Step 1: Weathering (Destroying Rock!)

The rocks that make high ridges are much more resistant to weathering than the surrounding rocks. Click on image for full size (53K JPG) Courtesy of Jerome Wyckoff

Rocks are broken apart by two types of weathering.

By chemical weathering, the minerals in rocks are dissolved into rainwater or changed from one type of mineral into another. Climate is an important factor determining the rate of chemical weathering. Warm, moist environments have more chemical weathering because water is needed for the chemical changes and warmth facilitates the reactions. Not all minerals are susceptible to chemical weathering. Rocks made of the mineral calcite, like limestone and marble, are very likely to dissolve when in contact with weakly acidic rain water. Feldspar and quartz, the most common minerals in the igneous rock granite, have very different levels of resistance to chemical weathering. Quartz doesn’t chemically weather very easily, but feldspar does. It chemically changes into clay minerals over hundreds and thousands of years.

By physical weathering, rocks are mechanically broken apart into smaller pieces. A variety of agents can be responsible for breaking up rocks including water and wind. Tree and plant roots often push rocks apart, especially when they grow in areas with little topsoil. If water freezes into cracks in a rock it will expand as it freezes, opening the crack even more. Large amounts of ice were responsible for physical weathering of rocks during the last Ice Age. They scraped the surface of vast areas of the land, removing bits of the rocks they moved across. Smaller glaciers continue this process in some areas of the Earth today.

Silicate minerals that form at high temperatures in Bowen’s series are less stable when they are at the cool Earth’s surface. They will weather away before silicate minerals that formed at lower temperatures.

Weathered rock fragments are either transported by water or wind, or they become a part of the soil. Soil is formed as rocks are weathered at the surface and combined with organic material like plant and animal remains. There are many different types of soil depending on rock weathering and the amount of organic material available.

The rocks that make high ridges are much more resistant to weathering than the surrounding rocks. Click on image for full size (53K JPG) Courtesy of Jerome Wyckoff

Over time, rocks exposed at the Earth’s surface wear away. This is called weathering. There is two types of weathering that break rocks apart.

By chemical weathering, the minerals in rocks are dissolved into rainwater or changed from one type of mineral into another. Chemical weathering happens quickly in warm, moist environments because water is needed for the chemical changes and warmth speeds up the process. Not all minerals are susceptible to chemical weathering. For instance, feldspar and quartz, the most common minerals in the igneous rock granite, have very different levels of resistance to chemical weathering. Quartz doesn’t weather very easily, but feldspar does. Over a long time, it chemically changes into clay minerals.

By physical weathering, rocks are broken apart into smaller pieces. There are many ways that rocks are physically weathered. Water flowing in rivers and streams can break rock apart, as can ocean waves on a rocky coastline. Tree and plant roots often push rocks apart, especially when they grow in areas with little topsoil. If water freezes into cracks in a rock it will expand as it freezes, opening the crack even more. Large amounts of ice were responsible for physical weathering of rocks during the last Ice Age. They scraped the surface of vast areas of the land, removing bits of the rocks they moved across. Smaller glaciers continue this process in some areas of the Earth today.

Weathered rock fragments are either moved away by water or wind, or they become a part of the soil. Soil is formed as rocks are weathered at the surface and combined with plant and animal remains. There are many different types of soil depending on rock weathering and the amount of plants and animals in the environment.

The rocks that make high ridges are much more resistant to weathering than the surrounding rocks. Click on image for full size (53K JPG) Courtesy of Jerome Wyckoff

Over time, rocks wear away just like your favorite jeans or the bottom of your shoes. This is called weathering. There are two ways that it can happen. Either rocks wear away chemically, or they physically break apart.

Wearing away chemically
Sometimes the minerals in a rock are dissolved into rainwater just like table salt dissolves in a glass of water. Other times, minerals are chemically changed from one type into another. Not all minerals are likely to be weathered in this way. For instance, the mineral quartz doesn’t weather very easily, but feldspar does. Over a long time, feldspar changes into clay.

Breaking apart physically
Sometimes rocks are broken apart into smaller pieces. This can happen when water flowing in rivers and streams tumble rocks and when ocean waves hit the rocks along a rocky coastline. Tree and plant roots can push rocks apart as they grow. If water freezes into cracks in a rock it will expand as it freezes, opening the crack even more.

The little pieces of rock are either moved away by water or wind, or they become a part of the soil.