Chapter 7- Igneous Rocks (Week 5) Flashcards
(97 cards)
1
Q
How do igneous rocks form?
A
Igneous rocks form when melted rock cools. The melted rock, known as magma, originates within Earth.
2
Q
What are the eight main elements found in magma compositions?
A
The eight main elements in magma compositions are oxygen, silicon, aluminum, iron, calcium, sodium, magnesium, and potassium. These elements are also the most abundant in Earth’s crust.
*image 1
3
Q
What happens to lighter elements in magma as it cools?
A
Lighter elements in magma, such as hydrogen, carbon, and sulfur, are converted into gases like water vapor, carbon dioxide, hydrogen sulfide, and sulfur dioxide as the magma cools.
4
Q
How does magma composition vary?
A
Magma composition depends on the composition of the rocks that melted to form the magma and the conditions under which the melting happened.
5
Q
What is partial melting, and how does it influence magma composition?
A
Partial melting occurs when only some minerals within a rock melt due to different melting temperatures. The resulting melt, less dense than the surrounding rock, percolates upward, creating magma with a different composition than the original rock. In the Earth’s crust, most igneous rocks come from magmas formed through partial melting.
partial melting in the real world is more complex. Many rocks are more intricate than the example, with some mineral components having similar melting temperatures and changing in the presence of other minerals. The process of rocks melting can take millions of years.
6
Q
Why is magma produced by partial melting less dense than the surrounding rock?
A
Magma produced by partial melting is less dense than the surrounding rock, allowing it to rise upward through the mantle and potentially pool at the base of the crust or ascend through the crust.
7
Q
How does moving magma influence surrounding rocks?
A
Moving magma carries heat, and some of that heat is transferred to surrounding rocks. If the melting temperature of a rock is lower than the temperature of the magma, the rock will begin to melt.
8
Q
What happens when rocks melt due to the influence of magma?
A
When rocks melt due to the influence of magma, the composition of the magma may change, reflecting a mixture of sources. The melted rock may contribute to the overall composition of the rising magma.
9
Q
Is adding heat the only way to trigger melting in rocks?
A
No, adding heat is not the only way to trigger melting in rocks. The influence of moving magma can also lead to melting if the temperature of the magma exceeds the melting temperature of the surrounding rocks.
10
Q
Why doesn’t the Earth’s mantle, despite high temperatures, melt like rock at the Earth’s surface?
A
The Earth’s mantle remains almost entirely solid at high temperatures due to the high pressure it experiences.
11
Q
How can melting in the Earth’s mantle be triggered without adding heat?
A
Melting in the Earth’s mantle can be triggered without adding heat if the rock is already hot enough, and the pressure is reduced. This process is known as decompression melting.
12
Q
What is decompression melting?
A
Decompression melting is the process by which melting is triggered in solid rock when the pressure is reduced, even if heat is not added. This phenomenon is observed in the Earth’s mantle, where high temperatures exist, and a reduction in pressure leads to melting.
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13
Q
How is pressure reduced in the Earth’s mantle, leading to decompression melting?
A
Pressure is reduced in the Earth’s mantle when mantle rocks move upward due to convection or rise as a plume within the mantle. Additionally, pressure is reduced in areas where the crust thins, such as along rift zones.
14
Q
What are the two main processes that contribute to pressure reduction in the mantle?
A
Pressure reduction in the mantle occurs through the upward movement of mantle rocks due to convection or the rise of plumes within the mantle. Additionally, pressure is reduced in regions where the crust thins, such as along rift zones.
15
Q
What is flux-induced melting?
A
Flux-induced melting occurs when a substance like water is added to hot rocks, causing the melting points of minerals within the rocks to decrease. This can trigger partial melting, especially if the rock is close to its melting point.
16
Q
What is a flux in the context of flux-induced melting?
A
A flux, in the context of flux-induced melting, refers to a substance like water that is added to hot rocks, reducing the melting points of minerals within the rocks.
17
Q
Where does flux-induced partial melting of rock commonly occur?
A
Flux-induced partial melting of rock commonly occurs in subduction zones, where minerals undergo chemical transformations under high pressures and temperatures, producing water as a by-product.
18
Q
How much water is required to trigger partial melting in laboratory studies of the Japanese volcanic arc?
A
In laboratory studies of the Japanese volcanic arc, rocks with only 0.2% of their weight consisting of water were found to melt by up to 25% under the conditions of partial melting.
19
Q
What does viscosity refer to, and how is it related to the flow of a substance?
A
Viscosity refers to the ease with which a substance flows. A substance with low viscosity flows more easily than a substance with high viscosity.
20
Q
Why is most magma entirely liquid at temperatures over 1300°C?
A
At temperatures over 1300°C, most magma is entirely liquid because there is too much energy for the atoms to bond together.
21
Q
What happens to magma as it loses heat to the surrounding rocks and its temperature drops?
A
As magma loses heat to the surrounding rocks and its temperature drops, silicon and oxygen combine to form silica tetrahedra. With further cooling, these tetrahedra start to link together into chains, a process known as polymerization. This linking increases magma viscosity.
22
Q
How do silica chains affect magma viscosity?
A
Silica chains formed by the linking of silica tetrahedra during cooling make magma more viscous.
23
Q
What are the implications of magma viscosity for volcanic eruptions?
A
Magma viscosity has important implications for the characteristics of volcanic eruptions. The viscosity of magma influences the explosiveness and behavior of volcanic eruptions.
24
Q
What is the process of crystallization in the context of magma?
A
Crystallization in the context of magma refers to the solidification or freezing of minerals that make up igneous rocks at various temperatures.
25
Why can cooling magma have both crystals and remain predominantly liquid?
Cooling magma can have both crystals and remain predominantly liquid because the minerals within the magma crystallize at different temperatures.
26
What is the sequence in which minerals crystallize from magma as it cools known as?
The sequence in which minerals crystallize from magma as it cools is known as Bowen's reaction series.
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27
What is Bowen's reaction series?
Bowen's reaction series is the sequential order in which minerals crystallize from a magma as it cools. This series helps explain the range of temperatures at which different minerals solidify.
28
How does Bowen's reaction series contribute to understanding the crystallization of magma?
Bowen's reaction series provides insight into the order in which minerals crystallize during the cooling of magma, helping to explain the variety of temperatures at which different minerals solidify.
29
What was Bowen's focus in his experiments, and what type of magmas did he primarily work with?
Norman Levi Bowen is responsible for the development of Bowen's reaction series.
Bowen focused on how magma cools and primarily worked with mafic magmas, which are rich in iron and magnesium.
30
Describe the process Bowen followed in his experiments to determine the order of crystallization of minerals.
Bowen melted the rock completely in a specially made kiln, allowed it to cool slowly to a specific temperature, and then quenched it quickly to prevent the formation of new minerals. The resulting rocks were studied under the microscope and analyzed chemically. This process was repeated at progressively lower temperatures.
it remains an important basis for understanding igneous rocks, describing the sequence in which minerals form as magma cools.
31
What are the two pathways in Bowen's reaction series for minerals to form as magma cools?
Bowen's reaction series has two pathways for mineral formation as magma cools: the discontinuous series and the continuous series.
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What characterizes the discontinuous series in Bowen's reaction series?
The discontinuous series in Bowen's reaction series is characterized by the transformation of one mineral into a different mineral through chemical reactions as magma cools.
33
What characterizes the continuous series in Bowen's reaction series?
The continuous series in Bowen's reaction series is characterized by the transformation of plagioclase feldspar from being rich in calcium to being rich in sodium as magma cools.
34
At what point in Bowen's reaction series does plagioclase feldspar begin to crystallize, and what characterizes its composition at that temperature?
Plagioclase feldspar begins to crystallize at about the point where pyroxene begins to crystallize. At that temperature, the plagioclase is calcium-rich, toward the anorthite end-member.
35
How does the composition of plagioclase feldspar change as the temperature drops, provided there is sodium left in the magma?
As the temperature drops, and if there is sodium remaining in the magma, the plagioclase that forms becomes a more sodium-rich variety, toward the albite end-member.
36
Why is the continuous series in Bowen's reaction series considered continuous?
The continuous series in Bowen's reaction series is considered continuous because the mineral is always plagioclase feldspar, but the series involves a transition from calcium-rich to sodium-rich composition.
37
What happens when cooling occurs relatively quickly in the context of the continuous series?
When cooling happens relatively quickly, individual plagioclase crystals can be zoned, ranging from calcium-rich in the center to more sodium-rich around the outside. This zoning occurs as calcium-rich early-forming plagioclase crystals become coated with progressively more sodium-rich plagioclase as the magma cools
38
At what temperature does olivine begin to form in Bowen's reaction series, and what happens as the temperature drops?
Olivine begins to form just below 1300°C, but as the temperature drops, olivine becomes unstable. Early-forming olivine crystals react with silica in the remaining liquid and are converted into pyroxene.
39
Describe the sequence of minerals formed in the discontinuous series as magma cools.
In the discontinuous series, olivine reacts to form pyroxene, and pyroxene reacts to form amphibole. Under the right conditions, amphibole may transform into biotite. Finally, if the magma is silica-rich, potassium feldspar, quartz, and possibly muscovite mica will form at lower temperatures.
40
What is the trend in the complexity of silica tetrahedra arrangements as you move down the discontinuous series?
The sequence of minerals formed in the discontinuous series goes from isolated tetrahedra (olivine) toward increasingly complex arrangements of silica tetrahedra. Pyroxene consists of single chains, amphibole has double chains, mica has sheets of tetrahedra, and potassium feldspar and quartz at the bottom of the series have tetrahedra connected to each other in three dimensions.
41
Why don't igneous rocks normally contain both olivine (at the top of the series) and quartz (at the bottom)?
Igneous rocks typically do not contain both olivine (at the top of the series) and quartz (at the bottom) because the first minerals to form are usually completely used up in later chemical reactions. However, exceptions can occur when rocks that crystallized early in the series come into contact with magmas representing compositions later in the series.
42
Provide an example of an exception where both olivine and quartz are found in the same rock.
Exceptions can occur when rocks that crystallized early in the series come into contact with magmas representing compositions later in the series. For instance, dark green olivine-rich xenoliths may be included within quartz- and feldspar-rich rocks, as seen in image4. The dark line around the xenoliths is amphibole, which formed as the olivine reacted with the melt. In some smaller xenoliths, olivine has been completely transformed into amphibole
*image 4
43
What determines how far the reaction process can continue before all the magma is used up, and what does it determine in terms of mineral formation?
The composition of the original magma determines how far the reaction process can continue before all the magma is used up. It also determines which minerals will form during the cooling process.
44
How are magma compositions reported, and what are the key oxides mentioned in image 5?
Magma compositions are reported in terms of the fraction of mass of oxides (e.g., Al2O3 instead of just Al). Key oxides mentioned include silicon dioxide (SiO2), iron, magnesium, calcium, sodium, and potassium.
*image 5
45
What is the approximate SiO2 content of mafic magma, and what are the major oxides found in mafic magmas?
On average, mafic magma is approximately half SiO2 by mass. Major oxides in mafic magmas include more than 25% iron, magnesium, and calcium oxides.
46
What is the approximate SiO2 content of felsic magma, and what are the major oxides found in felsic magmas?
On average, felsic magmas are closer to 75% SiO2 by mass. Major oxides in felsic magmas include approximately 5% iron, magnesium, and calcium oxides. Sodium and potassium oxides account for about 10% of felsic magmas by mass.
47
What is the term used to describe magmas falling between mafic and felsic magmas in terms of composition?
Magmas that fall between mafic and felsic magmas in terms of composition are described as having an intermediate composition.
48
What determines the composition of magma?
The composition of magma is determined by the original rock that melts to form the magma. Different rocks have different mineral compositions, and as they melt, the resulting magma will reflect the proportions of these minerals.
49
Why is there no ultramafic magma encountered in modern volcanic environments?
Although Earth was once hot enough to have ultramafic magma, it is no longer hot enough to melt ultramafic rocks. As a result, ultramafic magma is not encountered in modern volcanic environments, and ultramafic rocks are relatively rare at Earth’s surface.
50
What characterizes ultramafic rocks in terms of MgO and SiO2 content?
Ultramafic rocks have higher MgO than mafic rocks and even less SiO2.
51
What is the predominant composition of rocks in Earth’s lithosphere, and why is ultramafic magma not encountered in modern volcanic environments?
The vast majority of silicate rocks in Earth’s lithosphere are ultramafic rocks, as the mantle is composed of ultramafic rock. However, ultramafic magma is not encountered in modern volcanic environments because Earth is no longer hot enough to melt ultramafic rocks. The youngest known ultramafic volcanic rocks, called komatiites, are around 2 billion years old.
52
What is partial melting, and how does it affect the composition of magma?
Partial melting is a process where some components of a mixture melt before others. In the case of mafic magma, it is produced when ultramafic rocks undergo partial melting. Silicate minerals with more silica will melt before those with less silica during partial melting, resulting in a partial melt that has more silica than the rock as a whole.
53
How does fractional crystallization affect the composition of magma?
Fractional crystallization is a process within a magma chamber where crystals that form early, such as olivine, slowly settle towards the bottom. This removes iron- and magnesium-rich components, making the overall composition of the magma near the top more felsic.
54
What is the consequence of the formation of olivine during fractional crystallization?
The formation of olivine during fractional crystallization removes iron- and magnesium-rich components, altering the composition of the magma and making it more felsic near the top of the magma chamber.
55
How does the settling of crystals during fractional crystallization impact the lower part of the magma chamber?
The crystals that settle may either form an olivine-rich layer near the bottom of the magma chamber, or, if the lower part of the magma chamber is hotter, the crystals might remelt. If remelting occurs, crystal settling will make the magma at the bottom of the chamber more mafic than it was initially.
*image 6
56
How can the composition of magma change when other rocks are melted and mixed in?
The composition of magma can change when other rocks, especially the country rock in which the magma chamber is located, are melted and mixed in. This process adds to the existing magma in the magma chamber.
57
What is the term for the rock in which a magma chamber is located?
The rock in which a magma chamber is located is called the country rock.
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58
What happens if the country rock is more felsic than the magma in the magma chamber?
If the country rock is more felsic than the magma, the country rock may melt and mix with the magma in the magma chamber, altering its composition.
59
What are xenoliths, and how can they affect the composition of magma?
Xenoliths are fragments of unmelted rock carried by magma. Melting of xenoliths within the magma can alter its composition.
60
How can the re-melting of crystals that have settled out of the magma impact the composition of magma?
The re-melting of crystals that have settled out of the magma can alter the composition of the magma in the magma chamber.
61
How can igneous rocks be classified based on their chemical composition, and what are the four categories?
Igneous rocks can be classified based on their chemical composition into four categories: felsic, intermediate, mafic, and ultramafic.
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62
How are igneous rocks given names based on mineral abundance, and what is used to decide the name of an igneous rock?
Igneous rocks are given names based on the proportion of different minerals they contain. Which represents the minerals from Bowen's reaction series, is used to decide the name of an igneous rock.
63
How does image 8 work in deciding the name of an igneous rock?
Image 8 has a vertical scale representing the percentage of minerals within a rock by volume. An igneous rock can be represented as a vertical line drawn through the diagram, and the vertical scale is used to determine the proportion of each mineral it contains.
For instance, if the arrows in the ultramafic field of the diagram represent a rock containing 48% pyroxene and 52% plagioclase feldspar, an igneous rock at the boundary between the mafic and ultramafic fields would have approximately 20% olivine, 50% pyroxene, and 30% Ca-rich plagioclase feldspar by volume
64
How does the classification of igneous rocks by grain size affect their names, and what are the two main categories based on where the rock cools?
The classification of igneous rocks by grain size depends on whether they cool within Earth (intrusive or plutonic igneous rocks) or on the Earth's surface after erupting from a volcano (extrusive or volcanic igneous rocks). For example, a felsic intrusive rock is called granite, while a felsic extrusive rock is called rhyolite.
65
What is the key difference between intrusive and extrusive igneous rocks, and how is it related to grain size?
The key difference is the size of crystals making up the rocks, which is related to how rapidly melted rock cools. Intrusive rocks have larger crystals because magma cools much slower within Earth, while extrusive rocks have smaller crystals because they cool rapidly on Earth's surface.
66
What is the term for a rock with individual crystals that are visible to the unaided eye, and what does it mean?
A rock with individual crystals that are visible to the unaided eye has a phaneritic or coarse-grained texture. This term indicates that the crystals are large enough to be seen and identified.
*image 9
67
What is the term for a rock with crystals that are too small to see with the unaided eye, and what does it mean?
A rock with crystals that are too small to see with the unaided eye has an aphanitic or fine-grained texture. This term indicates that the crystals are so small that individual crystals cannot be distinguished.
68
Provide examples of igneous rocks with the same mineral composition but different names due to grain size differences.
A rock of intermediate composition is diorite if it is coarse-grained and andesite if it is fine-grained. A mafic rock is gabbro if it is coarse-grained and basalt if fine-grained. The coarse-grained version of an ultramafic rock is peridotite, and the fine-grained version is komatiite. Different names are used because rocks of different grain sizes form in different ways and geological settings.
69
Can an igneous rock have more than one grain size, and what causes this phenomenon?
Yes, an igneous rock can have more than one grain size. This occurs when there is a change in the rate at which melted rock is cooling. If magma is cooling in a magma chamber, slow cooling allows some minerals to crystallize before others, resulting in larger crystals. However, if the magma is suddenly erupted from a volcano, the lava cools rapidly, and larger crystals are surrounded by much smaller ones.
70
What is the term for an igneous rock with crystals of distinctly different sizes, and what are the larger crystals and smaller ones called?
An igneous rock with crystals of distinctly different sizes has a porphyritic texture, or might be referred to as a porphyry. The larger crystals are called phenocrysts, and the smaller ones are referred to as the groundmass.
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How can the composition of an intrusive igneous rock be approximated if the minerals present are uncertain, and what is the general trend regarding dark minerals in igneous rocks?
The composition of an intrusive igneous rock can be approximated by estimating the proportion of light and dark minerals. In general, igneous rocks have an increasing proportion of dark minerals as they become more mafic.
72
What are dark-colored minerals in igneous rocks, and why are they sometimes collectively referred to as ferromagnesian minerals?
Dark-colored minerals in igneous rocks are those higher in iron and magnesium (e.g., olivine, pyroxene, amphibole, biotite). They are collectively referred to as ferromagnesian minerals due to their higher iron and magnesium content.
73
How is the proportion of light and dark minerals estimated in a sample, and how is this information used to classify the rock?
By estimating the proportion of light minerals to dark minerals in a sample, it is possible to place that sample on a graphical scale, such as Figure 7.16. This information helps classify the rock based on its composition
*image 11
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What is a potential issue with estimating the proportion of dark minerals in identifying igneous rocks?
One issue is that plagioclase feldspar, which is not ferromagnesian, can appear darker if it is calcium-rich even though it is light-colored when sodium-rich. This can lead to inaccuracies in estimating the proportion of dark minerals in identifying igneous rocks.
75
How is the classification of igneous rocks affected when individual crystals are not visible, and what are some cases where this occurs?
The method of estimating the percentage of minerals, which works well for phaneritic igneous rocks with visible crystals, is not as effective when individual crystals are not visible. This occurs in cases such as volcanic rocks without phenocrysts and glassy igneous rocks.
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What is the characteristic feature of phaneritic igneous rocks, and how is their mineral composition estimated?
Phaneritic igneous rocks have individual crystals that are visible with little to no magnification. Their mineral composition is estimated by estimating the percentage of light and dark minerals.
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How can the minerals present in porphyritic but otherwise aphanitic igneous rocks be determined?
In porphyritic but otherwise aphanitic igneous rocks, the minerals present as phenocrysts provide clues to the identity of the rock.
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How are volcanic rocks without visible crystals or phenocrysts classified, and what characteristics are used for their classification?
In the absence of visible crystals or phenocrysts, volcanic rocks are classified based on color and other textural features. The color of volcanic rocks goes from light to dark as the composition goes from felsic to mafic.
79
What are the typical colors of rhyolite, andesite, and basalt, and how do they relate to their composition?
Rhyolite is often a tan or pinkish color, andesite is often grey, and basalt ranges from brown to dark green to black. The color of volcanic rocks correlates with their composition, with felsic rocks being lighter and mafic rocks being darker
80
What textural features are commonly observed in basalt, and what causes these features?
Basalt often shows textural features related to lava freezing around gas bubbles. When magma erupts, gases dissolved in the lava are released, leading to the formation of vesicles. If these vesicles are later filled by other minerals, they are called amygdules.
*image 12
81
How does the color of volcanic rocks change with the composition, and what does this imply?
The color of volcanic rocks goes from light to dark as the composition goes from felsic to mafic. This implies that the color of volcanic rocks can be indicative of their mineral composition and, by extension, their classification.
82
What is the relationship between crystal size and cooling rate in igneous rocks?
Crystal size is a function of cooling rate. The faster magma or lava cools, the smaller the crystals it contains.
83
What is volcanic glass, and how does it form?
Volcanic glass is formed when lava cools so rapidly that no crystals can form. This can result in smooth glass like obsidian or vesicular glass like scoria (mafic) and pumice (felsic).
84
Provide examples of volcanic glass, and what are their characteristics?
Provide examples of volcanic glass, and what are their characteristics?
Answer: Examples of volcanic glass include smooth obsidian and vesicular glass like scoria (mafic) and pumice (felsic). Pumice, due to its low-density felsic composition and enclosed vesicles, can float on water.
*image 13
85
What are the characteristics of pumice, and why can it float on water?
Pumice has a low-density felsic composition and enclosed vesicles, allowing it to float on water.
86
What are the ways in which hot magma tends to move upward toward the surface, and what is the process called when magma forces itself into cracks, breaks off pieces of rock, and envelops them?
Hot magma tends to move upward toward the surface by filling and widening existing cracks, melting the surrounding rock, pushing the rock aside (where the rock is hot enough and under enough pressure to deform without breaking), and breaking the rock. When magma forces itself into cracks, breaks off pieces of rock, and envelops them, this process is called stoping. The resulting fragments are xenoliths.
87
What is the resulting body of rock called when magma cools within the crust, and what are the different shapes and relationships with the surrounding rock that plutons can have?
The resulting body of rock when magma cools within the crust is called a pluton. Plutons can have different shapes and relationships with the surrounding country rock. Large, irregularly shaped plutons are called stocks or batholiths, depending on their size. Tabular plutons are called dikes if they cut across existing structures and sills if they are parallel to existing structures. Laccoliths are similar to sills but have caused the overlying rocks to bulge upward. Pipes are cylindrical conduits.
*image 14
88
What are xenoliths, and how do they form?
Xenoliths are fragments of rock that result from magma forcing itself into cracks, breaking off pieces of rock, and enveloping them. They may appear as dark patches within a rock.
89
What are large irregularly shaped plutons called, and what determines whether they are classified as stocks or batholiths?
Large irregularly shaped plutons are called either stocks or batholiths, depending on their area. If an irregularly shaped body has an area greater than 100 km2, then it is classified as a batholith; otherwise, it is a stock.
90
How is the classification of stocks and batholiths affected by the limitations of surface observations?
The classification of stocks and batholiths can be affected by the limitations of surface observations. A body with an area of less than 100 km2 exposed at the surface might be classified as a stock initially, but it could be much larger at depth. Mapping out its true extent is necessary for accurate classification.
91
How are batholiths typically formed, and can you provide an example of a large batholith?
Batholiths are typically formed when a number of stocks coalesce beneath the surface to create one large body. An example of a large batholith is the Coast Range Plutonic Complex, which extends from the Vancouver region to southeastern Alaska.
*image 15
92
How are tabular plutons classified based on their concordance with existing layering in the country rock, and what are the designations for concordant and discordant tabular intrusions?
Tabular plutons are classified based on whether they are concordant with (parallel to) existing layering or not. A sill is concordant with existing layering, while a dike is discordant. If the country rock has no bedding or foliation, then any tabular body within it is a dike.
93
What is the difference between a sill and a dike, and how is their designation determined?
A sill is concordant with existing layering, while a dike is discordant. The designation is not determined simply by the orientation of the feature; a dike could be horizontal, and a sill could be vertical, depending on the orientation of features in the surrounding rocks.
94
What is a laccolith, and how does it differ from a sill?
A laccolith is a sill-like body that has expanded upward by deforming the overlying rock. It differs from a sill in that it has caused the overlying rocks to bulge upward.
95
What is a lopolith, and how does it form?
A lopolith is formed when a sill-like body forms, but magma pools and sags downward, creating a depressed structure.
96
What is a pipe in the context of igneous rocks, and what is its function?
In the context of igneous rocks, a pipe is a cylindrical body with a circular, elliptical, or irregular cross-section that serves as a conduit or pipeline for the movement of magma from one location to another. Pipes may feed volcanoes, but they can also connect plutons.
97
What is the most obvious effect of country rock on magma during the intrusion of plutons, and how does it manifest?
The most obvious effect of country rock on magma during the intrusion of plutons is the formation of a chilled margin along the edges of the pluton. The country rock, being much cooler than the magma, causes the magma in contact with it to cool rapidly compared to the magma toward the interior of the pluton. This rapid cooling leads to smaller crystals, resulting in a different texture and possibly a darker color along the edges of the pluton compared to its interior.
*image 16
