THE ROCK CYCLE

Unit Overview

This unit investigates the processes that create the various types of rocks and rock formations found on Earth. There are different methods to determine relative and absolute age of some rock layers in the geologic record. Key terms will be in bold.

GLOSSARY OF KEY TERMS

acid rain: any precipitation that has an unusually high concentration of sulfuric or nitric acids resulting from chemical pollution of the air

erosion: process by which rock and/or the products of weathering are removed

fossils: the traces or remains of a plant or an animal found in sedimentary rock

igneous rock: any rock formed from cooled and hardened magma or lava

lithosphere: the thin outer shell of Earth, consisting of the crust and the rigid upper mantle

magma: the molten rock within Earth

mantle: the layer of rock between Earth’s crust and its core

metamorphic rock: any rock formed from other rocks as a result of heat, pressure, or chemical processes

mineral: a natural, inorganic solid with a definite chemical composition and a characteristic internal structure

sedimentary rock: any rock formed from compressed or cemented deposits of sediment

weathering: the change in the physical form or chemical composition of rock materials exposed at Earth's surface

Study Suggestion: Make vocabulary cards to review the words for this unit. Try putting the word on one card and the definition on another. Then play a matching game!

Structure and Origins of Rocks

· All rocks are composed of minerals.

· Minerals are natural, inorganic solids with definite chemical composition and internal structure. EXAMPLE: Quartz (SiO2)

· There are around 3500 known minerals in the Earth’s crust. Only about 20 of these minerals make up 95% of all rocks.

Six most common rock-forming minerals:
1. Feldspar: Approximately 60% of the earth's crust is composed of feldspar. It is generally dull to opaque with a porcelain-like appearance. Color varies from red, pink, and white (orthoclase) to green, grey and white (plagioclase). Feldspar is also hard but can be scratched by quartz. Feldspar in igneous rocks forms well developed crystals which are roughly rectangular in shape, and they cleave or break along flat faces.
2. Pyroxene: Green to black in color. Pyroxenite, an igneous rock composed totally of pyroxene minerals, is related to ultramafic terrains and is therefore relatively rare at the surface of the earth's crust.
3. Mica: Mica is easily distinguished by its characteristic of peeling into many thin flat smooth sheets or flakes. It is translucent to black in color.
4. Quartz: Quartz is a glassy looking, transparent or translucent mineral which varies in color from white and grey to smoky. When there are individual crystals they are generally clear, while in larger masses quartz looks more milky white. Quartz is hard, durable, and relatively inert.
5. Amphibole: Amphibole is mostly black and forms long, slender crystals. Amphibolite is a metamorphic equivalent of basalt, and can contain extremely coarse grained specimens of hornblende.
6. Olivine: Olivine is olive green to black, translucent, with a conchoidal fracture. Olivine phenocrysts are relatively common in some basaltic rocks (like those found in Hawaii), and make an extremely pretty contrast with the black groundmass of the basalt. A semi-precious variety occurs (peridot), which can be cut and faceted like any other gemstone.

Other common rock-forming minerals

● Calcite

● Clays

● Magnetite

● Pyrite

● Talc

Combinations of the rock-forming minerals result in rocks with unique properties such as hardness, density, color, porosity, and granular make-up. The appearance of a rock depends on the mineral make-up and how the rock was formed.

Comparison of Rock Texture

· Rock textures and types depend on how the rocks were formed.

Molten Rock Cools to Form igneous rock.

· As molten rock cools, the minerals in the rock form crystals. If the rock cools quickly, the crystals have less time to grow and the rock will have a smooth texture. Obsidian is an example of this type. If the rock cools slowly, the crystals grow large and the rock will have a granular, porous texture like granite.

· Igneous rocks are those that form from cooled and hardened lava or magma.

· Igneous rock that cools on the Earth’s surface is called extrusive igneous rock. Basalt is the most common example of this type.

· Igneous rock that cools while trapped beneath the Earth’s surface is called intrusive igneous rock. Granite is the most common form of this type.

· The composition of magma is limited to the eight common elements of the earth's crust. These elements combine within a melt to form silicate minerals, the most common minerals of igneous rocks. These silicate minerals include feldspars (plagioclase feldspar, potassium feldspar), quartz, micas (muscovite, biotite), pyroxenes (augite), amphiboles (hornblende), and olivine. These minerals make up over 95% of the volume of the common igneous rocks, making igneous rocks easy to identify.

The photo to the right shows a volcanic (extrusive) igneous rock. Minerals that compose the rock are clearly much smaller and many could be seen with a magnifying lens or microscope.

(Photo taken on beach at north edge of Lake Huron, Ontario, Canada).

Just to show that not all igneous rocks are either plutonic or volcanic, the photo to the right shows one that exhibits characteristics of both.  Large greenish crystals, called phenocrysts, are clearly visible.  These crystals are surrounded by much smaller crystals.  The phenocrysts formed by slow cooling within a deeply buried magma chamber.  Rapid eruption of the magma brought these crystals to the surface where they were surrounded by magma that cooled rapidly during the eruption.

(Photo taken on beach at north edge of Lake Huron, Ontario, Canada)

Sedimentary Rocks

• Remains of older rocks and organisms form sedimentary rock. These particles of rocks and deposits of dead organisms (plants and animals) are pressed together and form a cement-type composition.
• Old rocks on the Earth’s surface can be broken down by weathering. They can be washed by rain or constantly eroded by wind. This is called physical or mechanical weathering. Some rocks are broken down by chemical weathering, such as that caused by acid rain.

Examples of Sedimentary Rock

◦ Rocks made of pebbles and small bits of rocks are called conglomerates.

The picture at the left shows a quartz-rich sandstone from a beach on the north edge of Lake Huron in Ontario, Canada.  The image on the right shows a microscopic image of a sandstone at varying magnification levels.  At the left, the upper one-half of this boulder consists of well-sorted, small sand grains.  The lower half consists of more poorly sorted pebbles and grains.  The poorly sorted mixture in the lower half qualifies this part of the rock as a conglomerate, whereas the well-sorted character of the upper half qualifies it as a sandstone.

◦ Rocks made of compressed sand are called sandstone.

As the most common mineral in continental rocks, quartz sand grains are being transported and deposited in numerous geologic settings. Two are shown below.
Great Sand Dunes National Monument, Colorado.   Geology by Light Plane Web site, courtesy of Dr. Lou Maher.	Lower Wisconsin River.  Geology by Light Plane Web site, courtesy of Dr. Lou Maher.
Although sandstones frequently accumulate in horizontal layers, wind- and water-deposited sandstones often exhibit cross-bedding.  Cross bedding preserves the tilted sides of dunes and sand ripples.  Due to the migration of sand dunes and ripples, cross-bedded sandstones are distinctive and easily recognized.  They are also excellent indicators of the local environment in ancient times (flowing water, beach, and desert).
Vermillion Cliffs, Arizona, Photo credit– Thomas F. Osborne. Cross bedding of these sandstones is exhibited dramatically in this landscape, which has been sculpted by flowing water.

◦ Rocks formed from a highly compressed fine mud are called mudstone.

Mudstone is made up of tiny clay particles (less than 0.05mm) that can’t be seen with the naked eye. These tiny particles are deposited in quiet low-energy environments like tidal flats, lakes, and the deep sea.	When mudstone is buried beneath many layers of sediment deposited on top, it may be compressed to form shale, which breaks easily into thin flakes.
Uses of Mudstone: Pottery, bricks and cement

Metamorphic Rock

• Rock that undergoes pressure and heating without melting form metamorphic rock.
• The most common types of metamorphic rock are formed by heat and pressure deep in the crust.
• Limestone will form marble under heat and pressure.

Marble column formed from Limestone	When mudstone or shale is heated and pressured, it will form slate.

The Rock Cycle

Determining the Age of Rocks

• It is difficult to know how old rocks are because they form over millions of years and can later form into new rocks.
• Scientists use the Principle of Superposition to tell how old rocks are in relationship to the rocks around them:

Assuming no change in the position of the rock layers, the oldest will be on the bottom, and the youngest will be on the top.

• Reading the layers of rock can not only tell you something about rock age, but also about what was happening at the time of deposit. The layering of rock is referred to as the stratigraphy of the rock.

Radioactive Dating

• Absolute age of a rock can be determined by radioactive dating. (carbon dating)
• Radioactive elements will decay over billions of years at a constant rate.
• Scientists look at the decaying elements around the rocks to determine how many years have passed since the rock has formed.

The Role of Thermal Energy

• The Earth is made up of several layers of matter. Each layer gets warmer with depth.
• The topmost layer of Earth, the crust, is relatively cool and made up of solid rock.
• The layer underneath the crust is the mantle. The area of the mantle directly under the crust is mostly solid rock, but as the mantle deepens, the temperature increases and the rock becomes molten. Scientists refer to the rock as being “plastic” which means it is soft and easily deformed. This lower layer of the mantle is in constant motion and causes pressure against the crust.
• The Earth’s core is solid even though it is very hot. Pressure due to the weight of the mantle and crust is so great that the atoms in the core are forced together as a solid.
• As the mantle pushes against the crust, motion occurs in the Earth’s plates and many times, molten rock can ooze up between cracks in the crust.

• Rocks at the boundary of the crust and mantle are constantly melting and reforming. This is also the area where sedimentary rock gets heated and undergoes extreme pressure to form metamorphic rock.
• As the mantle continues to move, it exerts forces on the crust and the Earth’s plates. These plates move also, and mantle material can reach the surface by means of volcanoes and cracks in the crust.
• Because the Earth’s mantle is in constant motion, the Earth’s crust is continually reforming. This process continually destroys and reforms rock.