Rocks are everywhere in our world. They’re used for buildings, sidewalks, pool decks, landscaping, in jewelry and in nature to create some of the most astoundingly beautiful natural features in the world. We might think of one rock existing in its current state into infinity. In truth, rocks form part of an endless cycle controlled by the Earth’s forces of weather, compression, melting and exposure to chemical compounds. Rocks travel through a rock cycle that takes the newest rocks in the Earth in liquid form, transforms them into what we see above the surface and reabsorbs these rocks back into liquid form again.
Volcanoes contain liquid rock called magma. This liquid rock moves beneath the earth’s surface in areas of convergence of oceanic plates. These areas typically represent areas of high volcanic activity. When pressure increases, an eruption causes magma to flow upward and out onto the Earth’s surface. Lava refers to liquid rock at the Earth’s surface. Lava can harden as it’s exposed to the cooling air to form igneous rock.
Igneous rock forms from cooling magma. This type of rock can cool and form beneath the surface as well as above ground. The intense pressure and heat beneath the earth causes the formation of these rocks that often feature fine-grained volcanic rock or open-pored rocks such as pumice. Igneous rock can become the precursor for both sedimentary and metamorphic rocks in the rock cycle due to the forces of weathering, erosion, heating and compression.
Despite our impression that rock is a very hard substance, it’s subjected to the extreme forces of nature. An integral part of the rock cycle occurs in the sediment phase where igneous, sedimentary and metamorphic rock break down into smaller fragments. These fragments face weathering, erosion and the extreme forces of compression to form new rocks and land features.
Sedimentary rocks form from the millions of fragments of rock sediment that pile at the base of rivers and streams over time. Layers form with repeated additions of all types of rocks, including metamorphic and igneous rocks. As wind and water carry these particles, erosion occurs to the rock itself. Deposition at a final resting-place results in layered rocks such as those seen on the cliff faces of river gorges. Sedimentary rocks aren’t always smooth and can contain a variety of shapes and sizes of fragments embedded in the stone. According to the University Corporation of Atmospheric Research, 75 percent of the rocks at the Earth’s surface are sedimentary rocks. Sedimentary rocks can change into igneous or metamorphic rock.
The rock cycle continuously breaks up, moves and reuses rock fragments. Metamorphic rocks represent the part of the rock cycle involved with intense heat and high pressure. This environment actually changes the mineral composition of a rock to form new crystals. Metamorphic rocks can contain the same substances before heating or can have new minerals added during this stage. Both igneous and sedimentary rocks can be subjected to heat and pressure to make metamorphic rocks.