Rocks Unit Notes
What is the Rock Cycle?
Like most Earth materials, rocks are created and destroyed in cycles. The rock cycle is a model that describes the formation, breakdown, and reformation of a rock as a result of sedimentary, igneous, and metamorphic processes. All rocks are made up of minerals. A mineral is defined as a naturally occurring, crystalline solid of definite chemical composition and a characteristic crystal structure. A rock is any naturally formed, nonliving, firm, and coherent aggregate mass of solid matter that constitutes part of a planet.
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How are rocks classified? igneous rock - All igneous rocks start out as melted rock (magma) and then crystallize. Its properties are the result of mineral composition (silica (SiO
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) and cooling (crystallizing) environment. There are two basic types:
 intrusive
– slow cooling magma (beneath the Earth’s surface) which produces large mineral crystals
 extrusive – rapid cooling lava (magma above the Earth’s surface) which produces small mineral crystals
Porphyritic rock – It is igneous rock which is characterized by large, well formed crystals surrounded by finer grained crystals of the same or different minerals. This dual texture is the result of complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly. sedimentary rock – All are formed from weathered, transported, deposited, compacted, and cemented solid material or chemical precipitation. They generally occur in layers or beds that range in thickness from inches to thousands of feet and thus preserve a record of the environments that existed when they formed. Fossils of ancient living things are preserved in sedimentary rocks too. Sedimentary rocks make up 75% of the rocks at the Earth’s surface. There are three basic types:
 clastic/detrital – made from the broken pieces of other rock
 chemical – evaporites that are formed when mineral crystals precipitate out of water as it evaporates.
The most common chemical sedimentary rock is limestone which is composed mostly of the mineral calcite (CaCO
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).
 organic – formed from the remains of once living plants and animals (coal) metamorphic rock – They are formed when high temperature and pressure are combined to alter the texture, mineralogy, or chemical composition of a rock without melting it. There are two basic types:
 foliated – high pressure causes minerals with flat or needlelike crystals to form with their long axes perpendicular to the pressure.
 nonfoliated – metamorphic rocks that lack mineral grains with long axes in one direction
What are the types of metamorphism?
Regional metamorphism results when high temperature and pressure affect large regions of Earth’s crust. When molten rocks, such as those in an igneous intrusion, come in contact with solid rock, a local effect called contact metamorphism occurs due to high temperature and moderate-tolow pressure. Hydrothermal metamorphism occurs when very hot water reacts with rock and alter it chemistry and mineralogy.
What is weathering and what are the different types?
Weathering is a set of mechanical and chemical processes that break rock into smaller pieces.
Mechanical weathering is the gradual breakdown of rock to sand, and then to silt or powdered rock, and finally to clay through physical means. Mechanical weathering occurs in a variety of ways. Heat and cold may
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cause minerals within a rock to expand and shrink at different rates, creating cracks. Water may seep into those cracks and freeze, expanding and splitting the rock. A raging river or ocean waves can smash rocks against each other, wearing the outsides smooth and turning rocks into sand. Or sand carried by the wind can act like sandpaper, slowly wearing rock away. A glacier can rub debris against the rock beneath it, also acting like sandpaper. Even plants can force rocks to split as roots creep into cracks and grow.
Chemical weathering is the breakup of rock caused by a change in its chemical makeup. Rain is the most common producer of chemicals that can weather rock. Rain absorbs carbon dioxide and sulfur dioxide from the atmosphere, forming carbonic acid and sulfuric acid, two liquids capable of dissolving other materials. The levels of carbonic and sulfuric acids in rain, while generally weak, can over time dissolve rock such as limestone, freeing other types of rock. Over very long periods, rain can even dissolve enough limestone to create caves and unusual rock formations. Plants and plant-like organisms also play a role in chemical weathering. For example, lichens, which are plant-like organisms, often appear on rock as green, gray, or yellow patches. Lichens secrete a weak acid that helps roughen the surface of the rock, allowing moss and other plants to take root there.
What factors affect the rate of weathering in rocks?
The climate of an area—including precipitation, temperature, and evaporation—is a major influence on the rate of chemical weathering. The interaction between temperature and precipitation has the greatest effect on a region’s rate of weathering. Chemical weathering occurs more readily in climates with warm temperatures, abundant rainfall, and lush vegetation. Physical weathering occurs readily in cool, dry climates. Physical weathering rates are highest in areas where water undergoes repeated freezing and thawing of water (frost wedging) and other matter, along with thermal expansion (increasing size, caused by heat). The characteristics of rocks, including how hard or resistant they are to being broken down, depend on their structure and mineral composition . In general, sedimentary rocks are more easily weathered than harder igneous and metamorphic rocks. Mechanical weathering breaks up rocks into smaller pieces. As the pieces get smaller, their surface area increases. The greater the total surface area, the more weathering that occurs. Topography (the surface characteristics of the land) may affect weathering. Earth materials on level areas are likely to remain in place as they undergo changes. Materials on slopes have a greater tendency to move as a result of gravity, thereby exposing underlying rock surfaces and thus providing more opportunities for weathering to occur. All of these factors work together over time.
How do different climatic factors affect weathering?
The water cycle and climate are intimately involved in the processes of weathering. Hot and wet climate conditions enhance chemical weathering, and cold, wet conditions promote physical weathering. Weathering processes are typically slower in dry regions, whether cold or hot. Visit http://ees.as.uky.edu/sites/default/files/elearning/module07swf.swf
for further explanation.
How are weathered sediments typically transported?
Erosion (transportation) is the removal and movement of surface materials from one location to another. The agents of erosion include water, ice, wind, and gravity.
Erosion due to gravity - movement of sediment, rock, etc. down-slope from an area of higher elevation to an area of lower elevation due to the force of gravity. Without gravity, glaciers would not move downslope and streams would not flow.
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With few exceptions, water has more power to move large particles of weathered material than wind does.
Stream erosion is greatest when a large volume of water is moving rapidly. Swiftly flowing water can carry material over a greater distance. Each year, streams and rivers carry billions of metric tons of sediments to coastal areas. When a river enters a large body of water, the water slows down and deposits large amounts of sediments which form deltas. Ocean currents, waves, and tides carve out cliffs, arches, and other features along the continents’ edges. The constant movement of water and the availability of accumulated weathered material result in a continuous erosional process, especially along ocean shorelines. Sand along a shoreline is repeatedly picked up, moved, and deposited by ocean currents. Sandbars form from offshore sand deposits and can become barrier islands. Ocean currents, waves, and tides carve out cliffs, arches, and other features along the continents’ edges. The constant movement of water and the availability of accumulated weathered material result in a continuous erosional process, especially along ocean shorelines. Sand along a shoreline is repeatedly picked up, moved, and deposited by ocean currents.
As a glacier (large chunks of ice) moves differences in pressure and melting/refreezing ice within the glacier breaks pieces off and incorporate them into the glacier (plucking). Thrusting occurs when rock is frozen to the bottom of the glacier. And is thrust forward with the glacier and then deposited elsewhere. Glaciers also cause abrasion. As the glacier moves over the underlying rock with the pieces it has picked up, it gouges, carves, and grinds up the underlying rock. This type of erosion results in non-sorted sediments.
Wind is a major erosional agent in areas on Earth that experience both limited precipitation and high temperatures. The abrasive action of wind-blown particles can damage both natural features and human-made structures. Shore areas also experience wind erosion. Whether or not wind erodes sediment depends on the size of the material, the velocity (speed) of the wind, the duration of the wind, and the length of the area over which it can blow unobstructed. Smaller particles, (grains of silt or clay) may be carried great distances before being deposited. Larger-sized particles move by rolling or bouncing along the ground. This type of erosion results in sorted sediments (largest sediments at the bottom with progressively smaller sediments towards the top).
How are transported sediments deposited? Deposition occurs when sediments are laid down on the ground or sink to the bottoms of bodies of water. When several events of deposition in quiet water occur, each involving a mixture of sediments, vertical sorting will take place, and graded beds of sediment will be formed.
The largest, most dense and rounded sediments are deposited at the bottom and the sediments become progressively smaller towards the top. In every erosional-depositional system erosion occurs whenever the medium (erosional agent) is gaining speed, and deposition occurs whenever it is losing speed. Horizontal sorting occurs when a stream begins to slow down, the largest of particles will be left behind first. As the velocity becomes slower the next smallest size will deposit. The smallest particles will be carried the farthest.
When a river enters the sea or any other large body of water its velocity suddenly drops. This causes deposition to begin. Because the current doesn’t stop completely at the mouth of a stream horizontal sorting occurs. The largest, roundest, and most dense, particles are deposited closest to the shoreline. As you move out from the shoreline the pattern will show a gradual change from coarse to fine, from roundest to flattest, and from most to least dense. This deposition forms a triangular shaped pattern call a delta .
Water in meandering streams moves faster on the outside of the meander than on the inside of the meander.
Erosion occurs on the outside of the curve while deposition occurs on the inside. The sediment in the inside of curves produces point bars. See the illustration below. The blue indicates deposition.
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The total sediment carried by a stream is referred to as load and there are three types: dissolved load - material which is transported in solution suspended load - material which is carried along in the water without settling to the bottom bed load - material too large to be carried in suspension, it bounces or rolls along without being lifted causing more erosion.
Wind deposited sediment usually consists of well sorted, small particles in layers that may be tilted with respect to one another. This is called Cross Bedding .
In a solid erosional system such as a glacial, sediments of all shapes, sizes, and densities are deposited together when the glacier melts. This deposition results in unsorted and unlayered piles of rubble called till.
Gravity acting alone could produce deposits of unsorted, angular sediments of all sizes at the bases of hills, cliffs, and mountain sides. This is called Mass Wasting.
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Key rock terms and concepts: bedding – the sedimentary process where layers of sediment are layered down in well-defined planes horizontally to the land surface. The sediments that are the lowest were the earliest to be deposited, and are therefore the oldest. lithification is the process that begins to compaction (water is squeezed out). Sediments that are buried 3 to
4km deep experience temperatures high enough to start the chemical and mineral changes that cause cementation (bonding sediments together). uplift
– a structurally high area in the crust, produced by movements that raise the rocks, as in a broad dome or arch. It is the process in the rock cycle by which once deeply buried rock (all types) is again exposed to the weathering and erosion process as earth is removed from over and around it or is push upward by the interaction of tectonic plates. Hills and mountain are often formed by uplifting.
The melting of rocks typically occurs in the lower lithosphere or upper asthenosphere. There is a considerable range of melting temperatures for different compositions of magma. Magma temperatures usually fall somewhere in the range of 700-1300 degrees Celsius, which is about 1200-2400 degrees Fahrenheit.
Crystallization is the formation solid crystals due cooling.
Porphrytic texture is characterized by large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals. It is indicative of a complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly.
Soil Formation and Types
Soils are complex mixtures of minerals, water, air, organic matter, and countless organisms that are the decaying remains of once-living things. It forms at the surface of land – it is the “skin of the earth.” Soil is capable of supporting plant life and is vital to life on earth.
The soil-forming process begins when weathering breaks solid bedrock into smaller and smaller pieces.
Bacteria, fungi, and insects begin to live in the weathered materials. More nutrients are added to the weathered materials by the death and decay of organisms.
Soil performs many critical functions in almost any ecosystem (whether a farm, forest, prairie, marsh, or suburban watershed). There are seven general roles that soils play:
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Soils serve as media for growth of all kinds of plants.
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Soils modify the atmosphere by emitting and absorbing gases (carbon dioxide, methane, water vapor, and the like) and dust.
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Soils provide habitat for animals that live in the soil (such as groundhogs and mice) to organisms (such as bacteria and fungi), that account for most of the living things on Earth.
4.
Soils absorb, hold, release, alter, and purify most of the water in terrestrial systems.
5.
Soils process recycled nutrients, including carbon, so that living things can use them over and over again.
6.
Soils serve as engineering media for construction of foundations, roadbeds, dams and buildings, and preserve or destroy artifacts of human endeavors.
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7.
Soils act as a living filter to clean water before it moves into an aquifer
What is the composition of soil?
The particles that make up soil are categorized into three groups by size – sand, silt, and clay. Sand particles are the largest and clay particles the smallest. Most soils are a combination of the three. The relative percentages of sand, silt, and clay are what give soil its texture. A clay loam texture soil, for example, has nearly equal parts of sand, slit, and clay.
Sand (2.0 to 0.05 mm) - S and particles are essentially small rock fragments, and as such, have little or no ability to supply grass with nutrients or to retain them against leaching. As rock fragments, sandy soils feel gritty between the fingers. The sand grains have little ability to stick together. It is well known that sandy soils are droughty soils because they retain little water when wetted. Nevertheless what water is retained is released to plants easily. When rain or irrigation occurs the water readily penetrates the soil surface, the excess moves through rapidly and the soil remains well aerated. These properties make sands a desirable medium for growing sports turf where there is no limitation in applying water and nutrition, as needed, throughout the season.
Name of Soil Particle Diameter Limits (mm)
Clay less than 0.002
Silt ( 0.05 to 0.002 mm) – The particles classified as silt are intermediate in size and chemical and physical properties between clay and sand. The silt particles have limited ability to retain plant nutrients, or to release them to the soil solution for plant uptake. Silt tends to have a spherical shape. Because of the spherical shape, silt also retains a large amount of water, but it releases the water readily to plants. While silt soils are generally considered very fertile for the growth of plants, largely due to their water characteristics and ease of cultivation, engineers dread working with them due to their relatively easy release of water and lack of ability for the particles to stick together.
Silt
Very Fine Sand
Fine Sand
Medium Sand
Coarse Sand
Very Coarse Sand
Fine Gravel
Medium Gravel
Coarse Gravel
0.002 - 0.05
0.05 - 0.10
0.10 - 0.25
0.25 - 0.50
0.50 - 1.00
1.00 - 2.00
2.00 - 10.0
10.0 - 25.0
greater than 25
Clay (less than 0.002 mm) – Clay size particles are the source of most of the chemical properties of soil. They are responsible for the retention of many of the plant nutrients in the soil, such as calcium, magnesium, potassium, trace elements and some of the phosphorus. Clays react with the breakdown products of organic matter to stabilize the humus in the soil. A soil without clay particles can be a very infertile soil. Clays, because of their very small size and very large surface area, are able to retain greater amounts of water than sandy soils.
On the other hand, clays hold the water more closely and do not release the water as readily to grass roots as sands. Clay particles have a vastly greater tendency to stick together than sand, thus it is common farmer knowledge that soils high in clay are difficult to till.
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How are soil textures classified?
See the diagram below.
There are 12 soil textural classes represented on the soil texture triangle on the right. This triangle is used so that terms like “clay” or “loam” always have the same meaning. Each texture corresponds to specific percentages of sand, silt, or clay. Soil that has 30% clay, 10% silt, and 60% sand is classified as sandy clay loam.
What is the composition of the sequence of soil layers in a soil profile?
Soil Profile is a vertical section of the earth's crust from the ground surface up to the bedrock. Soil profile reveals various layers known as soil horizons. Each horizon differs from the other in terms of texture, color, and composition. See illustration below.
See explanation below.
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O Horizon (organic matter): The top, organic layer of soil, made up mostly of leaf litter and humus
(decomposed organic matter).
A Horizon (surface layer): This layer is commonly called topsoil. Seeds germinate and plant roots grow in this dark-colored layer. It is made up of humus (decomposed organic matter) mixed with mineral particles.
B Horizon (subsoil): It contains clay and mineral deposits (like iron, aluminum oxides, and calcium carbonate) that it receives from layers above it when mineralized water drips from the soil above.
C Horizon (parent rock): It consists of slightly broken-up bedrock. Plant roots do not penetrate into this layer; very little organic material is found in this layer.
R Horizon (bedrock): The unweathered rock (bedrock) layer that is beneath all the other layers.
How does geographical location affect soil types?
Polar soils form at high latitudes and high elevations in places such as Greenland, Canada, and Antarctica. These soils have good drainage but no distinct horizons because they are very shallow, sometimes only a few centimeters deep. Permanently frozen ground, called permafrost, is often present under thin polar soils. Temperate soils vary greatly and are able to support such diverse environments as forests, grasslands, and prairies. The specific amount of rainfall in an area determines the type of vegetation that will grow in temperate soils. Grasslands, which have an abundance of humus, are characterized by rich, fertile, soils. Forest soils are characterized by less deep and less fertile soils that contain aluminum-rich clays and iron oxides. Deserts receive low levels of precipitation. Desert soils often have a high level of accumulated salts and can support only a limited amount of vegetation. Desert soils have little or no organic matter and a very thin A horizon, but they often have abundant nutrients. Desert soils are also lightcolored, coarse, and may contain salts and gypsum. Tropical soils : Tropical areas experience high temperatures and heavy rainfall, leading to the development of intensely weathered and often infertile soil. The intense weathering combined with a high degree of bacterial activity leave tropical soils with very little humus and very few nutrients. These soils experience much leaching of soluble materials, such as calcite and silica, but they have high concentrations of iron and aluminum.
What factors influence soil formation?
Parent Rock – The parent rock material is the rock material that breaks down into rock particles and may influence the nature of the soil in terms of fertility, mineral composition, depth, color and the final soil profile.
Parent rock may be hard or it may be soft. Hard parent rock is normally resistant to weathering and as a result skeletal soils are formed. On the other hand relatively soft rocks are easily broken down into soil particles and the results into a higher rate of soil formation.
Living Organisms – These include bacteria, insects, mammals (animals), human beings and plants. Bacteria play an important role in the breakdown of rocks through complex processes. Organisms such as earthworms, termites also play an important role in the breakdown of rocks into simple or smaller substances that constitute soil. Rodents e.g. rats, moles, squirrels etc physically breakdown rocks as they dig holes into the ground. Man also influences soil formation through activities like mining/Quarrying and digging. As a result masses of rock are physically weathered by man to produce soil. On the other hand plant roots physically break rocks as they grow into the ground. Plant roots also secrete substances that chemically decompose rocks to produce soil.
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Plant leaves or branches may fall down and decay to form humus that is added to the soil through the soil forming processes of humification and consequently mineralisation. That is why in areas of thick vegetative over the soils are rich in organic content while in desert areas or areas of limited vegetation cover the soils have limited humus.
Slope – Landscape Slope influences soil formation through erosion and deposition. The nature of the landscape influences the rate and nature of the soils formed. Steep slopes are easily eroded and this implies that the weathered material on the steep slope soils tend to be skeletal because of erosion. However the rate of soil formation is high because erosion exposes the parent rock to further weathering. On the other hand in the gentle slopes, soils tend to be deep, mature and with a well developed profile. In the lowlands or flatlands where rainfall is high leaching takes place and may lead to the formation of Laterite soils that are poor in terms of plant nutrients.
Time – This refers to the duration of the interaction of soil forming processes and factors. Soil formation requires adequate time, time is important in that the nature of the soils depends on how long the processes and factors have been interacting. If a parent rock has been exposed to the weathering processes for a long time, soil formation will complete as compared to a parent rock exposed for a relatively shorter period.
Climate – Climate influences soil formation through its role in weathering that leads to the formation of soil. In areas of heavy rainfall adequate moisture is provided for the process of chemical weathering. In addition, in the desert areas soil formation through physical weathering processes like exfoliation are common. High temperatures accelerate chemical weathering leading to high rate of soil formation unlike in areas of lower temperatures where soil formation through chemical weathering is limited. In very cold regions like mountain tops, the nature of soil formation is through physical weathering processes like frost action or freeze and thaw.
Climate also determines the nature of vegetation and animal life that consequently contribute to the soil formation through the addition of humus.
In general the age of a soil is not considered in years but in how much development the soil has undergone.
Thus young soils have minimal soil development and few horizons while old soils have well developed horizons. Soil forming factors that hasten the rate of soil development are:
 permeable, unconsolidated, parent material,
 warm, humid, climate
 forest vegetation
 landscape position that is well drained.
Conditions that are prone to retard soil development are:
 impermeable, hard, consolidated, parent material

 cold, or dry, climate
 prairie vegetation steeply sloping landscape
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