Rock type and formation
The rocks that are found at or near the Earth’s surface can be broadly divided into three types: igneous rocks, sedimentary rocks, and metamorphic rocks. There are many varieties of each of these three main types of rock and the exact type of rock that is formed can depend on factors such as temperature, pressure, and the minerals present at the time of formation.
Igneous rock
Igneous rocks are formed from molten rock (rock that has melted) which cools and hardens. If the molten rock erupts at a volcano, then the subsequent cooled and hardened rock is very fine-grained with crystals that can only be seen under a microscope. There are a number of fine-grained rocks that erupted in this way including basalt, andesite, and rhyolite (depending on the amount of silica in the magma) and volcanic glasses which are so fine-grained they look and behave like glass.
This type of rock generally cools quickly because it is exposed to the cooling air or water at the Earth’s surface upon eruption. Basalt covers the ocean floor (which covers over two-thirds of the Earth’s surface). If the rock cools more slowly then there is time for crystals to grow within the molten rock, and a coarse-grained rock is formed. Granites or gabbros are igneous rocks of this type containing large crystals. The Sierra Nevada mountains, New Mexico, and Dartmoor, UK, are made up primarily of granite. In addition to grain size, igneous rocks are often divided into acid rocks (often light-colored) which are rocks formed from the melting of continental rocks and basic rocks (often dark-colored) formed most commonly by the melting of oceanic rocks.
Sedimentary rock
Sedimentary rocks are formed from the products of the chemical and/or physical weathering of rocks exposed to the Earth’s surface. The sediment produced from such weathering can accumulate over time and eventually build up a deposit which, over time, can harden to form rock. Many sedimentary rocks are formed after the weathering products such as sand, silts, or clays are transported by rivers and deposited downstream in coastal regions. Sediments are cemented together and compacted and hardened over time by the weight and pressure of the sediments above them and by the precipitation of chemical cement such as calcium carbonate or silica. These processes result in rocks such as sandstones, siltstones, or mudstones.
Sedimentary rocks often contain structures that represent a record of the physical conditions present when the rocks were deposited. In addition, there are sedimentary rocks formed by the accumulation of the remains of either the skeletons or the organic remains of microscopic animals. These can be as fossils within other rocks or can represent most of the rock itself such as chalks, which are almost entirely the remains of coccoliths (dead microscopic plants called algae). It is amazing to think that in some areas such as the southeast of England, there are thick layers of the Earth’s surface that are made up almost entirely of the skeletons (tests) of coccoliths.
Other sedimentary rocks are formed when the concentration of a dissolved mineral in water is so great that mineral precipitates are formed. This can often happen when the water evaporates to leave behind solid minerals. Halite or gypsum are formed, for example, when seawater is evaporated towards dryness.
Metamorphic rock
Metamorphic rocks form as a result of partial melting and recrystallization of existing sedimentary or igneous rocks. These changes usually take place where there is also high pressure such as under hundreds of meters of bedrock or where rock is crushed at the junction of tectonic plates. As a result, many metamorphic rocks have a layered structure caused by this external pressure. Metamorphic rocks tend to be harder than sedimentary rocks and are more resistant to weathering and erosion. For example, limestone and mudstone change to marble and slate when metamorphosed.
The rock cycle
Over time all rock types can convert into other forms and this has often been termed the rock cycle. Igneous and sedimentary rocks can become metamorphic rocks under pressure and heat. All rock types can erode to form layers of sediment that can eventually become sedimentary rocks, and all rocks can be completely melted. When molten rock eventually cools and hardens at or near the Earth’s surface it will form igneous rock.
MCQ based on the above article
Which of the following is not a type of rock found at or near the Earth’s surface?
a. Igneous rocks
b. Sedimentary rocks
c. Metamorphic rocks
d. All of the above are types of rocks found at or near the Earth’s surface
Answer: d. All of the above are types of rocks found at or near the Earth’s surface
Which type of rock is formed from molten rock that cools and hardens?
a. Sedimentary rock
b. Igneous rock
c. Metamorphic rock
d. None of the above
Answer: b. Igneous rock
Which type of rock is formed from the products of the chemical and/or physical weathering of rocks exposed to the Earth’s surface?
a. Sedimentary rock
b. Igneous rock
c. Metamorphic rock
d. None of the above
Answer: a. Sedimentary rock
What type of rock is formed as a result of partial melting and recrystallization of existing sedimentary or igneous rocks?
a. Sedimentary rock
b. Igneous rock
c. Metamorphic rock
d. None of the above
Answer: c. Metamorphic rock
What is the name given to the process by which all rock types can convert into other forms?
a. Rock transformation
b. Rock evolution
c. Rock transition
d. Rock cycle
Answer: d. Rock cycle
Which of the following rock types is formed from the products of the chemical and/or physical weathering of rocks exposed to the Earth’s surface?
a. Igneous rock
b. Sedimentary rock
c. Metamorphic rock
d. Both a and c
Answer: b. Sedimentary rock
Which of the following is NOT a factor that can affect the type of rock that is formed?
a. Temperature
b. Pressure
c. Minerals present at the time of formation
d. Distance from the equator
Answer: d. Distance from the equator
Which type of rock is formed from molten rock that cools and hardens?
a. Igneous rock
b. Sedimentary rock
c. Metamorphic rock
d. Both b and c
Answer: a. Igneous rock
What type of rock covers over two-thirds of the Earth’s surface?
a. Granite
b. Basalt
c. Gabbro
d. Andesite
Answer: b. Basalt
Which of the following is a sedimentary rock formed from the accumulation of the remains of either the skeletons or the organic remains of microscopic animals?
a. Granite
b. Limestone
c. Slate
d. Basalt
Answer: b. Limestone
Which type of rock tends to be harder than sedimentary rocks and is more resistant to weathering and erosion?
a. Igneous rock
b. Sedimentary rock
c. Metamorphic rock
d. Both a and c
Answer: c. Metamorphic rock
What term is used to describe the process by which all rock types can convert into other forms over time?
a. Rock formation
b. The rock cycle
c. Metamorphism
d. Erosion
Answer: b. The rock cycle