Earth s crust is made mostly of Igneous rocks. There are 3 main types of Sedimentary Rocks: 1. Clastic 2. Chemical 3. Organic

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1 Sedimentary Rocks Earth s crust is made mostly of Igneous rocks. But, most rocks on Earth s s surface are Sedimentary Rocks.. (75%) Sedimentary Rocks Sedimentary rocks are rocks that are made of broken-down materials from other preexisting rocks on Earth. There are 3 main types of Sedimentary Rocks: 1. Clastic 2. Chemical 3. Organic Clastic: rocks that formed from sediment fragments of other rocks. Chemical: rocks formed when dissolved minerals drop out of solution. Organic: rocks that form from the remains of once-living organisms. 1

2 Sedimentary Rocks Clastic Sedimentary Rocks Clastic Sedimentary Rocks: Weathering Rocks on Earth slowly break down under forces of nature. Wind, water, and ice break rock apart. This is called weathering. Think of weathering like a sand-blaster. When rock weathers, it breaks into fragments or pieces. These rock fragment are called sediment. Sediment includes: gravel, pebbles, sand, slit, and clay. 2

3 Clastic Sedimentary Rocks: Sediment Sediment is given a name based on it s size Gravel Sand Silt Clay From biggest to smallest Based on its size Clastic Sedimentary Rocks: Erosion Erosional currents (moving wind and water) have energy to carry sediment. If the sediment is large, it will need a stronger erosional current to carry it away. As a current slows, the largest sediments begin to drop out first. Strong currents carries clay, silt, sand, and gravel. Medium currents carries clay, silt, and sand. Gentle currents carries clay and silt. Quiet waters carries mostly clays and muds. 3

4 Clastic Sedimentary Rocks: Erosion These sediments are carried away by wind and water. This process is known as erosion. Wind and water have energy to carry sediments. Large sediments needs strong current to move it. You have experienced erosion: - At the beach when the wind blows and sand gets into your eyes. - In the ocean when the waves come in and you feel sand suspended in the water. - In a farm field where gullies form from water running over the land. Clastic Sedimentary Rocks: Erosion As these sediments are carried away, they are broken down more. The sediments start out large and jagged. As wind and water bounces them around, they get smaller. They also become more rounded the further away they are carried. The movement of the sediment acts like sandpaper to grind it down and smooth it out. 4

5 Clastic Sedimentary Rocks: Deposition Eventually these sediments will be deposited. Deposition: when sediments settle out of moving currents. Deposition occurs when erosional currents slow down. If the currents slows down, there is not enough energy to carry the sediments any longer. - When a stream enters a lake. - When a river enters and ocean. Clastic Sedimentary Rocks: Deposition 5

6 Clastic Sedimentary Rocks: Deposition Sediments are usually deposited in layers that build up. - Just like when you make deposits in a bank. - Your money builds-up over time. - Or, when you deposit your books in your locker throughout the day. Clastic Sedimentary Rocks: Deposition After sediment is deposited, clastic sedimentary rocks from by: 1. Compaction 2. Cementation 6

7 Clastic Sedimentary Rocks: Compaction As sediments build up, they begin to compact. Pressure from the layers above push down on the lower layers. - Think of a trash compactor - Compacting snow to make a snowball. If sediments are really small,, (silt( & clay) they will stick together forming rock. This rock forming process is known as COMPACTION. Clastic Sedimentary Rocks: Compaction Clay sediments compacts to make the sedimentary rock shale. Silt sediments compacts to make the sedimentary rock siltstone. Siltstone. Shale. 7

8 Clastic Sedimentary Rocks: Cementation If sediments are really large (sand and gravel) they will not stick together just by compaction. Larger sediment cannot just be squeezed together to make rock. There needs to be something else to hold the sediment together. - Water runs through the pore spaces between the sediment. - Dissolved minerals in the water act like glue (quartz and calcite). - The minerals harden and cement the sediment together as rock. This rock forming process is known as CEMENTATION. Clastic Sedimentary Rocks: Cementation Sand makes Sandstone Sand and Gravel makes Conglomerate Large and sharp rock fragments make Breccia Sandstone Breccia Conglomerate Sidewalk 8

9 Clastic Sedimentary Rocks: Cementation Think of cementation like fruit cake Fruit cake has sediments: - Nuts - Cherries - Raisins - Pineapple - Cloves But those sediments do not stay together on their own. The sediments would be loose if they were not cemented together with Batter Clastic Sedimentary Rocks: Classification Sedimentary rocks are classified by sediment size: - Clay, silt, sand, and gravel are the major types of sediment. - The are compacted and cemented to form sedimentary rocks. - Each sediment forms a different type of rock. 9

10 Clastic Sedimentary Rocks Summary of Clastic Sedimentary Rocks: 1. Sedimentary rocks form from pre-existing existing rocks. 2. Rocks on Earth weather and break into sediments. 3. The sediment is eroded and carried away. 4. The carried away sediment is later deposited. 5. As sediment is deposited, it builds up layer upon layer. 6. The layers of sediment exert pressure on each other. 7. The layers compact over time. 8. Minerals dissolved in water start to glue sediment together. 9. Finally, a Clastic Sedimentary Rock is formed. Sedimentary Rocks Chemical Sedimentary Rocks 10

11 Chemical Sedimentary Rocks Chemical sedimentary rocks form from dissolved minerals. - Water contains dissolved minerals. - When water evaporates, those minerals precipitate. - Precipitate means to come out of solution. - The mineral crystals grow together to make the rock. Types of Chemical Sedimentary Rocks: 1. Limestone (most common). 2. Rock Salt Chemical Sedimentary Rocks: Limestone Limestone Forms when Calcium Carbonate precipitates from solutions. The Calcium Carbonate is essentially the mineral Calcite. How can we tell if a rock is Limestone?? Travertine Limestone: Limestone often found in caves. 11

12 Chemical Sedimentary Rocks: Rock Salt Rock Salt Water rich in salt evaporates to leave salt crystals behind. Those crystals are especially the mineral Halite. Halite forms the rock, Rock Salt. 12