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Igneous Rock
Many diamonds are found in a kind of rock called kimberlite. Kimberlite rocks are generally only found in very old parts of Earth’s crust. Kimberlite is a kind of igneous rock. Igneous rocks are often found around volcanoes because they form when hot semi- solid or liquid rock (either magma or lava) from Earth’s mantle cools into solid form. The magma either cools slowly below Earth’s surface or is released as lava in volcanoes.
When magma spews from a volcano, it cools quickly when it is exposed to the cooler temperature of Earth’s oceans or atmosphere. The cooled lava hardens into igneous rock. Rocks formed this way are fine-grained or glassy.
However, not all magma erupts out of a volcano. Some magma is pushed slowly toward Earth’s surface over many years. This magma will cool more slowly than magma erupting from a volcano. Igneous rocks formed this way are coarser.
Categories of Rock
Sedimentary rock forms from layers of sand, soil, clay, gravel, and other sediment that built up in one location over time. Sediment is a collection of particles and pieces of living and nonliving things. It collects in layers over time. Over thousands of years, the pressure of more and more top layers of sediment compresses the bottom layers of sediment into solid rock. The oldest sediment forms the bottom-most layers of the rock. Newer layers replace older layers at the top.
Metamorphic rock is a category of rock formed in chemical reactions where one type of rock is changed by pressure or heat into a new type of rock with different properties. The movement of the tectonic plates can cause rocks to get pushed down deep into the crust. There, the heat and pressure of all the weight on top of it cause chemical reactions in the rock to change it into a new rock with different properties. In a chemical reaction, the molecules of a substance are rearranged into a new substance with new properties. The word “metamorphose” means “to change or to transform.” It can take millions of years for a metamorphic rock to form.
The Rock Cycle
Rocks do not remain the same forever. The matter that makes them up is cycled and transformed into different categories over time. For example, rocks on Earth’s surface break down into sediment. That sediment can collect in layers. Over time, heat and pressure can compress the layers of sediment into new sedimentary rock.
Or the sediment can get pushed deep into Earth’s crust by the movement of tectonic plates. There, it can undergo chemical reactions that change its properties, turning it into metamorphic rock. Eventually, any rock pushed deep into Earth’s interior will melt into magma.
If magma reaches the surface, it will cool and harden into rock again. The processes that form, break down, and re-form rock from one category to another are called the rock cycle.
Weathering Rock
All rocks on Earth’s surface are broken down over time through a natural process called weathering. Weathering is the breakdown of rock into small pieces from exposure to wind, water, changes in temperature, and/or biological forces. For example, when kimberlite rocks reach the surface, they are immediately weathered. Sometimes weathering wears down the rock so much that the diamond becomes exposed. There are two kinds of weathering: mechanical and chemical.
Mechanical weathering takes place when rocks are broken down by physical force without any change in their chemical structure. Mechanical weathering is a physical change because it does not affect the chemical structure of a substance. Whenever rocks and sediment grind against each other, they mechanically weather surfaces. This kind of weathering is called abrasion.
Chemical weathering occurs when chemical reactions change the composition of rocks, rearranging the atoms or molecules into new substances with different properties. Chemical weathering generally occurs gradually over time.
Eroding Rock
Wind, water, or gravity then transports the sediment to new locations in a process called erosion. Kimberlite rocks containing diamonds have been found in areas away from volcanoes, eroded by wind, water or gravity.
Weathering and erosion usually work together. For example, wind and water carry pieces of sediment that grind against rock and wear it away. Water can also seep into cracks between rocky particles and expand, making cracks larger. over it. Over time, the rock wears down from the inside and crumbles into sediment.
Weathering and erosion are both processes that can occur rapidly or over many years. For example, it can take many years for some rocks to be broken down into smaller pieces of sediment and to then be transported through erosion to a new place. However, erosion can also occur rapidly, such as through landslides caused by flooding.
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