The Rock Cycle & The Law of Superposition

Sabrina, Carlie, Olivia and Emily

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

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.

Metamorphic Rock

Fascinating Fact The oldest known rock lies in Canada. The Acasta gneiss, a metamorphic rock, is 3.96 billion years old.

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.

Igneous Rock

Fascinating Facts Some igneous rocks float (pumice) because of a large number of trapped air bubbles.

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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.

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.

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.

Transportation

Deposition

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.)

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!

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Law of Superposition

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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.

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

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.

Nonconformity

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.

Angular Unconformity

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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.

Disconformity

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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.

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CROSSCUTTING RELATIONSHIPS

The Geologic ColumnoThe 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.

oRock 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.

Geologic Column EMILY

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

Carlie, Sabrina, Olivia and Emily

The End