Unit D Chapter 2 “Rocks and the Rock Cycle” Lesson 2 Pgs D48-D53 Sedimentary Rocks

Formation of Sedimentary Rocks Of all the rock on Earth’s surface, 75% of it is sedimentary rock. But sedimentary rock is only about 5% of the crust. Unlike igneous rock which forms from material deep in the Earth, sedimentary rock forms from materials on the surface.

Formation of sedimentary rocks There are five steps to forming a sedimentary rock. Weathering is the process of breaking rock into smaller pieces. Moving water and freezing can cause this. Erosion is the movement of sediment to new locations by wind or water. Water which was trapped in small cracks kept freezing and expanding, slowly cracking this boulder into smaller pieces.

Formation of sedimentary rocks Deposition is the dropping of sediment from wind, water, or ice that slows down. Sedimentation is the process of layers of sediment building up for millions of years. The weight of the sediment compacts or packs down the sediment. Cementation is the process of gluing the particles together. Here you can see the layers of sediment that were deposited over millions of years.

The Two Types of Sedimentary Rocks: Clastic Rocks Clastic Rocks are made from sediments that were weathered, transported and deposited in layers. Clastic Rocks are classified by the size of the particles that form them. Fine particles form shale, medium particles form sandstone. Shale is the most common sedimentary rock. 75% of sedimentary rock is shale or siltstone.

Clastic Rocks continued Rocks that have coarse, pebble-sized particles with rounded edges form conglomerates. The shape of the particles in the rock can help you infer how far from the source the particles were deposited. If the particles are rounded and smooth, they traveled farther and were worn down. Particles with sharp edges did not travel as far. This conglomerate’s particles are rounded and smooth. This type of conglomerate (Breccia) has sharp-edged particles.

The two types of sedimentary rocks: Chemical rocks Chemical Rocks are formed from minerals that were dissolved in water, came out of a solution and were then deposited. Example: Mix salt and water together to create a solution. If you leave the saltwater out, it will eventually evaporate. When it does, it leaves behind salt crystals at the bottom. Stalactites and stalagmites are chemical rocks. They are formed from billions of drops of water leaving behind trace amounts of minerals each time they fall.

The two types of sedimentary rocks chemical rocks Chemical sedimentary rocks can also form from animal shells and skeletons. Coral is actually a chemical rock made of limestone from the shells of millions of coral. Shells can also pile up on the bottom of a lake or ocean. With compaction and cementation it can form limestone. Limestone is the most common chemical sedimentary rock.

Fossil Formation Fossils are usually found in sedimentary rock. They are destroyed in igneous rocks. They can sometimes be found in metamorphic. Fossils can form in different ways, but it does not happen easily. In one method, the bones of an animal are covered with sediment and the calcium is slowly replaced with minerals from the ground. This is “Sue”, one of the most complete T-Rex skeletons discovered. 80% of the original bones were found. Her remains are on display in the Field Museum in Chicago, Illinois.

Fossil Formation In another method, a dead plant or animal may fall into soft sediment and become buried. If the organic remains decay completely, other minerals can fill the empty space creating a mold or cast. This has let us observe remains from ancient species that did not have bones.

Uses of Sedimentary Rocks Sedimentary Rocks are used for a large variety of resources. Sandstone is a common building material and used to make glass. Limestone and its forms helps make concrete, building materials, chalk and antacids. (That’s why some medicines taste chalky…..they are chalk.) Shale and coal are sources of oil and natural gas for energy. This temple in Petra, Jordan was carved out of a wall composed of sandstone. Anyone recognize it from a certain movie with an archaeologist with a bullwhip?

So what have we learned? 1. Sedimentary rock makes up ___% of the rock on Earth’s surface. 75% 2. There are two types of sedimentary rock _______ and _______. Clastic and chemical 3. What are the five steps to forming a clastic rock? Weathering, erosion, deposition, sedimentation and cementation. 4. Why would you never find a fossil in a chunk of obsidian? Obsidian is an igneous rock, the fossil would be destroyed from the lava. 5. Explain how a chemical rock like stalactites form. Dripping water leaves behind deposits of minerals eventually building up to form a rock.