Weathering of Rocks & Soil Formation

Earth Science - Middle School

Chemical Weathering Different soils can support a variety of living organisms. Soils with rich A horizons are able to support an abundant plant and animal life, as well as fungi, bacteria, insects and worms. Step 1 Mechanical and chemical processes weather the bedrock and turn it into soil particles that form the C horizon. Step 2 The A horizon layer forms as plants grow adding organic material to the soil and breaking up the rocks further. Step 3 The B horizon forms as clay and other soil particles percolate downward from the A horizon. Photos courtesy of USGS & NPS. bedrock bedrock bedrock C horizon bedrock C horizon bedrock A horizon bedrock C horizon bedrock B horizon bedrock A horizon bedrock bedrock Chemical weathering is the decomposition of rocks by chemical reactions. The three main chemical reactions that decompose rocks are acid reactions, hydrolysis, and oxidation. In the presence of water and natural chemicals, feldspar in granite changes to clay which crumbles easily, weakening the rock and causing it to break down. The rock eventually becomes sand. granite Mechanical Weathering Weathering is a collection of natural processes that, over time, break large rock into smaller and smaller pieces. Mechanical weathering is the physical decomposition of rocks. There are a number of physical processes that break rock down into smaller and smaller pieces. Abrasion The physical grinding of rock fragments is abrasion. Wind blows sediment against rocks which slowly sands the rocks down, much like sandpaper sands wood. Plant Growth & Animal Action Roots of plants can grow and expand within cracks in rocks, eventually prying the rocks apart. Freezing & Th awing As temperatures heat up during the day rocks slightly expand and as the temperatures cool down at night they contract. This repeated expanding and contracting can cause some rocks to ake apart at the surface. This process is called exfoliation. Release of Pressure Small cracks can ll with water. When the water freezes it expands which pushes the rock apart causing it to split. Repetitive cycles of freezing and thawing pushes the pieces of rock further apart from each other. This process is called frost wedging. Soil Formation Soil is a combination of decomposed rock and organic materials. There are different types of soil depending on the climate in which they form. Geologists study soil by studying the soil profi le. The soil profi le is a cross-section of the soil and is subdivided into horizons. The A horizon is the uppermost layer and consists primarily of top soil - decaying organic material. The B horizon is also known as the subsoil. The C horizon contains partially decomposed bedrock. The bedrock is layer of solid rock. All soil begins with this bedrock. Weathering of Rocks & Soil Formation © Copyright NewPath Learning. All Rights Reserved. 94-4107
\|xiBAHBDy01797kzU Chemical Weathering Mechanical Weathering What is weathering? ____________________________________________________ ______________________________________________________________________ What is mechanical weathering? __________________________________________ ______________________________________________________________________ Abrasion Describe: __________________________ ___________________________________ ___________________________________ Plant Growth & Animal Action Describe: __________________________ ___________________________________ ___________________________________ Freezing & Th awing Describe: __________________________ ___________________________________ ___________________________________ Release of Pressure Describe: __________________________ ___________________________________ ___________________________________ Soil Formation What is soil? ___________________________________ ______________________________________________ ______________________________________________ Describe: The A horizon __________________________________ ______________________________________________ ______________________________________________ The B horizon __________________________________ ______________________________________________ ______________________________________________ The C horizon __________________________________ ______________________________________________ ______________________________________________ The bedrock ___________________________________ ______________________________________________ ______________________________________________ Step 1 ____________________ ____________________ ____________________ ____________________ Step 2 ____________________ ____________________ ____________________ ____________________ Step 3 ____________________ ____________________ ____________________ ____________________ How is soil formed? ________________________________________________ __________________________________________________________________ __________________________________________________________________ abrasion acid reaction bedrock chemical weathering exfoliation frost wedging horizon hydrolysis mechanical weathering Key Vocabulary Terms oxidation soil soil profi le subsoil top soil weathering Photos courtesy of USGS & NPS. What is mechanical weathering? ___________________________ ___________________________ ___________________________ ___________________________ ___________________________ ___________________________ ___________________________ ___________________________ Weathering of Rocks & Soil Formation © Copyright NewPath Learning. All Rights Reserved. 94-4107