There are three main types of rocks: sedimentary, igneous, and metamorphic. Each of these rocks are formed by physical changes, such as melting, cooling, eroding, compacting, or deforming, that are all part of the rock cycle. The rock components of the Earth's crust are slowly but constantly being changed from one form to another, as illustrated in the chart below: The rock cycle is driven by two forces: Earth's internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust the water cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun Sedimentary Rocks Sedimentary rocks are formed from pieces of other existing rock or organic material. The formation of some sedimantary rock begins with the weathering of exposed rock into small fragments. Erosion by wind, water, ice, or biological activity helps transport these fragments to a new location. Once the sediment settles somewhere, and enough of it collects, the lowest layers become compacted so tightly that they form solid rock. Other sedimentary rocks form from chemical precipitation, when the solution it is dissolved in, usually water, evaporates and leaves the compound behind. This occurs as water travels through Earth’s crust, weathering the rock and dissolving some of its minerals, transporting it elsewhere. These dissolved minerals are precipitated when the water evaporates. Metamorphic Rocks Metamorphic rocks have been changed from their original form by very high heat or pressure. When a rock with flat or elongated minerals is put under immense pressure, the minerals line up in layers, getting squeezed into flat sheets. When granite, for example, undergoes this process it turns into gneiss. Other metamorphic rocks are formed without the obvious layering. For example, bituminous coal, limestone, and sandstone, given enough heat and pressure, can turn into anthracite coal, marble, and quartzite. Igneous Rocks Igneous rocks form when molten hot rock called magma cools and solidifies. Magma can cool slowly within the crust, over centuries to millions of years, forming intrusive igneous rock. It can also erupt onto the surface and cool quickly, within seconds to years, forming extrusive igneous rock. Granite and diorite are examples of common intrusive igneous rocks. They have a coarse texture with large grains, indicating that they spent thousands or millions of years cooling down inside the earth, a length of time that allowed large mineral crystals to grow. Alternatively, rocks like basalt and obsidian have very small grains and a fine texture. When magma erupts into lava, it cools more quickly than it would if it stayed inside the earth, giving crystals less time to form. These extrusive igneous rocks can also have a 'holey' texture, when the magma still has gases inside of it so when it cools, the gas bubbles are trapped and end up giving the rock a bubbly texture. An example of this would be pumice. The rock cycle is affected by various environmental and human activities, including: The Earth’s internal heat and pressure, which can cause rock to melt completely or transform it into metamorphic rock. The uplift of land caused by tectonic processes, which expose rock that was underground to weathering and erosion. The rate of weathering, which is affected by climatic conditions such as precipitation and temperature. Weathering increases in the presence of water and under warmer temperatures. Plant growth can physically break up rocks, and change the soil chemistry, increasing the rate of chemical weathering. The kind of rock that is weathered determines soil quality and nutrient levels. Rates of erosion caused by water, wind, ice, or gravity, and driven by the water cycle The size and depth of the bodies of water where sediment is deposited. Slower rates of water flow lead to the deposition of finer grained sediments. Urbanization, specifically, the paving of land with asphalt and concrete, increases water runoff, increasing erosion and decreasing soil quality in surrounding areas. Hydraulic fracking to remove oil and gas, which uses water, sand, and chemicals to create new or expand existing cracks in rocks that allow oil and gas to flow into drill holes for extraction. This also triggers earthquakes Human land and water use, including deforestation and agricultural activities, removing trees and other plants, plowing fields, and overgrazing livestock destabilizes soils and can increase rates of erosion by 10 to 100 times. Damming rivers and extracting water from freshwater ecosystems for human use changes where and how much sedimentation occurs, which affects soil quality and causes changes in habitats. Plants can trap sediment that otherwise might be deposited elsewhere. Extreme weather events can cause accelerated rates of erosion due to flooding or wave action. This is ocurring more frequently now becaus of climate change. Resources HTML & design by Bill Willis 2024