Chapter 7- Igneous Rocks (Week 5)

Question 1

How do igneous rocks form?

Answer 1

Igneous rocks form when melted rock cools. The melted rock, known as magma, originates within Earth.

Question 2

What are the eight main elements found in magma compositions?

Answer 2

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.

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Question 3

What happens to lighter elements in magma as it cools?

Answer 3

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.

Question 4

How does magma composition vary?

Answer 4

Magma composition depends on the composition of the rocks that melted to form the magma and the conditions under which the melting happened.

Question 5

What is partial melting, and how does it influence magma composition?

Answer 5

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.

Question 6

Why is magma produced by partial melting less dense than the surrounding rock?

Answer 6

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.

Question 7

How does moving magma influence surrounding rocks?

Answer 7

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.

Question 8

What happens when rocks melt due to the influence of magma?

Answer 8

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.

Question 9

Is adding heat the only way to trigger melting in rocks?

Answer 9

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.

Question 10

Why doesn’t the Earth’s mantle, despite high temperatures, melt like rock at the Earth’s surface?

Answer 10

The Earth’s mantle remains almost entirely solid at high temperatures due to the high pressure it experiences.

Question 11

How can melting in the Earth’s mantle be triggered without adding heat?

Answer 11

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.

Question 12

What is decompression melting?

Answer 12

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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Question 13

How is pressure reduced in the Earth’s mantle, leading to decompression melting?

Answer 13

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.

Question 14

What are the two main processes that contribute to pressure reduction in the mantle?

Answer 14

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.

Question 15

What is flux-induced melting?

Answer 15

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.

Question 16

What is a flux in the context of flux-induced melting?

Answer 16

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.

Question 17

Where does flux-induced partial melting of rock commonly occur?

Answer 17

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.

Question 18

How much water is required to trigger partial melting in laboratory studies of the Japanese volcanic arc?

Answer 18

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.

Question 19

What does viscosity refer to, and how is it related to the flow of a substance?

Answer 19

Viscosity refers to the ease with which a substance flows. A substance with low viscosity flows more easily than a substance with high viscosity.

Question 20

Why is most magma entirely liquid at temperatures over 1300°C?

Answer 20

At temperatures over 1300°C, most magma is entirely liquid because there is too much energy for the atoms to bond together.

Question 21

What happens to magma as it loses heat to the surrounding rocks and its temperature drops?

Answer 21

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.

Question 22

How do silica chains affect magma viscosity?

Answer 22

Silica chains formed by the linking of silica tetrahedra during cooling make magma more viscous.

Question 23

What are the implications of magma viscosity for volcanic eruptions?

Answer 23

Magma viscosity has important implications for the characteristics of volcanic eruptions. The viscosity of magma influences the explosiveness and behavior of volcanic eruptions.

Question 24

What is the process of crystallization in the context of magma?

Answer 24

Crystallization in the context of magma refers to the solidification or freezing of minerals that make up igneous rocks at various temperatures.

Question 25

Why can cooling magma have both crystals and remain predominantly liquid?

Answer 25

Cooling magma can have both crystals and remain predominantly liquid because the minerals within the magma crystallize at different temperatures.

Question 26

What is the sequence in which minerals crystallize from magma as it cools known as?

Answer 26

The sequence in which minerals crystallize from magma as it cools is known as Bowen’s reaction series.

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Question 27

What is Bowen’s reaction series?

Answer 27

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.

Question 28

How does Bowen’s reaction series contribute to understanding the crystallization of magma?

Answer 28

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.

Question 29

What was Bowen’s focus in his experiments, and what type of magmas did he primarily work with?

Answer 29

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.

Question 30

Describe the process Bowen followed in his experiments to determine the order of crystallization of minerals.

Answer 30

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.

Question 31

What are the two pathways in Bowen’s reaction series for minerals to form as magma cools?

Answer 31

Bowen’s reaction series has two pathways for mineral formation as magma cools: the discontinuous series and the continuous series.

Question 32

What characterizes the discontinuous series in Bowen’s reaction series?

Answer 32

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.

Question 33

What characterizes the continuous series in Bowen’s reaction series?

Answer 33

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.

Question 34

At what point in Bowen’s reaction series does plagioclase feldspar begin to crystallize, and what characterizes its composition at that temperature?

Answer 34

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.

Question 35

How does the composition of plagioclase feldspar change as the temperature drops, provided there is sodium left in the magma?

Answer 35

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.

Question 36

Why is the continuous series in Bowen’s reaction series considered continuous?

Answer 36

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.

Question 37

What happens when cooling occurs relatively quickly in the context of the continuous series?

Answer 37

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

Question 38

At what temperature does olivine begin to form in Bowen’s reaction series, and what happens as the temperature drops?

Answer 38

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.

Question 39

Describe the sequence of minerals formed in the discontinuous series as magma cools.

Answer 39

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.

Question 40

What is the trend in the complexity of silica tetrahedra arrangements as you move down the discontinuous series?

Answer 40

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.

Question 41

Why don’t igneous rocks normally contain both olivine (at the top of the series) and quartz (at the bottom)?

Answer 41

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.

Question 42

Provide an example of an exception where both olivine and quartz are found in the same rock.

Answer 42

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

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Question 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?

Answer 43

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.

Question 44

How are magma compositions reported, and what are the key oxides mentioned in image 5?

Answer 44

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.

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Question 45

What is the approximate SiO2 content of mafic magma, and what are the major oxides found in mafic magmas?

Answer 45

On average, mafic magma is approximately half SiO2 by mass. Major oxides in mafic magmas include more than 25% iron, magnesium, and calcium oxides.

Question 46

What is the approximate SiO2 content of felsic magma, and what are the major oxides found in felsic magmas?

Answer 46

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.

Question 47

What is the term used to describe magmas falling between mafic and felsic magmas in terms of composition?

Answer 47

Magmas that fall between mafic and felsic magmas in terms of composition are described as having an intermediate composition.

Question 48

What determines the composition of magma?

Answer 48

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.

Question 49

Why is there no ultramafic magma encountered in modern volcanic environments?

Answer 49

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.

Question 50

What characterizes ultramafic rocks in terms of MgO and SiO2 content?

Answer 50

Ultramafic rocks have higher MgO than mafic rocks and even less SiO2.

Question 51

What is the predominant composition of rocks in Earth’s lithosphere, and why is ultramafic magma not encountered in modern volcanic environments?

Answer 51

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.

Question 52

What is partial melting, and how does it affect the composition of magma?

Answer 52

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.

Question 53

How does fractional crystallization affect the composition of magma?

Answer 53

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.

Question 54

What is the consequence of the formation of olivine during fractional crystallization?

Answer 54

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.

Question 55

How does the settling of crystals during fractional crystallization impact the lower part of the magma chamber?

Answer 55

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.

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Question 56

How can the composition of magma change when other rocks are melted and mixed in?

Answer 56

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.

Question 57

What is the term for the rock in which a magma chamber is located?

Answer 57

The rock in which a magma chamber is located is called the country rock.

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Question 58

What happens if the country rock is more felsic than the magma in the magma chamber?

Answer 58

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.

Question 59

What are xenoliths, and how can they affect the composition of magma?

Answer 59

Xenoliths are fragments of unmelted rock carried by magma. Melting of xenoliths within the magma can alter its composition.

Question 60

How can the re-melting of crystals that have settled out of the magma impact the composition of magma?

Answer 60

The re-melting of crystals that have settled out of the magma can alter the composition of the magma in the magma chamber.

Question 61

How can igneous rocks be classified based on their chemical composition, and what are the four categories?

Answer 61

Igneous rocks can be classified based on their chemical composition into four categories: felsic, intermediate, mafic, and ultramafic.

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Question 62

How are igneous rocks given names based on mineral abundance, and what is used to decide the name of an igneous rock?

Answer 62

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.

Question 63

How does image 8 work in deciding the name of an igneous rock?

Answer 63

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

Question 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?

Answer 64

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.

Question 65

What is the key difference between intrusive and extrusive igneous rocks, and how is it related to grain size?

Answer 65

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.

Question 66

What is the term for a rock with individual crystals that are visible to the unaided eye, and what does it mean?

Answer 66

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.

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Question 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?

Answer 67

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.

Question 68

Provide examples of igneous rocks with the same mineral composition but different names due to grain size differences.

Answer 68

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.

Question 69

Can an igneous rock have more than one grain size, and what causes this phenomenon?

Answer 69

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.

Question 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?

Answer 70

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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Question 71

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?

Answer 71

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.

Question 72

What are dark-colored minerals in igneous rocks, and why are they sometimes collectively referred to as ferromagnesian minerals?

Answer 72

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.

Question 73

How is the proportion of light and dark minerals estimated in a sample, and how is this information used to classify the rock?

Answer 73

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

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Question 74

What is a potential issue with estimating the proportion of dark minerals in identifying igneous rocks?

Answer 74

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.

Question 75

How is the classification of igneous rocks affected when individual crystals are not visible, and what are some cases where this occurs?

Answer 75

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.

Question 76

What is the characteristic feature of phaneritic igneous rocks, and how is their mineral composition estimated?

Answer 76

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.

Question 77

How can the minerals present in porphyritic but otherwise aphanitic igneous rocks be determined?

Answer 77

In porphyritic but otherwise aphanitic igneous rocks, the minerals present as phenocrysts provide clues to the identity of the rock.

Question 78

How are volcanic rocks without visible crystals or phenocrysts classified, and what characteristics are used for their classification?

Answer 78

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.

Question 79

What are the typical colors of rhyolite, andesite, and basalt, and how do they relate to their composition?

Answer 79

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

Question 80

What textural features are commonly observed in basalt, and what causes these features?

Answer 80

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.

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Question 81

How does the color of volcanic rocks change with the composition, and what does this imply?

Answer 81

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.

Question 82

What is the relationship between crystal size and cooling rate in igneous rocks?

Answer 82

Crystal size is a function of cooling rate. The faster magma or lava cools, the smaller the crystals it contains.

Question 83

What is volcanic glass, and how does it form?

Answer 83

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).

Question 84

Provide examples of volcanic glass, and what are their characteristics?

Answer 84

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.

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Question 85

What are the characteristics of pumice, and why can it float on water?

Answer 85

Pumice has a low-density felsic composition and enclosed vesicles, allowing it to float on water.

Question 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?

Answer 86

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.

Question 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?

Answer 87

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.

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Question 88

What are xenoliths, and how do they form?

Answer 88

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.

Question 89

What are large irregularly shaped plutons called, and what determines whether they are classified as stocks or batholiths?

Answer 89

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.

Question 90

How is the classification of stocks and batholiths affected by the limitations of surface observations?

Answer 90

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.

Question 91

How are batholiths typically formed, and can you provide an example of a large batholith?

Answer 91

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.

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Question 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?

Answer 92

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.

Question 93

What is the difference between a sill and a dike, and how is their designation determined?

Answer 93

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.

Question 94

What is a laccolith, and how does it differ from a sill?

Answer 94

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.

Question 95

What is a lopolith, and how does it form?

Answer 95

A lopolith is formed when a sill-like body forms, but magma pools and sags downward, creating a depressed structure.

Question 96

What is a pipe in the context of igneous rocks, and what is its function?

Answer 96

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.

Question 97

What is the most obvious effect of country rock on magma during the intrusion of plutons, and how does it manifest?

Answer 97

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.

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