Igneous, Sedimentary, or Metamorphic? Quiz

Answer: Sedimentary

Sandstone is literally composed of sand that has cemented together under the influence of water, pressure and mild heat. In the early stages of this sintering, biological processes also play a role, later on physical and chemical changes take over. Fully formed sandstone usually requires that the material reaches about 2000 meters of depth before being brought back to the surface. Being made of sand, the primary composition of sandstone is silicon dioxide.

Answer: Sedimentary

Limestone is sedimentary calcium carbonate. This compound precipitates from water when the pH value becomes high enough (more alkaline). Once carbonate mud has formed, pressure and temperature, primarily once these mud beds dry out.

Answer: Sedimentary

Unlike most other sedimentary rocks, shale is not of a specific primary chemical composition but rather consists of fine particles of various mineral compounds. The sediments forming shale typically deposit long after those forming other sedimentary rocks. Being deposited and formed in relatively superficial layers, shale exhibits prominent layering, creating a direction in which the material is split much more easily than in others.

Answer: Sedimentary

Halite - rock salt or sodium chloride - is not a typical sedimentary rock in that it does not require a lengthy formation process. Instead, it is formed directly when saline solutions evaporate and thus some of the salt precipitates. Halite can easily dissolve again if exposed to water, so most halite rocks of significant age are found underground, interspersed within other sedimentary rocks.

Answer: Igneous

Obsidian is also called volcanic glass. It is composed of many of the lightest elements present in lava. It forms when, during an eruption, the newly extruded material cools so quickly that it has no chance to form significant crystals. Obsidian is usually black in color and not fully stable - even without outside influences, it will slowly degenerate into crystalline rocks over millions of years. Like common glass, it often breaks with sharp edges, thus many stone age tools such as arrowheads or crude knives were made from obsidian.

Answer: Igneous

Diabase, often referred to as dolerite outside North America, is a basalt-like rock composed mostly of microcrystals. Deposits are often relatively small compared to those of other igneous rocks and can take the form of dikes and domes, among others. Some diabase resembles granite.

It is a common material for headstones although most of it is used in construction to create, among others, road and rail beds.

Answer: Igneous

Granite forms a subgroup of a wide class of igneous rocks composed of quartz and different types of feldspar. For such a rock to be considered granite, it must contain between one fifth and three fifth quartz and at least a third of the feldspar must be of the alkaline kind. Mica is also present in most granites and contributes to the typical sparkle of cut and polished granite.

In the 21st century, granite has become a very popular material for kitchen countertops, because it looks very good and is very easy to keep clean.

Answer: Igneous

Basalt is by far the most common igneous rock type, composing almost 90 percent of all igneous rocks. It is formed from mafic lava (the kind low on silica) and forms the majority of material deposited in most volcanic eruptions. Basalt also forms a major part of the surface of most other planets and moons - in particular, most of the surface of Venus and the maria (darker areas) on Earth's moon are made of basalt.

Answer: Metamorphic

Slate is a metamorphic rock derived from a sedimentary base - either shale or a similar rock. While it also shows layering just like shale, the orientation of those layers is not derived from the original shale but rather from the direction of compression throughout the metamorphic process. Slate is a traditional and eco-friendly roofing material with a long lifespan - slate roofs can last a century or more.

Answer: Metamorphic

Gneiss can form both from igneous and sedimentary rocks. It is most easily recognized by a characteristic banded appearance. Its formation requires high temperatures and pressures, often resulting in rocks significantly harder and more dense than the precursor rocks. Gneiss is among the oldest rocks on Earth, with specimens dated older than 4 billion years not being extraordinary.

Answer: Metamorphic

Schist develops at moderate pressures and temperatures and possesses a pronounced layer structure. This can be a challenge in some construction works because a schist foundation can suddenly fail if the direction of the layers happens to correspond with the direction of high loads.

In particular bridges and tunnels can be affected by this. This layering is the defining property of schist and most commonly develops when the original rocks already contain layered material.

Answer: Metamorphic

Marble is a metamorphic version of limestone or dolomite. The metamorph does not preserve the sructure of the original material but rather is a complete recrystallization. Completely white marble results from a pure parent rock while the commonly seen banded, colored, variants include various other minerals, often silicates. Like any carbonate-based rock, marble is highly sensitive to acid, so no acids (not even mild ones such as vinegar) should ever be used on marble.

Answer: Metamorphic

Quartzite should not be confused with quartz, although it shares the same chemical composition. Quartz as a mineral is a macrocrystalline variant of silica while quartzite is rather a metamorph resulting from sandstone and preserving the microcrystalline structure of its progenitor.

The differentiation between sandstone and quartzite is somewhat subtle - sandstone is more weakly cemented and breaks between the individual grains while the structure in quartzite is strongly cemented and breaks through individual grains.

Answer: Sedimentary

Coal is a class of sedimentary rock mostly composed of pure carbon, although, depending on the kind of coal, other elements such as nitrogen, oxygen and hydrogen are also present in various amounts. It is derived from organic deposits, mostly wood and peat. Lignite - a brown coal type - is the least pure type, often having just over 50% carbon and producing a lot of noxious emissions on combustion while bituminous coal and anthracite are higher grades with a higher carbon content.

When coal becomes almost pure carbon, it is considered graphite which is not suitable as a fuel due to being very difficult to ignite but has ample uses, for example as pencil leads and for some types of lubrication as well as being important in the construction of nuclear reactors.

Answer: Igneous

Pegmatite is an igneous rock usually derived from the last remaining liquid fraction of a cooling subterranean magma mass. As such, it is rich in lighter elements and often contains large crystals of minerals with higher melting points. In particular, gemstones such as corundum (ruby, sapphire) and semi-precious stones are often found embedded in pegmatite.

The mineral composition of pegmatite can be highly variable, even within a single rock measuring only a few meters across.