The Rock Cycle & The Law of Superposition Sabrina, Carlie, Olivia and Emily

Carlie

Introduction To Rock Cycle “ Rocks contain clues to the earth’s past, including evidence of ancient life- forms. Earth scientists can use the information found in the rock record to help them understand the geologic processes that have shaped the earth’s crust.” The three rocks that are seen and produced in the Rock Cycle are: metamorphic igneous sedimentary Sabrina

Metamorphic rocks form deep in the earth where high temperature, great pressure, and chemical reactions cause one type of rock to change into another type of rock. Metamorphic rocks begin to form at 12-16 kilometres beneath the earth's surface. They begin changing at temperatures of 100 degrees Celsius to 800 degrees Celsius. Olivia Metamorphic Rock Fascinating Fact The oldest known rock lies in Canada. The Acasta gneiss, a metamorphic rock, is 3.96 billion years old.

EMILY Igneous rocks are the product of the solidification of magma, which is molten rock generated by partial melting caused by heat and pressure in the deeper parts of the Earth's crust or in the upper mantle. These hot magmas have lower densities than their source rocks and rise buoyantly to the surface. On the way, they cool and may crystallize partially or completely. Olivia Igneous Rock Fascinating Facts Some igneous rocks float (pumice) because of a large number of trapped air bubbles.

Sedimentary rock is a type of rock that is formed by sedimentation of material at the Earth's surface and within bodies of water. Particles that form a sedimentary rock by accumulating are called sediment. Before being deposited, sediment was formed by weathering and erosion in a area, and then transported to the place of deposition by water, wind, mass movement or glaciers. Carlie Sedimentary Rock Fascinating Fact Seventy percent of all the rocks on earth are sedimentary rocks.

Steps of The Rock Cycle

Weathering & Erosion Igneous, sedimentary, and metamorphic rocks on the surface of the earth are constantly being broken down by wind and water. Wind carrying sand wears particles off rock like sandpaper. Rushing river water and crashing surf rub off all the rough edges of rocks, leaving smooth river rocks or pebbles behind. Water seeps into the cracks in mountain rocks, then freezes, causing the rocks to break open. The result of all this: large rocks are worn down to small particles. When the particles are broken off a rock and stay in the same area, it is called weathering. When the particles are carried somewhere else, it is called erosion. Sabrina

Transportation Deposition Eroded rock particles are carried away by wind or by rain, streams, rivers, and oceans. As rivers get deeper or flow into the ocean, their current slows down, and the rock particles (mixed with soil) sink and become a layer of sediment. Often the sediment builds up faster than it can be washed away, creating little islands and forcing the river to break up into many channels. Carlie

Compaction & Cementation Metamorphism As the layers of sediment stack up (above water or below), the weight and pressure compacts the bottom layers. Dissolved minerals fill in the small gaps between particles and then solidify, acting as cement. After years of compaction and cementation, the sediment turns into sedimentary rock. Over very long periods of time, sedimentary or igneous rocks end up buried deep underground, usually because of the movement of tectonic plates. While underground, these rocks are exposed to high heat and pressure, which changes them into metamorphic rock. This tends to happen where tectonic plates come together: the pressure of the plates squish the rock that is heated from hot magma below. (Tectonic plates are large sections of the earth's crust that move separately from each other. Their movement often results in earthquakes.) Olivia

EMILY Melting Metamorphic rocks underground melt to become magma. When a volcano erupts, magma flows out of it. (When magma is on the earth's surface, it is called lava.) As the lava cools it hardens and becomes igneous rock. As soon as new igneous rock is formed, the processes of weathering and erosion begin, starting the whole cycle over again! Emily

EMILY Law of Superposition

Law of Superposition Scientists use a basic principle called the law of superposition in attempting to determine the relative age of a layer of sedimentary rock. The principle states that an undeformed sedimentary rock layer is older than the layers above it and younger then the layers below it. Scientists also know that sedimentary rock generally forms in horizontal layers. Therefore, they can assume that most sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements after the layers were formed. Carlie

Unconformities Sometimes movements of the earths crust lift up rock layers that were buried and expose these layers to erosion. If the eroded surface is lowered or the sea level rises, sediments will again be deposited, forming new rock layers. The missing rock layers create a break in the geological record witch is called unconformity. A unconformity shows that for a period of time deposition stopped, rock was removed by erosion and then deposition resumed. Three types of unconformities are: Nonconformity Angular unconformity Disconformities Sabrina

Sedimentary rock is stratified or deposited in layers Sedimentary rock is stratified or deposited in layers. Metamorphic rocks are usually unstratified. When stratified rock rests upon unstratified rock, it is called non conformity. Olivia Nonconformity

Angular Unconformity EMILY The boundary between the tilted layers and the horizontal layer is called and angular unconformity. In step one, sediment weathered from land and carried to the sea accumulates on the sea floor and over millions of years turns to rock layers. Then the collision of plates, giant sections of the earth's crust that constantly shift, lift and tilt the layers until the layers rise above sea level and then weather and erode. They erode for millions of years until the edges of the tilted layers become a flattened plane. Finally, in step four, sea level rises or land sinks. Sediments wash down, forming new horizontal layers that cover the submerged, tilted layers. These four steps could take hundreds of millions of years to complete. Emily Angular Unconformity

EMILY The boundary between the older, eroded surface and the younger layers is nearly horizontal and is called a disconformity. Sediments on the ocean floor are lifted above sea level without folding or tilting. These layers are eroded because they are exposed to wind and running water. Carlie Disconformity

Law of Crosscutting Relationships Cross-cutting relationships are of several basic types. There are structural cross-cutting relationships where in a fault or fracture cuts through older rock. Stratigraphic cross-cutting relationships occur where an erosional surface (or unconformity) cuts across older rock layers, geological structures, or other geological features. Sedimentologic cross- cutting relationships occur where currents have eroded or scoured older sediment in a local area to produce, for example, a channel filled with sand. Paleontologic cross-cutting relationships occur where animal activity or plant growth produce truncation. This happens, for example, where animal burrows penetrate into pre- existing sedimentary deposits. Geomorphic cross-cutting relationships occur where a surficial feature, such as a river, flows through a gap in a ridge of rock. In a similar example, an impact crater excavates into a subsurface layer of rock. Sabrina

... Continued TURN PAGE 326 Olivia

CROSSCUTTING RELATIONSHIPS Emily

The Geologic Column The geologic column represents a time line of the earth’s history, with the oldest rocks at the bottom and the most recent at the top. Rock layers in the geologic column are distinguished from each primarily by the kinds of fossils they contain and by the rock type. Fossils in the upper, more recent layers resemble modern plants and animals. Most of the fossils in the lower, older layers are from species that have been long extinct. Carlie

EMILY Geologic Column

Divisions of Geological Time The geological history of the earth is punctuated by major changes in the earths surface or climate and by the extinction of various species. Geologists use these events as the basis for dividing the geologic time scale into smaller units. A very large unit of geological time is era. There are four geological eras. Precambrian time Paleozoic era Mesozoic era Cenozoic era Sabrina

Carlie, Sabrina, Olivia and Emily The End Carlie, Sabrina, Olivia and Emily