is illustrated using the rock cycle (see the figure below). The three types of rock are depicted within rectangles, some of the intermediate forms of rocky materials (magma, outcrop and loose sediments) are shown as ellipses, and the processes that are important in the transformations are shown with arrows. Those processes include:

• Uplift (such as mountain formation) that results in rock that was present at depth in the crust being brought to surface in an "outcrop" and exposed to weathering,
• Erosion and transportation of weathered products (e.g., sand, clay, and ions in solution) and then deposition as sediments (e.g., within rivers or the ocean),
• Burial of those sediments beneath other sediments, followed by compaction (squeezing) and cementation to make sedimentary rock,
• Further burial which results in heating and more squeezing and then mineral transformations to form metamorphic rock,
• Further heating or other changes in conditions that result in melting to form magma, and
• Movement of magma towards surface where it can cool slowly to make intrusive igneous rock, or to surface where it can cool quickly to make extrusive igneous (volcanic) rock.

The rock cycle can be disrupted at any point by a change in conditions brought about by uplift, or burial (blue and red arrows), or in rare cases by the nearby presence of magma. The rock cycle is something you should continually refer to as you read this section of the text. It is not usually understood in one reading, so keep referring to it.

Figure \(\PageIndex{1}\): The Rock Cycle

Igneous Rocks

Igneous rocks form from the cooling of magma (molten rock at depth) slowly underground IN the crust, or from the cooling of lava (molten rock on the surface) quickly ON the crust. In general, the longer that cooling process takes (up to millions of years), the larger the crystals will be. Rocks formed from lava typically have mineral crystals that are less than 0.1 mm across (because they can cool in seconds or minutes). They are often called volcanic rocks as they often create volcanoes. They are also called extrusive rocks because they are formed on the exterior of the Earth. Rocks formed from magma have crystals that are typically larger than 1 mm across. They are called intrusive rocks because they are formed on the interior of the Earth. Classification of igneous rocks is based mainly on the texture (size of the grains) and rock composition.

Figure \(\PageIndex{2}\): General Classification of Igneous Rocks

In the figure above, the two textural categories are fine and coarse. The fine-grained rocks have small grains and form from lava. The coarse-grained rocks have large grains and form from magma.

Three broad compositional classes of igneous rocks are shown in the figure above, namely felsic, intermediate, and mafic. These are determined by the amount of silica in the rock Since pure silica is white or clear, rocks with high silica tend to be lighter and those with low silica tend to be darker. Accordingly, since felsic rocks have ~70% silica, they are light in color as there is little room for the darker minerals. Mafic rocks have ~50% silica so they have plenty of room for darker minerals and are typically dark in color. Intermediate rocks are in between compositionally and are typically medium-dark (typically grey) in color. The three main types of coarse-grained (or intrusive) igneous rocks are granite, diorite and gabbro. The equivalent fine grained, or volcanic or extrusive rocks, are rhyolite, andesite and basalt. Igneous rocks with close to 100% dark minerals (not shown on the figure) are known as ultramafic; these are rare on the Earth’s surface, but common in the mantle.

Mafic igneous rocks (e.g., basalt or gabbro) are denser than felsic igneous rocks. Basalt has a specific gravity of about 3 g/cm³, while for granite the value is about 2.6 g/cm³. This small difference becomes very important in the context of plate tectonics because it helps us understand why oceanic crust (made of basalt) subducts beneath continental crust (made of granite). In comparison, the ultramafic rock of the mantle has a density of about 3.3 g/cm³.

Sedimentary Rocks

Sedimentary rocks form near to the Earth’s surface following the accumulation of fragments of rocks and minerals that have been weathered (broken down) and eroded (removed) from outcrops (rocks sticking out of the ground), transported by gravity, rivers, waves, wind, or glacial ice, and then deposited as sediments. The sedimentary grains (e.g., grains of sand) are known as clasts, and the resulting sedimentary rocks are called clastic if they are composed mostly of such grains.

Figure \(\PageIndex{3}\): Cross-Bedded River-Deposited Sand on Vancouver Island. This is not sandstone because the deposit is soft and loose and can be scraped away with fingers. Instead, it is just a deposit of sediment that happens to be sand-sized.

Clasts can range in size from tiny (invisible) clay fragments to boulders the size of buildings. We classify clastic sedimentary rocks based on the clast sizes. The key piece of information to remember is that a grain of sand ranges in size from 1/16^th mm to 2 mm, and that means from about 1/4 the size of the period at the end of this sentence, to about the size of this capital O. (That depends, of course, on the type of device that you are reading this on.) Very fine sand will feel gritty (not slippery) between your fingertips. Clasts smaller than 1/16^th mm are classified as silt and clay, and those larger than 2 mm are classified as gravel. Gravel is sometimes broken down into granules, pebbles, cobbles, and boulders (in order of increasing size). Sand grains can be transported by rivers with medium flow, by strong winds, and by waves, and so sand deposits tend to accumulate in rivers and deserts and on beaches. Silt and clay can be transported in similar environments, but they tend not to be deposited unless the medium slows, so they will be deposited in lakes, and the ocean. Gravels can typically only be transported and deposited by fast-flowing water, and so they are commonly deposited in high-energy parts of streams. These sediments must then become buried beneath other layers of sediments and compressed and cemented before they can become sedimentary rock, as illustrated on Figure \(\PageIndex{1}\).

Three important types of clastic sedimentary rocks are illustrated below.

Figure \(\PageIndex{4}\) Examples of the Elastic Sedimentary Rocks Conglomerate, Sandstone and Mudstone. The pen is 15 cm long, and the coin in the mudstone photo is 2 cm across.

Sedimentary rocks can also form from the crystallization of ions that were transported in water as dissolved ions. These are known as chemical sedimentary rocks. For example, minerals form when water evaporates from an inland sea or lake because the ions get too concentrated to remain in the solution. Examples are rock salt (halite) and gypsum (which is used to make dry wall).

Sedimentary rocks can also form from materials associated with organisms. For example, marine organisms extract bicarbonate and calcium ions (HCO₃^– and Ca²⁺) from ocean-water to make calcite shells (CaCO₃) which then accumulate on the sea floor (typically in tropical areas around reefs) to form calcite mud and sand that later gets buried and becomes limestone. Other organisms make their shells out of silica, and those can accumulate on the sea floor to make the rock chert.

The final way sedimentary rocks can form is from the remains of plants, animals, and other biological materials that are compressed over time to form rock. These are called organic sedimentary rocks, and the most common example is coal which forms when the plant and animal materials decay in swampy or marshy environments.

Metamorphic Rocks

Metamorphic rocks form when pre-existing sedimentary or igneous rocks are heated and squeezed in such a way that one or more of the minerals present becomes unstable. The result might be that crystals of those minerals are converted into different minerals or into larger crystals of the same type.

One way to understand this process is to consider the sedimentary rock mudstone, which is mostly made up to clay minerals. Clays are low-temperature minerals; they are not stable at temperatures higher than about 150⁰ C. As a clay-rich rock is heated the clay minerals tend to break down and are converted into micas. At even higher temperatures those might be converted to minerals such as quartz, feldspar and amphibole. There is no melt involved as that would be an igneous process. This all happens as minerals recrystallize in the solid state.

Most metamorphism takes place at depth in the crust in areas that have experienced mountain-building (and so crustal thickening) and where there is compression due to plate convergence. That means that the rocks get heated (because of burial) and squeezed (because of the converging plates) at the same time. New minerals that form under these conditions are typically forced to grow perpendicular to the direction of that pressure, and so the metamorphic rock becomes foliated meaning that it takes on a fabric of aligned minerals or aligned bands of minerals.

Examples of foliated metamorphic rocks are shown below. Slate forms from mudstone at relatively low metamorphic temperatures and pressures, as clay minerals are turned into tiny (invisible) mica crystals. The alignment of these gives slate a layered look and the tendency to split into sheets. Schist forms at higher temperatures that allow the micas to become large enough to see. The mica crystals are generally parallel to each other, but the rock doesn’t split into sheets so easily. Gneiss forms at temperatures thar are typically beyond the stability of micas, and so is characterized by minerals like quartz, amphibole and feldspar that have segregated into dark and light bands.

Figure \(\PageIndex{5}\) Examples of Foliated Metamorphic Rocks. (From left to right: slate, schist and gneiss.)

It is very important not to confuse foliation with bedding. The slate pictures above is not splitting along pre-existing bedding planes in the parent mudstone, and the layers in the gneiss have no relationship to what bedding (if any) might have existed in the starting rock.

Some other metamorphic rocks include quartzite and marble. These form from the metamorphosis of the sedimentary rocks sandstone and limestone respectively. They tend not to be foliated even if they did form under directional squeezing due to the minerals making them up. Quartzite, for example, is mostly made up of quartz crystals, and they tend not to take on a directional fabric.

Media Attributions

• Figure \(\PageIndex{1}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{2}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{3}\): Steven Earle, CC BY 4.0
• Figure \(\PageIndex{4}\): (all photos) Steven Earle, CC BY 4.0
• Figure \(\PageIndex{5}\): (all photos) Steven Earle, CC BY 4.0