Chapter 4 Igneous Rocks: Solids From Melts

Exercise 9: Why do melts migrate upward?

density

porphyry

A _____is an igneous rock that has a mixed texture in which large crystals "float" in a predominantly finegrained matrix. _______textures are important to geologists because they show that different minerals crystallize at different rates

What are the forms of igneous intrusions?

Large intrusive igneous bodies are called plutons. The largest plutons are batholiths, which are thick horizontal masses extending from a funnel-shaped central region. Stocks are smaller plutons. Less massive than plutons are sills, which lie parallel to the layers of bedded country rock, and dikes, which cut across those layers. Veins form where water is abundant, either in the magma or in the surrounding country rock

pumice

One volcanic rock type is ____a frothy mass of volcanic glass in which a great number of spaces remain after trapped gas has escaped from the solidifying melt

Exercise 8: In which plate tectonic settings would you expect magmas to form?

Spreading Centers, Subduction zones, and Mantle Plumes.

Batholiths,

________ the largest plutons, are great irregular masses of coarse-grained igneous rock that, by definition, cover at least 100 km2. They are thick, horizontal, sheetlike or lobe-shaped bodies extending from a funnel-shaped central region. Their bottoms may extend 10 to 15 km deep, and a few are estimated to go even deeper Other, smaller plutons are called stocks. Both ______and stocks are discordant intrusions; that is, they cut across the layers of the country rock that they intrude.

Dikes

_____are the major route of magma transport in the crust. _____, like sills, are sheetlike igneous bodies, but _____ cut across the layers in bedded country rock (Figure 4.11b) and so are discordant intrusions.

Viscosity

_____the measure of a liquid's resistance to flow.

a rock's melting point depends on: chemical and mineral composition, and temperature & pressure.

a rock's melting point depends on its ____ and ____composition and on conditions of ____ and _____.

concordant intrusions

boundaries lie parallel to the country rock layers, whether or not those layers are horizontal.

magma chambers

large pools of molten rock, called ___ _____ form in the lithosphere as rising magmas melt and push aside surrounding solid rock. We can think of them ____ ____ as large, liquid-filled cavities in solid rock, which expand as more of the surrounding rock melts or as magma migrates through cracks and other small openings.

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1. Some magma rises through the narrow cracks that open where the plates are separating and erupts into the ocean, forming the basaltic pillow lavas that cover the seafloor. 2. Some magma solidifies in the cracks as vertical, sheeted dikes of gabbro. 3. The remaining magma solidifies as massive intrusions of gabbro as the underlying magma chamber is pulled apart by seafloor spreading. These igneous units—pillow lavas, sheeted dikes, and massive gabbros—are the basic layers of the crust that geologists have found throughout the world's oceans.

How do Geologists today classify igneous rock samples?

1. Texture. & 2. Mineral and chemical composition.

obsidian

Another wholly glassy volcanic rock type is _____; unlike pumice, it contains only tiny vesicles and so is solid and dense. Chipped or fragmented _____ produces very sharp edges, and Native Americans and many other hunting groups used it for arrowheads and a variety of cutting tools.

How does magmatic differentiation account for the variety of igneous rocks?

Because different minerals crystallize at different temperatures, the composition of magma changes as it cools and various minerals are withdrawn by crystallization.

Exercise 1: Why are intrusive igneous rocks coarse-grained and extrusive rocks fine-grained?

Coarse-grained igneous rocks, which are intrusive and therefore cooled slowly; and fine-grained rocks, which are extrusive and cooled rapidly.

Exercise 3: What kinds of igneous rocks contain quartz?

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende, and sometimes quartz.

Exercise 4: Name two intrusive igneous rocks with a higher silica content than that of gabbro.

Granite, Diorite (check)

How are igneous rocks classified?

Igneous rocks can be divided into two broad textural classes: coarse-grained rocks, which are intrusive and therefore cooled slowly; and fine-grained rocks, which are extrusive and cooled rapidly. Igneous rocks can also be classified on the basis of their silica content using a scale that runs from felsic (rich in silica) to ultramafic (poor in silica).

Melt

It is the effect of pressure that explains why the rocks in most of the crust and mantle do not ____. Rock can _____ only when both temperature and pressure conditions are right.

Decompression Melting

Just as an increase in pressure can keep rock solid, a decrease in pressure can make rock melt, given a sufficiently high temperature. Because of convection currents in the mantle, mantle material rises to Earth's surface at midocean ridges at a more or less constant temperature. As the material rises and the pressure on it decreases below a critical point, the solid rock melts spontaneously, without the introduction of any additional heat. This process is known as _____ ____, and it produces the greatest volume of magma anywhere on earth. It is the process by which most basalts form on the seafloor.

magmatic stoping

Magma rising through the crust makes space for itself in three ways that may be referred to collectively as ______ ______: 1. Wedging open the overlying rock. As the rising magma lifts the great weight of the overlying rock, it fractures that rock, penetrates the cracks, wedges them open, and so flows into the rock. Overlying rocks may bow upward during this process. 2. Melting surrounding rock. Magma also makes its way by melting country rock. 3. Breaking off large blocks of rock. Magma can push its way upward by breaking off blocks of the invaded crust. These blocks, known as xenoliths (from the Greek for "foreign rocks"), sink into the magma, where they may melt and blend into the liquid

How do plate tectonic processes affect magma production?

Magmas are produced at two types of plate boundaries. At spreading centers, peridotite rises from the mantle and undergoes decompression melting to form basaltic magma. At subduction zones, subducting oceanic lithosphere undergoes fluid-induced melting to generate magmas of varying composition. Mantle plumes within lithospheric plates are also sites of decompression melting that produce basaltic magmas.

How and where do magmas form?

Magmas form at places in the lower crust and mantle where temperatures are high enough for partial melting of rock. Because the minerals within a rock melt at different temperatures, the composition of magmas varies with temperature. Pressure raises the melting temperature of rock, and the presence of water lowers it. Because melted rock is less dense than solid rock, magma rises through the surrounding rock, and drops of magma come together to form magma chambers.

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Magmatic differentiation is a more complex process than fi rst recognized. Some magmas derived from rocks of varying compositions may mix, whereas others are immiscible. Crystals may be transported to various parts of a magma chamber by turbulent flow in the liquid magma. Magmatic differentiation can begin with the partial melting of mantle and crustal rocks with a range of water contents over a range of temperatures. Magmas do not cool uniformly; they may exist transiently at a range of temperatures within a magma chamber. Differences in temperature within and among magma chambers may cause the chemical composition of magma to vary from one region to another. A few magma types are immiscible—they do not mix with one another, just as oil and water do not mix. When such magmas coexist in one magma chamber, each forms its own fractional crystallization series. Magmas that are miscible—that do mix—may follow a crystallization path different from that followed by any one magma type alone.

fluid-induced melting

Melting of rock induced by the presence of water that lowers its melting point is referred to as ___-____ ____.

Intermediate Igneous Rocks

Midway between the felsic and mafic ends of the scale are_____ _____ _____. these rocks are neither as rich in silica as the felsic rocks nor as poor in it as the mafic rocks. Ex; Granodiorite a light-colored rock that looks something like granite. It is also similar to granite in having abundant quartz, but its predominant feldspar is plagioclase, not orthoclase. Ex; To its right is diorite, which contains still less silica and is dominated by plagioclase feldspar, with little or no quartz. Diorites contain a moderate amount of the mafic minerals biotite, amphibole, and pyroxene. They tend to be darker than granite or granodiorite. Also includes Dacite and Andesite: The volcanic equivalent of granodiorite is dacite. To its right in the extrusive series is andesite, the volcanic equivalent of diorite.

extrusive igneous rocks

Rapid cooling at Earth's surface produces the finegrained texture or glassy appearance of ___ ___ ___ . These rocks, formed partly or largely of volcanic glass, are formed from material that erupts from volcanoes. For this reason, they are also known as volcanic rocks. They fall into two major categories based on the type of erupted material from which they are formed: Two types of these rocks are: (Mafic)Basalt and (Felsic)Rhyolite.

Lava

The term that we apply to magma flowing out onto Earth's surface is _____.

Intro

Today we know that deep in Earth's crust and mantle, rock melts and rises toward Earth's surface. Some magmas solidify before they reach the surface, and some break through and solidify on the surface. Both processes produce igneous rocks. In this chapter, we will examine the wide range of igneous rock types, both intrusive and extrusive, and the processes by which they form. We will explore the forces that cause rock to melt and form magmas and the ways in which those magmas reach the locations at and below Earth's surface where they solidify. We will then take a more detailed look at the igneous processes associated with specific plate tectonic settings

Pegmatites

Veins of extremely coarse grained granite cutting across much finer grained country rock are called _____. _____ crystallize from a water-rich magma in the late stages of solidification.

intrusive igneous rock

___ ___ ___ is one that has forced its way into the surrounding rock, called country rock, and solidified without reaching Earth's surface. Slow cooling of magma in Earth's interior allows adequate time for the growth of the large, interlocking crystals that characterize ___ ___ ___. Two types are: (Mafic)Gabbro, (Felsic)Granite.

Volcanic ash

___ is made up of extremely small fragments, usually of glass, that form when escaping gases force a fine spray of magma from a volcano.

Pyroclasts

___: In more violent eruptions, _____ form when fragments of lava are thrown high into the air. They are one of the two major categories of extrusive igneous rocks. Includes Volcanic ash, Bombs, & Pumice.

Mafic rocks

____ ____ contain large proportions of pyroxenes and olivines. These minerals are relatively poor in silica but are rich in magnesium and iron, from which they get their characteristic dark colors. Ex; Basalt: it's the most abundant igneous rock of the crust, and it underlies virtually the entire seafloor Ex; Gabbro: is a coarse-grained, dark gray intrusive igneous rock.

Felsic rocks

____ _____ are poor in iron and magnesium and rich in felsic minerals that are high in silica. Such minerals include quartz, orthoclase feldspar, and plagioclase feldspar. ex; Granite: one of the most abundant intrusive igneous rocks, contains about 70 percent silica ex; Rhyolite is the extrusive equivalent of granite. This light brown to gray rock has the same felsic composition and light coloration as granite, but it is much more fine-grained.

Plutons

____ are large igneous bodies formed deep in Earth's crust. They range in size from a cubic kilometer to hundreds of cubic kilometers

Bombs

____ are larger particles hurled from the volcano and streamlined by the air as they hurtle through it. As they fall to the ground and cool, these fragments of volcanic debris may stick together to form rocks

Lavas:

____: Volcanic rocks formed from flowing _____, _____ range in appearance from smooth and ropy to sharp, spiky, and jagged, depending on the conditions under which they are formed. They are one of the two major categories of extrusive igneous rocks.

country rock

_____ ____ is the rock that encloses a mineral deposit, igneous intrusion, or other feature.

Fractional crystallization

_____ _____ is the process by which the crystals formed in a cooling magma are segregated from the remaining liquid rock

Veins

_____ are deposits of minerals found within a rock fracture that are foreign to the country rock

ophiolite suites

______ ______ are unusual assemblages of rocks that were characteristic of the seafloor but were found on land. Known as ____ _____, these assemblages consist of deep-sea sediments, submarine basaltic lavas, and mafic igneous intrusions. Using data gathered from deepdiving submarines, dredging, deep-sea drilling, and seismic exploration, geologists now explain these rocks as fragments of oceanic lithosphere that were transported by seafloor spreading and then raised above sea level and thrust onto a continent in a later episode of plate collision

magmatic differentiation

______ _______a process by which rocks of varying composition can arise from a uniform parent magma

Ultramafic Rocks

_______ ______ consist primarily of mafic minerals and contain less than 10 percent feldspar. Ex: peridotite, a coarse-grained, dark greenish gray rock made up primarily of olivine with smaller amounts of pyroxene. Peridotites are the dominant rocks in Earth's mantle, they are the source of the basaltic magmas that form rocks at mid-ocean ridges.

Mantle Plume

_______, like spreading centers, are sites of decompression melting, but they differ from spreading centers by forming within lithospheric plates rather than along the margins of plates. These plumes of hot mantle material rise from deep within Earth, possibly as deep as the core-mantle boundary

sill

_____is a sheetlike body formed by the injection of magma between parallel layers of bedded country rock.

higher, crystallize

mafic minerals melt at ______ temperatures than felsic minerals. mafic minerals also _______ at higher temperatures than felsic minerals

Partial Melting

rocks do not melt completely at a given temperature instead rocks undergo _____ ______ because the minerals that compose them melt at different temperatures.

Exercise 10: Where on the ocean floor would you find basaltic magmas being extruded?

spreading centers