3.3 Sedimentary rocks form from earlier rocks.
REMEMBER: Rocks change into other rocks over time.
Some rocks form from rock particles.
· Sediment- something that settles
· Sediments- materials that settle out of water or air
Examples: pieces of rocks/minerals, plant and animal remains
· Sedimentary rocks- develop from layers of sediments that build up on land or underwater
Forming and Transporting Rock Particles
· Water and wind create and transport rock particles to streams and rivers, which eventually empty into lakes and oceans
· Strong winds pick up sand and rock dust and carry them to faraway places
· Sediments collect and form layers as large particles settle first, followed by smaller ones
Forming loose sediments into rocks
· Layers of rock get buried by other layers, which get buried by additional layers
· The above layers apply pressure to, or press down on, the sediments underneath
· Small sediment particles, such as silt and clay, form by pressure alone
· Other particles are held together by minerals that have crystallized between them, acting as cement
Some rocks form from plants or shells.
· Rocks formed from plants or shells undergo similar processes as those formed from rock particles
· These are fossils (remains of organisms from long ago)
· Coal is made from remains of plants, dead wood, bark, leaves, stems, and roots
· Contains no remains of earlier rocks
· Formed millions of years ago in swamps
· Plant remains -> rivers overflowing -> sediments buried -> peat -> lignite -> bituminous coal -> anthracite
· Made up of carbonate materials such as calcite
· Shells and skeletons of ocean organisms are made of calcite
· Organisms die and shells/skeletons settle to the ocean floor
· Over time, the sediment becomes buried, cemented, and forms limestone
· Limestone forms from material that came from older rocks
· White Cliffs (Dover, England)- chalk
Some rocks form when dissolved minerals re-form from water
· Some sedimentary rocks are made of minerals that crystallized as water dried up
· When water seeps through cracks on the Earth’s surface it may move through limestone. This dissolves some of the rock. This is how caves form. As water flows and drips through the cave, some of it evaporates. The new limestone that is formed may take odd and beautiful shapes (stalactites, stalagmites).
· Limestone towers in Mono Lake, CA- formed underwater as minerals crystallized and formed layers of sedimentary rock
Sedimentary rocks show the action of wind and water
· Sedimentary rocks are laid down in layers, with the oldest layers on the bottom
· Geologists studying the layers can tell something about what conditions were like in the past
· Fossils of fish or shells in a layer indicates the area was covered with water long ago
· Crossbeds- tilted layers in sandstone rocks that show the layers were once sand dunes
· Ripples- preserved ancient sand ripples
· Mud cracks- show that the rocks formed in areas where wet periods were followed by dry periods