Minerals and Rocks
Minerals are naturally occurring, solid, inorganic substances. They have a regular, repeating arrangement of atoms and molecules (see the Chemistry chapter). The study of minerals is known as mineralogy. People who study minerals are called mineralogists. Minerals can be described and classified according to their physical properties, such as their:
- Crystal structure and shape (called their habit)
- Hardness (measured using the Mohs scale of mineral hardness)
- Lustre (the way light reflects off them)
- Translucency (how see-through they are)
- Cleavage (how they break)
- Density (mass to volume ratio)
as well as how they react to substances such as acids or magnets. It is the properties of minerals that make them useful for important things such as manufacturing.
Over 5 400 different minerals have been acknowledged to exist by the International Mineralogical Association (IMA). This international group is responsible for naming minerals and verifying new mineral discoveries.
Rocks are made up of two or more minerals. Rocks containing valuable minerals are called ore. Minerals from ore are used to manufacture products that we use every day. This includes things like houses, stainless steel pots and pans, electronics, batteries, automobiles and fertilizer. Valuable minerals include base metals, industrial minerals and precious metals. Base metals are metals that do not contain iron, such as copper and nickel. Industrial minerals are minerals that do not contain any metals. Precious metals are metals of high value, such as gold, iron and platinum.
Rocks are classified as igneous, sedimentary, or metamorphic. Igneous rocks are rocks formed by the cooling and solidification of magma or lava. The word igneous comes from the Latin word “ignis” meaning fire. Igneous rock may form above or below the surface of the Earth. Igneous rock that forms below the surface is known as intrusive igneous rock. This type of rock cools slowly and has large crystals of different types of minerals which can be seen with the naked eye. Granite is a good example of this type of rock (A). Igneous rock that forms on the surface of the Earth is known as extrusive igneous rock. This type of rock cools quickly and has small crystals. Obsidian, a clear glass-like rock, is a type of extrusive igneous rock (B).
Sedimentary rocks are formed by the accumulation of sediments. They are made up of layers of minerals, rock particles or organic materials. The layers are formed over time as materials carried by water are deposited at the bottom of lakes, rivers and oceans or are transported by wind or ice along the Earth’s surface. Examples of sedimentary rocks include conglomerate, shale, limestone and sandstone. The shore of the Bay of Fundy (between Nova Scotia and New Brunswick) is a great place to see exposed sedimentary rock.
Did you know?
Fossils are only found in sedimentary rock!
Metamorphic rocks are rocks, which are formed because of a physical or chemical change to an existing rock through a process called metamorphosis. Metamorphosis means a change in form. You may have heard this word used for the life cycle process that butterflies undergo when they change from larvae to adults. Sedimentary, igneous and metamorphic rocks can become metamorphic rocks by being exposed to extreme heat and pressure, such as deep underground. Examples of metamorphic rocks include gneiss, marble, quartzite and slate. Marble is very strong, durable and beautiful. It has been a rock of choice for sculptors and builders for thousands of years.
Rocks can change from one form to another (igneous ↔ sedimentary ↔ metamorphic). This is known as the Rock Cycle (Figure 13). Rocks begin as magma deep underground (1). The magma cools (2) to form igneous rocks (3). Exposed rock can be worn away by wind, water and ice. This is known as weathering (4). The weathered material can move (erosion) and eventually settle at the bottom of a body of water (sedimentation) (5). Over time the layers of materials can be cemented together (lithified) to form sedimentary rocks (6). The layers can be shifted, folded and buried as a result of plate tectonics. This exposes them to heat and pressure (7). The heat and pressure transforms them into metamorphic rocks (8). Some of these rocks become so hot that they melt and form magma (9) and then the cycle starts again. The cycle may, however, be interrupted and follow any of the paths as shown in the image below. It can take millions of years for a rock to cycle through the rock cycle.
Illustration - Text Version
1. Magma begins underground
2. Magma cools
3. Forms igneous rocks
6. Lithification to form sedimentary rock
7. Exposed to heat and pressure
8. Some rock becomes metamorphic
9. Some rock becomes magma and cycle begins again
Weathering, Erosion, and Sediment Deposition
Wind and water can change the shape of the Earth over time. Weathering involves both the physical and chemical breakdown of rock. Physical weathering involves breaking the rock into smaller pieces. Freezing water can cause rock to become physically weathered. Since water expands as it freezes, water that freezes in a crack in a rock can split the rock apart. Chemical weathering involves chemical reactions that change the composition of the rock fragments. For example, acid rain can dissolve some types of sedimentary rock.
Erosion involves the transport of rock particles, sediments, and soils by water, wind, or glaciers. Eroded material that is transported from one place to another, such as from the upstream portion of a river to the downstream portion, will eventually be deposited (put down) on the river bed. Rock particles and sediment can be transported very dramatically in a landslide, which is the rapid downward sliding of a mass of soil and rock. Many kinds of events can trigger a landslide, such as erosion associated with rivers, glaciers, waves, heavy snowmelt or even earthquakes. Canada’s deadliest rockslide occurred on April 29th, 1903. On this day, 82 million tonnes of rock fell from the summit of Turtle Mountain, into the Crowsnest River valley below. The landslide buried mining buildings, homes, and the railway line, killing 90 people. You can still see rubble from the rockslide today.