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Sheila has a master's degree in geology and has taught middle school through university-level science courses.
Rocks are continually recycled, typically on timescales of millions of years, in what's known as the rock cycle. The three major types of rocks - igneous, metamorphic, and sedimentary - can each be turned into a new metamorphic rock when given enough heat and pressure. Each rock type can be turned into a new igneous rock if it is completely melted and then cooled again. Sedimentary rocks are made when other rocks are broken into fragments and those fragments are cemented together to form a new rock. One of the most common types of sedimentary rock is sandstone.
As the name implies, sandstone contains sand-sized grains of rock fragments and individual minerals broken down from other, older rocks. We all have a sense of how big sand grains typically are from our experience going to the beach or playing in a sand box. Geologists also know that sand-sized grains have a particular measurement, from about 1/16th of a millimeter to 2 millimeters.
We say source rocks, the original rocks that eventually make up the small grains of a sandstone, are weathered when they break down. The pieces coming off a source rock can then be eroded, or carried away from the source area. The longer the chunks of a source rock are carried by wind, water, or ice, the more likely they are to be broken down into very small fragments. Certain minerals found within rocks are also more likely to survive significantly long rock fragment travels. Because quartz is such a hardy mineral, unlikely to undergo chemical changes during erosion, it is the mineral found in the greatest amounts in many sands. On most beaches (and of course there are exceptions), you will find a lot of hard, almost clear-looking grains of sand, which are most likely quartz.
After a source rock is weathered and eroded, the resulting sand grains might fill a bowl-shaped basin on land or underwater. With the pressure from more sediments landing above and the movement of water through the grains, the sand becomes sandstone. Finer grains of rock and new mineral growth fill the spaces between the grains, cementing the new rock.
As we touched on in our description of the formation of sandstone, the properties of any given sandstone are quite variable. They can be formed under water or on land, holding clues in their coloring about their formation location. This is due to chemical differences that depend on oxygen being present in the rock-forming environment. Sand dunes can also be captured during rock formation, giving clues about the environment the sandstone formed in.
The makeup of the matrix (the finer grains of rock material) and the cement (the new mineral growth between sand grains that hold the rock together) is also variable. The distance the sand grains traveled can also be inferred from the types of minerals making up the sand grains, or the framework. For instance, we know that abundant quartz grains suggests that pieces of the source rock traveled great distances before becoming part of the new sandstone. The pore space in sandstone is also important information, particularly when considering how water moves through the rock.
Sandstone can be broken down into two major groups: arenites and wackes. At least 15% of a wacke is made up of matrix material between the sand grains. Arenites have little or no matrix material, with just spaces and new mineral growth cement between the grains.
Each group of sandstone can be described in more detail, depending on the minerals found within the rock. An arenite made up mostly of quartz grains, for instance, is called a quartz arenite. If an arenite has more than 25% feldspar, the second most common mineral found in sands (the white grains you see on the beach), it is called an arkosic arenite. The most common wacke is greywacke, but we can also describe the mineral content in a wacke by naming it a quartz wacke, arkosic wacke, or lithic wacke, meaning it mostly contains rock fragments.
Sandstone is a type of sedimentary rock. They are made up primarily of sand-sized grains of either rock fragments or individual minerals. The grain makeup within sandstone depends on both the type of source rock and the conditions and how far the pieces of the source rock traveled before forming the sandstone. The grains within a sedimentary rock and any matrix material between the grains are used to describe the type of rock.
Sandstone can be broken down into two major groups: arenites and wackes. At least 15% of a wacke is made up of matrix material between the sand grains. Arenites have little or no matrix material, with just spaces and new mineral growth cement between the grains.
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