Chapter 4—Rocks and Minerals: Documents that Record Earth’s History
CHAPTER OVERVIEW
This chapter offers a brief review of physical geology presenting the “documents” in which Earth’s geologic history is preserved. These documents are minerals, rocks, and the fossils embedded in rocks. The basics of Earth materials are presented to set the stage for understanding events that have shaped our planet. A comprehensive review of common rock forming minerals, their occurrence, chemical composition, and usefulness in the interpretation of ancient geologic environments is presented. A review of the three rock families—igneous, sedimentary, and metamorphic—is given using the rock cycle where geologic processes act continuously to change one rock type to another. Additional information on the three rock families is offered explaining how each rock type is reflective of the specific environmental conditions present during that rock’s formation.
LEARNING OBJECTIVES
By reading and completing information within this chapter, you should gain an understanding of the following concepts:
- Describe common minerals found in rocks and their properties.
- Explain how the common rock-forming minerals are formed.
- Explain the characteristics of silicate and non-silicate minerals and how they are grouped.
- Discuss the conditions under which rock-forming minerals occur.
- Describe the characteristics that differentiate igneous, sedimentary, and metamorphic rocks.
- Sketch and label the rock cycle, showing the alternative routes that can be taken.
- Describe the conditions under which igneous, sedimentary, and metamorphic rocks are formed and discuss how their formation reflects past environments.
CHAPTER OUTLINE
- Minerals as Documents of Earth History
- Minerals and Their Properties
- Common Minerals that Form Rocks
- Silicate Minerals
- Quartz
- Feldspar
- Mica
- Hornblende
- Augite
- Olivine
- Clay Minerals
- Nonsilicate Minerals
- Carbonates, Calcite, and Dolomite
- Evaporites
- Earth’s Three Great Rock Families and How They Formed
- Igneous Rocks: “Fire-Formed”
- What Does the Texture of Igneous Rocks Tell Us About Their History?
- What Does the Composition of Igneous Rocks Tell Us About Their History?
- Representative Igneous Rocks
- Some Minerals Form Early, Others Later
- Sequence of Mineral Crystallization
- Volcanic Activity Has Produced Enormous Volumes of Rock
- Sedimentary Rocks: Layered Pages of History
- Where Sediments Come From
- Classifying Sedimentary Rocks
- Clastic Sedimentary Rocks
- Carbonate Sedimentary Rocks
- Other Sedimentary Rocks
- What Do Sedimentary Rocks Tell Us About Earth’s History?
- Metamorphic Rocks: Changed Without Melting
- Contact Metamorphism and Regional Metamorphism
- Kinds of Metamorphic Rocks
- Foliated Metamorphic Rocks
- Nonfoliated Metamorphic Rocks
- What Do Metamorphic Rocks Tell Us About Earth’s History?
Key Terms (pages given in parentheses)
andesite (63): A volcanic rock that in chemical composition is intermediate between basalt and granite.
anhydrite (57): A granular, white, anhydrous (water-lacking) calcium sulfate (CaSO4).
aragonite (57):Calcium carbonate (the same composition as calcite, but a polymorph of calcite)
augite (55): A dark-colored, ferromagnesian mineral which is an important member of the pyroxene family of minerals (silicates of aluminum, calcium, magnesium, and iron). It is stumpy in shape, with good cleavage developed along two planes that are nearly at right angles.
basalt (61): A fine-grained extrusive igneous rock composed of ferromagnesian minerals and tiny rectangular grains of plagioclase feldspars.
Bowen’s Reaction Series (63): A series of minerals wherein any early-formed phase will react with the melt later in the differentiation to yield a new mineral further in the series.
calcite (56): Calcium carbonate, the predominant mineral in the carbonate rock limestone. One of the most common carbonate minerals; a polymorph of aragonite (see aragonite above).
carbonate clasts (71): An individual grain, fragment, or constituent of detrital sediment or sedimentary rock produced by physical breakdown of a larger mass.
carbonate spar (71): As viewed microscopically, the clear, crystalline carbonate that has been deposited in a carbonate rock as a cement between clasts or has developed by re-crystallization of clasts.
chert (71): A dense, hard sedimentary rock or mineral composed of sum-micro-crystalline quartz. Unless colored by impurities, chert is white, as opposed to flint, which is dark or black.
chlorite (72): Any of a group of greenish, platy, hydrous monoclinic silicates of aluminum, ferrous iron, and magnesium which are closely associated with and resemble the micas.
clastic sediment (69): Texture that characterizes a rock made up of fragmental grains such as sand, silt, or parts of fossils. Conglomerates, sandstones, and siltstones are clastic rocks; the individual clastic grains are termed clasts.
clay mineral (55): Silicates of hydrogen and aluminum with additions of magnesium, iron, and potassium. Their basic structure is similar to that of mica, but because individual flakes are extremely small, their mica-like form can only be seen with an electron microscope.
cleavage (51): The tendency of a mineral to break along certain directions in its crystal structure where the bonds between planes of ions are weakest.
color (50): A property of a mineral or rock that helps discern it from others. Color often reflects chemical compositions. Note variations in color exist in many minerals, rocks.
coal (72): A carbonaceous rock resulting from the accumulation of plant matter in a swampy environment combined with alteration of that plant tissue by both biochemical and physical processes until it is converted to a consolidated carbon-rich material. The stages are peat, lignite, sub-bituminous, bituminous, and anthracite, it is sedimentary in nature until it reaches the sub-bituminous stage, at which point it becomes metamorphic.
contact metamorphism (72): Alteration of rock immediately adjacent to an igneous intrusion. Compositional and textural changes occurring are largely the result of high temperatures and the emanation of chemically active vapors that accompany igneous intrusions.
continuous series (of Bowen’s Reaction Series ) (64): The branch of Bowen’s Reaction Series that comprise the plagioclase group of minerals in which reaction of early formed crystals with later liquids takes place without abrupt phase changes.
crystal habit (52): The way a mineral grows, through the regular addition of ions to its surfaces from a surrounding rock melt (magma), solution, or gas.
density (51): Is a mineral’s mass per unit volume (density = mass/volume).
diorite (63): A phaneritic, plutonic rock with granular texture composed largely of plagioclase feldspar with smaller amounts of dark-colored minerals; used occasionally as ornamental and building stone. Also know as black granite.
discontinuous series (of Bowen’s Reaction Series ) (64): The branch of Bowen’s Reaction Series that includes the minerals olivine, pyroxene, amphibole, and biotite. Each change in the series representing an abrupt phase change.
dolomite (57): A carbonate sedimentary rock that contains more than 50% of the mineral dolomite CaMg(CO3)2.
evaporite (57): Sediments precipitated from a water solution as a result of the evaporation of that water. Evaporite minerals include anhydrite gypsum (CaSO4) and halite (NaCl).
extrusive igneous rock (59): Igneous rocks formed from melts that have reached the Earth’s surface. Examples are rocks formed from lava erupted from volcanoes or lava that has welled out of fissures.
feldspar (52): A group of silicate minerals that make up about 60% of the outer 9 miles (15 kilometers) of the Earth’s crust.
ferromagnesian silicate mineral (=mafic silicate mineral) (55): Hornblende is designated a ferromagnesian or mafic silicate mineral because of its content of iron and magnesium.
foliation (in metamorphic rocks) (73): A textural feature especially characteristic of metamorphic rocks in which laminae develop by growth or realignment of minerals in parallel orientation. Examples includes, slate, phyllite, schist, and gneiss.
fractional crystallization (66): The separation of components of a cooling magma by sequential formation of particular mineral crystals at progressively lower temperatures.
gabbro (62): A group of dark-colored, intrusive, igneous rocks with granular texture, composed largely of basic plagioclase and clinopyroxene.
garnet (72): A family of aluminosilicates of iron and calcium that are particularly characteristic of metamorphic rocks.
granite (61): A silica-rich, relatively light-colored, intrusive igneous rock composed primarily of potassium feldspar, quartz (at least 25%), sodium plagioclase, hornblende, and mica.
granodiorite (62): Abundant quart-bearing, intrusive igneous rock, with plagioclase being the dominant feldspar.
greenstone (75): A non-foliated, low-grade metamorphic rock, which is dark green in color due to the presence of abundant chlorite, epidote, and biotite. It is derived from metamorphism of basalitic extrusive igneous rocks. Great linear outcrops of greenstones are termed green stone belts and are through to mark the locations of former volcanic island arcs. The texture is so fine that mineral components cannot be seen without magnification (except for larger crystals).
gypsum (57): A soft, hydrous, calcium sulfate. Referred to as an evaporate because it is often precipitated from bodies of water that have been subjected to intense evaporation.
halite (57): A non-silicate mineral. Also known as rock salt. This mineral has a salty taste and cleaves to form cubes. Referred to as an evaporate because it is often precipitated from bodies of water that have been subjected to intense evaporation.
hardness (51): A mineral’s resistance to scratching by another substance of known hardness.
hornblende (54): A vitreous, black or very dark green mineral. Most common member of the larger family of minerals called amphiboles. Designated as a ferromagnesian, or mafic mineral, because of its iron and magnesium content. It contains crystals that are long and narrow and shows two good cleavage planes parallel to the long axis which intersect each other at angles,
hornfels (75): A non-foliated metamorphic rock, which is very hard and fined grained. It is often studded with small crystals of mica and garnet having no preferred orientation. May form from shale or other fine-grained rocks that are intensely heated during contact metamorphism of intrusive igneous bodies.
igneous rock (58): A rock formed by the cooling and solidification of magma or lava. They constitute over 90% of the volume of the Earth’s crust.
intrusive igneous rock (59): Igneous rocks that have formed from magma that has penetrated into other rocks and solidified before reaching the surface. Very large masses of such rocks are called plutons.
lava (59): The term used to describe molten material that has reached the surface of the Earth.
lithification (67): The process by which loose sediment is converted to coherent solid rock by any of several processes: precipitation of a cementing material around individual grains, compaction, or crystallization.
luster (51): The way a mineral shines in reflected light.
mafic silicate (55): Rocks (or lava) dominated by dark iron and magnesium silicates, as is the oceanic crust.
magma (59): the term used to describe the mixture of molten silicates and gases while still beneath the surface.
marble (74): A non-foliated metamorphic rock composed of calcite or dolomite, with a fine to coarsely crystalline texture. Marble is derived from limestone or dolomite.
metamorphic index mineral (72): Metamorphic minerals that are known to form within a specific temperature and pressure range.
metamorphic rock (58): Any rock that has been changed from previously existing rocks by the action of heat, pressure, and associated chemical activity. The parent rock is subjected to high temperature and pressure but does not melt.
metamorphism (72): The transformation of previously existing rocks into new types by the action of heat, pressure, and chemical solutions. Metamorphism usually takes place at depth in the roots of mountain chains or adjacent to large intrusive igneous bodies.
mica (54): A silicate mineral easily recognized by its perfect and conspicuous cleavage along one directional plane. There are two chief varieties: the colorless or pale-colored muscovite mica, which is a hydrous potassium aluminum silicate, and the dark-colored biotite mica, which also contains magnesium and iron. Both are common rock-forming silicates.
micrite (71): An exceptionally fine-grained carbonate mud (limestone).
mineral (50): A naturally occurring, solid, inorganic substance having a specific chemical composition and a specific crystal structure.
nonsilicate mineral (56): Those minerals that do not contain the silicon-oxygen structures that characterize silicate minerals.
nonfoliated metamorphic rocks (74): A metamorphic texture in which there is no discernable preferred orientation of mineral grains. Examples are marble or quartzite.
obsidian (61): A jet-black volcanic glass, usually of rhyolitic composition, formed by rapid cooling of viscous lava; generally forms the upper parts of lava flows.
olivine (56): A glassy looking iron and magnesium silicate, often with an olive green color. Common mineral in dark, mafic rocks. The gem variety is called peridot.
oöid (71): Found in back-reef facies along with carbonate sands and muds.
oölite (71): Limestones composed largely of small, round or ovate calcium carbonate bodies called oöids.
partial melting (66): The general process by which a rock subjected to high temperature and pressure is partly melted and the liquid component is moved to another location. Partial melting results from the variations in melting points of different minerals in the original rock mass.
phyllite (74): Has a very fine texture, although some grains of mica, chlorite, garnet, or quartz may be visible. Surfaces often develop a wrinkled aspect and are more lustrous than slate. Phyllite is intermediate in degree of metamorphism between slate and schist. Parent rocks are commonly shale or slate.
pluton (59): A body of igneous rock that has formed beneath the surface of the earth by consolidation from magma.
plutonic igneous rock (59):Igneous rock that cools and crystallizes deep underground.
porphyritic texture (61): Texture shown due to a two-stage cooling of an igneous rock. The large crystals (phenocrysts) were formed slowly at depth and were then swept upward and incorporated in the lava as it hardened at the surface (more rapid cooling).
pumice (61): A rock froth, formed by the extreme puffing up of liquid lava by expanding gases liberated from solution in the lava prior to and during solidification. Also known as foam; pumice stone; pumicite; volcanic foam.
quartz (52): Most important of all the silicate minerals. In appearance, quartz is a glassy, colorless, gray, or white mineral. It is relatively hard and will scratch glass.
quartzite (75): A non-foliated metamorphic rock which is fine-grained and often sugary-textured. It is composed of inter-grown quartz and is very hard. Derived from quartz sandstone and may vary in color.
regional metamorphism (73): A type of metamorphic rock alteration that is really extensive and occurs under the conditions of great confining pressures and heat accompanying deep burial and mountain building.
rhyolite (61): A light-colored, aphanitic volcanic rock composed largely of alkali feldspar and free silica with minor amounts of mafic minerals; the extrusive equivalent of granite.
schist (74): Consists of platy or needle-like minerals sufficiently large to be visible to the unaided eye. Minerals tend to be segregated into distinct layers. Schists are named according to the most conspicuous mineral present. The parent rock is usually shale, although some are derived from fine-grained volcanic rocks.
sedimentary rock (58): A rock that has formed as a result of the consolidation (lithification) of accumulations of sediment.
sillimanite (72): A brown, pale-green, or white neosilicate mineral with vitreous luster crystallizing in the orthorhombic system; commonly occurs in slender crystals, often in fibrous aggregates; hardness is 6-7 on Mohs scale, and specific gravity is 3.23.
slate (74): Foliated metamorphic rock with the foliation being microscopic and caused by the parallel alignment of minute flakes of silicates. Planes of foliation are smooth and the rock may split along these planes of “slaty cleavage.” Derived from the regional metamorphism of shale.
strata (67): A feature of sedimentary rocks in which the rocks occur in beds or layers called strata.
streak (of mineral) (51): The color of a mineral when it is ground to powder, or rubbed against a streak plate.
texture (of rock) (58): The size, shape, and arrangement of constituent minerals in a rock.
tuff (61): Volcanic ash that has become consolidated into rock.
vesicles (61): A small cavity or cavities formed by the entrapment of gas bubbles or steam during the solidification of a rock.
Multiple-Choice Questions
1.The rock cycle shows us that
a.metamorphic rocks can only be derived from igneous rocks.
b.metamorphic rocks are derived from magma.
c.any rock type can be derived from any other rock type.
d.igneous rocks are composed of quartz and gabbro.
2.A textural term used to describe an igneous rock with large crystals called phenocrysts in a matrix of more finely crystalline rock is
a.phenocryptic.c.porphyritic.
b.polymorphic.d.dolomitic.
3.Which of the following is a foliated metamorphic rock?
a.schistc.gabbro
b.shaled.marble
4.A carbonate mineral that occurs in a different crystal form and is less common than either calcite or dolomite is
a.aragonite.c.calcite.
b.gypsum.d.silica.
5.An example of a sedimentary evaporite is
a.calcite.c.limestone.
b.dolomite.d.gypsum.
6.Rocks that cool from a melt are
a.metamorphic rocks.d.hard rocks.
b.igneous rocks.e.on the rocks.
c.sedimentary rocks.
7.Rocks that have recrystallized under high temperatures and pressures are
a.metamorphic rocks.d.granite.
b.igneous rocks.e.basalt.
c.sedimentary rocks.
8.The size, shape, and arrangement of mineral grains in a rock constitutes its
a.permeable origin.c.cement.
b.porosity.d.texture.
9.A clastic rock composed of water-worn, rounded particles larger than 2 mm in diameter is
a.mudstone.c.breccia.
b.siltstone.d.conglomerate.
10.The overwhelmingly predominant constituent of an igneous rock is
a.silicon.c.aluminum.
b.oxygen.d.calcium.
11.Which of the following is one of the eight most common elements in the earth’s crust?
a.nitrogend.thorium
b.potassiume.hydrogen
c.uranium
12.The fine extrusive equivalent of granite is
a.diorite.d.basalt.