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Geology Exam 2

Terms in this set (99)

Why do rocks have a range of melting/freezing rather than a single melting/freezing point?
Rocks are composed of minerals, each of which has its own melting/freezing point. Thus, the rock will melt over a range of temperatures rather than a single temperature.
How can a change in pressure trigger melting in rocks?
If a rock is under pressure, its melting point is increased and so it remains a solid at a higher temperature than it would at sea level pressure. If the pressure drops, the melting point decreases, and the already hot rock may start to melt.
How does the presence of water affect the melting point of rock?
The presence of water lowers the melting point of rock.
What is partial melting?
Because rocks are made of minerals that melt at different temperatures, a rock will melt only partially if the temperature is between the melting point of two of its minerals. The mineral that melts at the lower temperature will melt wheareas the mineral with the higher melting temperature will stay solid.
In what order do mafic and felsic materials melt?
In general, mafic minerals tend to melt at higher temperatures than felsic minerals. There is plenty of overlap, however.
What causes the difference between a phaneritic texture and an aphanitic texture?
These textures are caused by the rate of cooling. Slow cooling allows crystals to form larger, which produces a phaneritic texture. Fast cooling causes tiny crystals and a fine, aphanitic texture.
How are phenocrysts and groundmass related to each other?
Phenocrysts are the large crystals in a porphyritic rock wheareas groundmass is the apanitic rock that surrounds the phenocrysts.
How is granite related to rhyolite and gabbro related to basalt?
Granite and ryholite are both felsic rocks but have different textures. Granite is phaneritic and rhyolite is aphanitic. Likewise, gabbro and basalt are mineral equivalents with different textures: gabbro is phaneritic and basalt is aphanitic.
Why are magmas different?
Magmas contain different combinations of minerals and melt under different conditions. Partial melting causes the composition of magma to vary, depending upon the conditions under which the rock melted.
What type of magma forms at divergent boundaries?
Divergent boundaries produce hot fluid, basaltic magma.
What type of magma forms at convergent boundaries?
Convergent boundaries tend to produce cooler, more viscous felsic magmas
Why do more basaltic magmas make it to Earth's surface than felsic magmas?
Basaltic magmas contain mafic minerals that are less viscous when melted; that is, they have less resistance to flow. Thus, they can make the journey to Earth's surface relatively rapidly, before they cool enough to crystallize. Felsic magmas contain silica, which is more viscous when melted; thus these rocks have a higher resistance to flow. The increased viscosity impedes their journey to Earth's surface, and they often cool enough to crystallize before they get there.
What are plutons?
Plutons are bodies of rock that have solidified underground.
How do tabular plutons differ from massive plutons?
Tabular plutons are flat wheareas massive plutons have an irregular shape.
How does a dike differ from a sill?
Both are tabular plutonic bodies, but a dike is discordant and cuts across rock layers wheareas a sill is concordant and is parallel to rock layers.
How does a batholith differ from a stock?
They differ in the area that is exposed. A batholith has more than 100 square kilometers exposed whearas a stock has less than 100 square kilometers exposed.
How does a pahoehoe lava difer from a'a lava?
Pahoehoe is hotter and more fluid; it solidifies to form a smooth or ropy surface. A'a lava is cooler blocky; it forms a rough, chunky surface.
What does the presence of pillow lava mean?
It means that the lava was extruded underwater, where the lava formed sack-like structures before solidifying.
Why does a felsic magma produce an explosive eruption?
Felsic magma is viscous, which impedes the release of dissolved gases in the magma. As the magma moves toward the surface, the decreasing pressure allows the dissolved gases to expand; pressure builds, and when release comes, it does so explosively.
What is a lahar?
A lahar is a mudflow that occurs when melting ice at an erupting volcano's summit mixes with pyroclastic materials. The mixture rushes down the mountain following stream valleys, covering large areas with muddy ash.
How do calderas form?
A caldera forms when a volcano has released all of its magma in an explosive eruption. With the upward pressure of the magma gone, the crater collapses in on itself to form a caldera.
What is a tiltmeter and how does it indicate help to predict eruptions?
A tiltmeter is an instrument that measures the altitude, or tilt, of the side of a volcano. A change in tilt can mean that a bulge is forming, an indication of an accumulation of magma that often occurs prior to an eruption.
Compare frost wedging with crystal growth as mechnical weathering processes.
They are similiar in that expanding crystals pry apart cracks in rock, breaking the rock into fragments over time. They are also similiar in that the substance forming the crystals is carried into the rock as liquid water. They are different in that, with frost wedging, it is the water that forms the crystals as it freezes. With crystal growth, ions come together to form the crystals as the water evaporates.
Are you likely to ever see a basalt exfoliation dome?
No! As described in Chapter 6, basalt is an extrusive rock. The sheeting that forms an exfoliation dome is a weathering process distinctive to intrusive igneous rock, particularly granite. It occurs as the result of the pressure release when overlying layers in place when the rock solidified are eroded away as the rock uplifted. Basalt undergoes none of these processes, as it solidifies at or near the surface.
How is chemical weathering different from mechanical weathering?
Mechanical weathering breaks down rocks into smaller particles that have the same compositon as the original rock. Chemical weathering is the result of a chemical process that transforms the minerals in rock into different minerals.
How does dissolution weather rock?
Water is a polar molecule that exerts a strong attraction on the cations and anions in minerals, which pries them apart. They leave the rock to enter the water and remain there as ions until the water either evaporates or until another chemical process occurs that bonds them again.
What do hydrolysis and oxidation have in common as weathering processes?
Both hydrolysis and oxidation are chemical reactions. Oxidation combines the oxygen in the atmosphere, or in water, with cations in the minerals. During hydrolysis, a hydrogen atom from water replaces cations in minerals. Both processes change the original minerals into different substances.
Why is the rate of weathering faster in warm, wet climates than in cool, dry climates?
Heat speeds up chemical reactions, so chemical weathering processes occur at a faster rate in warm climates than in cooler climates. Because water is the chief agent of erosion, the presence of an abundance of water in warm, wet climates affords more opportunity for weathering to occur than in drier climates.
How are igneous rocks related to volcanism?
Current formation of igneous rock is in regions of active volcanism, which includes convergent and divergent boundaries.
How does spheroidal weathering produce rounded corners on rocks?
It's a matter of exposure. The corners of the rocks along joints have three sides exposed to wind and rain. The edges have only two exposed sides, and the surface of the rock exposes only one side. Thus, the corners weather faster, resulting in a rounded appearance.
What is the difference between the A horizon and the B horizon?
The A horizon is the zone of leaching from which water carries away dissolved substances. Those substances collect in the B horizon, which is called the zone of accumulation. Clay particles from the E horizon also accumulate in the B horizon.
What climatic factor plays the largest role in soil development?
The presence of water is the key factor in soil development.
Where would soil develop more quickly: on a shady slope or in a sunny valley?
The sunny valley would be more conducive to soil development. Erosion on a slope washes away upper layers of developing soil. In addition, the shade would make the slope cooler, inhibiting the organic proccesses that contribute to soil development.
How long does it take for soil to develop?
It can take hundreds of years for a well-developed soil profile to form. One reason our soil is a valuable asset is that it is not easily replaced.
How do forest and grassland soils compare to each other?
Forest soils are in moist areas and tend to pedalfers, which contain a large proportion of aluminum and iron in the B horizon. Forest soils have well developed O and A horizons due to the availability of large amounts of organic material. Grassland soils are in drier areas and tend to be pedocals, which contain a large proportion of calcium. Their O and A horizons are not as well developed because there is less organic material available.
Why is an oxisol the opposite of what would be expected?
An oxisol is a rainforest soil. It has a very thin O horizon because, with the warm, moist conditions, new plant material quickly decomposes and is available for fast-growing plants to use. There is little time for organic plant material to accumulate in the topsoil before plants take it up through their roots. The A horizon is also relatively thin and will become rock hard if exposed to the sun. Without A and O horizons, there is virtually no B horizon. Consequently, this is a poorly developed soil. Since water plays a major role in soil development and rain forests are wet, this soil profile is the opposite of what would be expected.
Explain the process by which mechanically weathered rock becomes clastic sedimentary rock.
Mechanical weathering produces clasts that are then transported to another location, where they are deposited. If they stay put, additional deposits will bury them. If they are buried deeply enough, the process of lithification will turn them into clastic sedimentary rocks.
Describe how clastic sedimentary rocks are classified.
They are classified primarily by the size of their clasts and by the shape of their grains.
Describe how evaporites are deposited, and give at least one example of an evaporite.
Evaporites are deposited when the water they are dissolved in evaporates. As the water becomes water vapor, the ions in the water find their partners and bond together again, forming mineral crystals. Examples are halite (often combined with other substances to form rock salt) and gypsum.
Describe how precipitates are deposited. What is the most common precipitated mineral?
Precipitates form when cations and anions from dissolved substances bond to form an insoluble substance that immediately drops out the water as a solid. Calcite is the most common precipitate.
What is the sequence of clasts in a graded bed?
In a graded bed, the largest clasts are the bottom and the finest clasts are at the top. The clasts were deposited in order of decreasing diameter as the energy of the water that carried them decreased.
How do geologists interpret sedimentary rock?
Geologists are aware of the types of environment each type of rock is likely to have deposited in. They look not only for the type of rock but for sedimentary features and fossils that can provide additional clues as to what the environment was like when the deposit was made.
Why are trace elements a concern with regard to weathering?
Weathering releases elements from rock. Some elements, such as arsenic, are toxic; other elements are necessary for good health in small amounts but toxic in large amounts.
How did the Dust Bowl occur?
Farmers turned over the sod in the Great Plains for agriculture, removing the protection the sod had provided the topsoil against wind erosion. When drought struck and agricultural crops failed, the topsoil was exposed to winds which picked it up, creating massive dust storms and permanently removing the topsoil.
What steps are taken today to avoid in a recurrence of the Dust Bowl?
New agricultural practices include planting windbreaks to block the wind, not till farming in which seeds are planted without turning over the soil, and contour plowing which helps to prevent soil erosion.
What are expansive soils, and why are they a concern?
Expansive soils are those that contain a lot of clay minerals. When wet, the water enters between the sheet structure and increases their volume. Expansive soils can crack foundations and destroy roads.
What is mass wasting?
Mass wasting is the downward movement of the materials composing a slope.
What is the driving force behind all mass wasting?
The driving force is gravity, which applies a downward force on slope materials.
Name the three main resisting factors to the pull of gravity.
Friction, cohesion, and the strength of the materials are the three main resisting factors.
What is an angle of repose and how does water affect it?
The angle of repose is one where resisting factors exceed the force of gravity, making it unlikely the slope will fail. An unstable slope is one in which the resisting factors are almost the same as the force of gravity, making it likely the slope will fail when a trigger occurs.
What is the role of a trigger?
A trigger is something that "tips the balance" between the driving force, gravity, and resisting factors to set slope movement in motion.
How are mass wasting features classified?
Mass wasting features are usually named after the type of material in motion, the angle and speed of the movement, and the amount of water present.
How is an avalanche different from a landslide?
An avalanche is a type of fall in which the materials are snow, rock, and debris. As a fall, the materials in an an avalanche move in a vertical or near-vertical path. A landslide involves materials that move along a plane at an angle that is less than vertical.
What is the difference between a landslide and a slump?
A landslide moves along a plane, whereas the materials in a slump rotate, resulting in a curved path and curved scarp.
What is the classification of a lahar, and how does it form?
A lahar is considered a type of mudflow. It occurs when the ash from a volcano mixes with meltwater from a glacier capping an erupting volcano, and rushes down the volcano's stream valleys.
What is the slowest type of mass wasting?
The slowest type of mass wasting is creep, whose movement is not perceptible and is evident only in its effects on plants, poles, and other objects.
What is a slope stability map?
It is a map produced by the U.S. Geological Service that describes the likelihood of a mass-wasting event in the areas covered by the map.
What are four signs that a mass-wasting event may be likely in the near future?
Cracks in roads, concrete walls, and foundations; a hummocky ground surface; other evidence of past mass-wasting events; titled trees or poles; and undercutting are all indications that a mass-wasting event may occur in the foreseeable future.
What are some measures engineers take to minimize the effects of mass wasting?
Engineers may build retaining walls of wood, concrete, or steel to hold back the slope. They may place gabions along the slope to stablize it. They may cover the rocky slopes or cliffs with mesh to protect roads and residences below. They may bench the slope so that the overall angle is less than the angle of repose for the slope materials. They may divert surface water so that it doesn't soak into the soil to add to the weight of the slope and reduce its cohesion. Finally, they may plant vegetation of a sort that will stabilize the slope.
Describe the distribution of water on Earth's surface.
Some 97 percent of the water on Earth's surface is contained in the ocean. The other three percent is freshwater.
What reservoir holds most of the Earth's freshwater?
Slightly more than 74 percent of Earth's freshwater is frozen in glaciers.
What are the two largest reservoirs of water available to us?
The largest reservoir of water available to us (not frozen in glaciers) is the atmosphere. Groundwater comes in second.
Describe the movement of water in the hydrologic cycle.
The hydrologic cycle describes how water moves between land and ocean. Evaporation turns water into water vapor; when it cools, the water vapor condenses and precipitation falls. If it falls on the ocean, the cycle is complete. If the precipitation falls on land, the water begins the process, under the influence of gravity, of flowing back toward the ocean through streams, rivers, and groundwater systems until it reaches the ocean and the cycle is complete. Along the way, some of the water will evaporate or be released by plants to begin the cycle again.
Describe the six main types of drainage patterns.
A dendritic stream pattern is the most common and resembles the veins of a leaf. A trellis stream pattern occurs when streams flow in parallel valleys that are located between parallel folds; the streams flow down the slopes on either side of the divide, then in parallel streams toward the main river. A rectangular stream pattern is found where the underlying rock has a rectangular fracture pattern of joints or faults; the stream will follow the fractures. Where there is domal uplifting, the streams form a radial pattern as they run off all sides of the dome away from the peak, then form an annular (ring) pattern as they continue to flow downhill around the dome to become a single stream. A reversed radial pattern drains the interior of large sinkholes as water from the edges flows down the center.
Name the factors that affect a stream's energy.
Discharge is a measurement of a stream's energy. The volume of water and its velocity determine a stream's discharge at any given point. Both volume and velocity depend on other factors: Volume is determined by width and depth of the stream's channel; velocity is determined by the stream's gradient and texture of the channel. These factors change from stream segment to stream segment. When a hydrologist talks about a stream's discharge, however, the reference is usually to the calculations made at the mouth of the stream.
What are laminar and turbulent flow, and how do they affect deposition?
In a laminar flow, all of the water and sediment particles in the stream flow in parallel lines. In a turbulent flow, the water and particles change directions both vertically and horizontally; they "bounce around." Turbulent waters have more energy and can more easily pick up and transport larger particles; a laminar flow has less energy and the larger particles will settle out.
What are the three types of load a stream carries in its total load, and how does a stream's load vary as it flows from its head to its mouth?
A stream's total load consists of its bed load (sediment moving along the bottom), dissolved load (minerals dissolved in its water), and suspended load (particles suspended but not dissolved in the water). Dissolved load may increase toward the mouth of the stream as more substances become dissolved in the water, especially if the stream flows through an area of limestone. The suspended load has more variety towards the head of the stream where the flow is turbulent enough to carry most particles, large and small. The suspended load changes character downstream as a reduction in turbulence allows large and intermediate particles, like gravels and sands, to settle out. Fine silts and clays tend to remain suspended until they reach the mouth of the stream where they may finally be deposited in a river delta.
What is meandering and what impact does it have on the surrounding landscape?
Meandering is the sinuous twisting and turning of a river's course. Erosion occurs on the outside bank of each turn while deposition occurs on the inside bank of the turn. The combination of erosion and deposition and filling it with sediment. The meander also migrates downstream, leaving behind features such as oxbow lakes.
Why are deltas found only at a stream's mouth?
The mouth of a stream or river is where the velocity of the water slows enough for fine silts and clays to settle out and accumulate. They do so in the triangular pattern characteristic of deltas.
How often is a river valley likely to have a 50-year flood? A 100-year flood?
While it is unlikely, a river valley could have floods of this magnitude every year for several years. The 50-year and 100-year designations refer not to actual time frames between floods but to the probability that a flood of a certain size will occur in any given year. That is, in any given year, there is only a two percent chance of floodwaters reaching the 50-year flood level. Under unusual circumstances, however, there is nothing to say that a river could not reach those stages several years i a row.
What factors determine when and where rocks will melt to become magma?
Pressure makes the melting temperature increase and water makes it decrease. A sudden drop in pressure can cause a decompression melting.
What is the relationship between partial melting and Bowen's order of crystalization?
A molten mineral crystallizes at the same temperature range at which it melted, so only some of the rock would crystallize
Crystallization
Melting in reverse...when temperature cools, the ions and tetrahedra of silicate minerals begin once again to feel their mutual attractions and to arrange themselves into a crystal lattice
Bowen's Reaction Series (Bowen's Order of crystallization)
1) When rock melts, the bonds holding tetrahedra in their silicate structure break and ions seperate. Later, when the rock begins to cool, silicon and oxygen ions find each other and bond to form isolated silica tetrahedra; then iron and and magnesium bond with the tetradhedra to form olivine, the first mineral to crystallize from cooling magma.

2) As the temperature continues to drop, some of the olivine is likely to bond with other silicate ions to form the single-chain silicates, the pyroxenes.

3) Further cooling coaxes the pyroxenes to bond with additional silicate ions to form the double-chain silicates, the amphiboles.

4) Continued cooling and more bonds among silicate ions produce the micas with their sheet structures.
What are the differences between mafic and felsic magmas?
Magmas that form at lower melting temperatures are felsic, and higher are mafic.
Does magma ever contain both mafic and felsic magmas?
Yes
Viscosity
the resistance to flow
What is the difference between magma and lava, and what type of rocks does each form?
Magma is molten rock that is underground and lava is on the surface. Magma usually forms peridotite. They both form igneous rock (rocks that solidify directly from molten rock.)
Magma formation
The vast majority of magma forms at plate boundaries, where much of it will ultimately produce new seafloor or volcanic arcs.
Aphanitic Rock
The fine-grained rock texture that results from crystals too small to be seen with the naked eye.
Phaneritic Rock
The coarse grained rock texture that results from larger crystals.
Porphyritic Rock
Type of rock composed of large crystals, called phenocrysts, that are cemented together by fine-grained rock, called groundmass.
Vesicular Rock
Type of texture that includes small air cavities called vesicles.
Aphanitic rock classification:
Komatiite (Ultramafic) Basalt (Mafic) Andesite (Intermediate) Rhyolite (Felsic)
Phaneritic rock classification:
Peridotite (Ultramafic) Gabbro (Mafic) Diorite (Intermediate) Granite (Felsic)
Describe how plutonic bodies are classified and give an example of each.
Magma that has reached the surface (very rare) They are classified by based on their shape and their association with the surrounding rock.

-Massive (irregular shape)
-Tabular (flat)
-Country Rock (igneous intrusion is injected)
-Concordant (lying parallel to existing rock layers)
-Discordant (cutting across existing rock layers)
Why is weathering such an important geological process?
It is nothing less than the geological process that makes life on earth possible. It produces the rock particles that develop into soil, the basis for plant life and, therefore, for animal life as well. Also, it releases from rock individual atoms and compounds necessary for the chemical processes of life to take place.
Explain the main difference between the products of mechanical and chemical weathering?
There is no change in composition or chemical makeup in mechanical weathering.
What is the most important agent in both chemical and mechanical weathering?
Rocks being broken down.
Why is soil considered an interface between the live and inanimate Earth?
Soil is where the nutrients released by chemical weathering are made available to land-based plants, which then make it available to animals, including us. Organisms provide the organic matter in humus, they mix the soil, and they convert minerals into forms that can be dissolved in water. Plants absorb the ions, adding these elements to the food chain where they ultimately nourish larger animals. Some microorganisms convert the nitrogen in soil's air content into nitrates, a key nutrient for plant growth. Elements that once came from deep beneath the earth find their way into the biochemical reactions that sustain life.
O horizon
The top layer of a soil profile; contains an accumulation of plant and other organic material in the process of decomposing.
A horizon
The second layer of a typical soil profile; contains humus and is part of the topsoil; also called the zone of leaching because of the transport of minerals dissolved in this layer.
Topsoil
The O and A horizons in a soil profile containing the organic material in a soil.
Zone of leaching
Another name for the A horizon where leaching begins, dissolving minerals and transporting them downward to the B horizon.
E Horizon
The third layer in a typical soil profile; also called the zone of leaching because water carries particles and dissolved minerals away from this layer.
Eluviation
The transport of fine mineral particles to lower soil horizons.
Why is climate such an important factor in the development of soils?
Wet provides more water which dissolves a great amount of the elements that support life and results in a greater abundance of plants. Drier can and do develop but slower. Cold soil remains frozen and soil development essentially stops all together.
What is the difference between a pedalfer and pedacol?
Pedalfer is wet climate and pedacol is dry.
Why is rainforest soil considered so poor for agriculture?
It causes a very thin O horizon and only three oxides are produced. Plant remains decay rapidly and tropical plants have shallow roots that absorb nutrients from decomposing plant litter.
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