Midterm

Question 1

Define historical geology:

Answer 1

It draws upon other sciences such as chemistry, physics, ecology, oceanography and biology etc.

Question 2

Why is geology an eclectic science?

Answer 2

It relies on qualitative, descriptive data to work out the most plausible explanation for a complex, time dependent event or set of events.

Question 3

Principle of superposition of strata:

Answer 3

Superposition states that in s succession of layered rocks, the oldest rocks are on the bottom and the youngest are on top

Question 4

Principle of original horizontality:

Answer 4

Original horizontality states that layered sedimentary rocks are horizontal when they are first deposited under the influence of gravity

Question 5

Principle of lateral continuity:

Answer 5

As originally deposited, strata extend in all directions until they terminate by thinning at the margin of the basin, end abruptly against some former barrier to deposition, or grade laterally into a different kind of sediment

Question 6

Uniformitarianism:

Answer 6

The scientific pillar upon which the study of Earth history rests. The laws of nature do not change through time. Law, process, rate, state.

Question 7

Principle of faunal (biological) succession:

Answer 7

This principle stipulates that the life forms of each age in the Earth’s long history were unique for particular periods, that the fossil remains of life permit geologists to recognize contemporaneous deposits around the world, and that fossils could be used to assemble the scattered fragments of the record into a chronologic sequence.

Question 8

Principle of cross-cutting relationships:

Answer 8

A geologic feature that cuts across another rock body must be younger than the rock body that it disrupts

Question 9

Nicolaus Steno:

Answer 9

He first discovered the principles of superposition, original horizontality and lateral continuity. He worked as a physician to the Duke of Tuscany.

Question 10

James Hutton:

Answer 10

Authored Uniformitarianism

Question 11

Georges Cuvier:

Answer 11

Expert in comparative anatomy and with that knowledge he became the most respected vertebrate paleiontologist of his day.

Question 12

Charles Lyell:

Answer 12

Popularized Uniformitarianism.

Question 13

Charles Darwin:

Answer 13

Wrote, “On the Origin of Species” and fathered evolution

Question 14

William Smith:

Answer 14

Discovered faunal succession

Question 15

Oldest to youngest periods:

Answer 15

Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian.

Question 16

Understand the general process by which the geological eras and periods were determined and named.

Answer 16

Units were named as they were discovered and studied. Sometimes they were named or borrowed by the geologist who discovered it; sometimes they were named by a certain rock from that age

Question 17

Distinguish between relative dating and absolute dating:

Answer 17

Relative dating is dating things by where they happened relatively to when something else happened. Absolute is being able to put an actual time and number on it.

Question 18

Discuss the role of radioactivity in constructing a quantitative geologic timescale:

Answer 18

It helped to determine absolute dating by using the rate that the radioactive elemental deteriorated the mineral sample

Question 19

Describe the general steps used by geologists and other scientists in their attempt to solve particular problems or explain natural phenomena:

Answer 19

A. Develop a question about an observation in nature;
B. Formulate a hypothesis or plausible explanation to answer the question;
C. Test the hypothesis by seeing if it fits the data; and
D. Accept, reject, or modify the hypothesis. If scientists modify the hypothesis, they test it again to see if it better explains the phenomenon in question.

Question 20

Discuss the principles the Steno, Lyell, and Smith formulated for the development of the geologic time scale:

Answer 20

Steno established superposition of strata, original horizontality, and lateral continuity. Lyell popularized uniformitarianism by showing how it applied to the real world. He did this in his famous book Principles of Geology, the founding document of the geological sciences. William Smith demonstrated equivalency of strata by noting consistencies in their stratigraphic position relative to overlying and underlying layers, and by the unique assemblage of fossils they bore. He is best remembered for recognizing that strata were often characterized by particular fossils and that there was a general progression toward more modern-looking fossils in successively younger strata (faunal succession).

Question 21

Oldest to youngest eons:

Answer 21

Hadean, Archean, Proterozoic and Phanerozoic.

Question 22

Oldest to youngest eras:

Answer 22

Paleozoic, Mesozoic, Cenozoic

Question 23

Explain the difference between a geochronologic term and chronostratigraphic term:

Answer 23

A geochronologic unit is an abstract unit because it is a unit of time. The basic geochronologic unit is the period (Cambrian, Ordovician, and so on). Other geochronologic units include eons, eras, epochs, and ages. Consider the Devonian period, a 56-million-year-long interval that began 418 million years ago and ended 362 million years ago. Just like the year 1999, this interval of time has come and gone, never to be revisited. So how do we know anything about the Devonian period if it happened over 362 million years ago? We know about the Devonian because during that time, geological and biological processes, such as sedimentation, plate tectonics, and organic evolution, were operating and leaving a record of their operation in Earth’s rocky crust. The rock strata formed by these processes during this 56-million-year interval comprise the chronostratigraphic unit known as the Devonian System. Hence, a geochronological unit is an increment of pure time, whereas a chronostratigraphic unit is a body of rock (a tangible, nonabstract entity) deposited or otherwise formed during a particular time interval.

Question 24

Why is the concept of half-life necessary?

Answer 24

Half-life is the time required for half of a quantity of radioactive atoms to have decayed into daughter elements. Because only half of the remaining radioactive atoms decay during the following half-life, the decay process goes on indefinitely making “whole life” impossible to determine. The half-lives of uranium 238, potassium 40, and carbon 14 are 4,500 million, 1,300 million, and 5,700 years, respectively.

Question 25

What is meant by uniformitarianism? Cite an example of a process occurring on the Earth today that did not occur in the geologic past.

Answer 25

The essence of uniformitarianism is embodied in the phrase, “The present is the key to the past.” In other words, we can apply our knowledge of modern geological and biological processes (and their products) to the rock and fossil record. Under uniformitarianism, we can assume that 400-million-year-old basalt flows were the product of volcanic eruptions just like those basalts forming on Hawaii today. Without uniformitarianism we have no intellectual basis for studying past events.

Question 26

How do isotopes of a given element differ from one another in regard to number of protons and neutrons in the nucleus?

Answer 26

Each of the known elements is defined by the number of protons in the nucleus of the atom. For example, each atom in the universe that possesses two protons in the nucleus is, by definition, helium. All atoms in the universe with six protons are carbon. For a given element, the number of protons is in the nucleus is fixed. The number of neutrons may, however, vary within the nucleus of atoms. Isotopes are atoms of a given element that vary in the number of neutrons. For example, some chlorine atoms have eighteen neutrons, while others have twenty. As long there are seventeen protons, these atoms are still chlorine (table 1–2 on page 20 of your text).

Question 27

Explain the difference between a rock and a mineral:

Answer 27

A mineral is a naturally occurring, inorganic solid with a definite chemical composition and a crystalline structure formed by geological processes. A rock is an aggregate of one or more minerals whereas a rock may also include organic remains and mineraloids. Some rocks are predominantly composed of just one mineral.

Question 28

Understand the nature and importance of silicate minerals (the most abundant of the common rock-forming minerals):

Answer 28

About 75% by weight of the Earth’s crust is composed of the two elements oxygen and silicon. These elements usually occur in combinations with such abundant metals as aluminum, iron calcium, sodium, potassium, and magnesium to form a group of minerals called the silicates. A single family of silicates, the feldspars, comprises about one half of the material of the crust, and a single mineral species called quartz represents a sizable portion of the remainder.

Question 29

Quartz

Answer 29

Silicon dioxide- Hardness of 7- Will not cleave- Specific gravity: 2.65

Question 30

Feldspar

Answer 30

Nearly as hard as quartz- good cleavage in two directions

Question 31

Mica

Answer 31

Aluminoscilicates of potassium with water- Hardens of 2-3- cleaves perfectly in one direction- Specific gravity 2.8-3.0

Question 32

Hornblende

Answer 32

Most common member of a larger family of minerals called amphiboles- Iron and magnesium- Designated a ferromagnesian or mafic mineral

Question 33

Augite

Answer 33

Member of a family of minerals called pyroxene- ferromagnesian mineral- good cleavage developed along two planes that are nearly at right angles.

Question 34

Olivine

Answer 34

Silicates of magnesium and iron- Hardness of 6.5-7.0-specific gravity: 3.2-4.3

Question 35

Calcite

Answer 35

Calcite is the main constituent of limestone and marble, and it is secreted as skeletal material by certain invertebrate animals from seawater, or formed as dripstone in caverns.

Question 36

Dolomite

Answer 36

Dolomite is used for carbonate minerals and in it, calcium and magnesium occur in equal proportions

Question 37

Aragonite

Answer 37

Aragonite is the stuff that’s inside the shells of clams where pearls are, and is a carbonate material that occurs in a different form and more rarely than calcite or dolomite.

Question 38

Halite

Answer 38

Halite is considered common rock salt and gypsum and is easily recognized by its salty taste and it crystallizes and cleaves to form cubes.

Question 39

Describe the different origins of igneous, metamorphic, and sedimentary rocks in the context of the rock cycle.

Answer 39

Igneous rocks are those that have cooled from a molten state. Sedimentary rocks consist of materials formed from the weathered products of pre-existing rocks that have been transported, deposited and lithified. Metamorphic rocks are any that have been changed from previously existing rocks by the action of heat, pressure and associated chemical activity.

Question 40

Understand how igneous rocks are classified:

Answer 40

Igneous rocks are extensively covered by sedimentary rocks and mountains. They are those that have cooled from a molten state. The word “igneous” comes from the Latin word, ignis, which means “fire.”

Question 41

Clastics

Answer 41

Clasts of pieces of rock that range from microscopic particles to huge boulders. These include conglomerates, sandstones and shales. Clastics= the small space between pieces of sand. Think: Calcite, aragonite, and limestone.

Question 42

Carbonates

Answer 42

Biogenic, from organisms that live in the ocean who secrete carbonate. Dolomites or limestones are carbonate.. reefs, which are built from organisms. Carbonates fracture more easily than clastics and have pores in them.

Question 43

Understand how metamorphic rocks are classified:

Answer 43

Metamorphic rocks are classified by the presence or absence of foliation. It is considered rocks that have been altered by physical and chemical changes in the environment that has converted it. They can be foliated or nonfoliated.

Question 44

What are the eight most abundant elements found in rocks and minerals?

Answer 44

• Oxygen
• Silicon
• Aluminum
• Iron
• Calcium
• Sodium
• Potassium
• Magnesium

Question 45

What igneous rock best approximates the composition of the continental crust?

Answer 45

Granite

Question 46

What igneous rock best approximates the composition of the oceanic crust?

Answer 46

Basalt

Question 47

List the classic sedimentary rocks in order of increasingly finger grain size:

Answer 47

Conglomerate, sandstone, siltstone, shale and claystone.

Question 48

What mineral groups discussed in this chapter are particularly common in sedimentary rocks?

Answer 48

• Slate
• Phyllite
• Schist
• Gneiss

Question 49

Explain the difference between the three structural (tectonic) elements of the North American continent (shield, platform, and orogenic belt).

Answer 49

1. Canadian Shield (flat area of exposed Precambrian igneous and metamorphic rocks)
2. The Platform (interior of continent where Precambrian crystalline rocks are covered with a thin veneer of layered sedimentary rocks)
3. Orogenic Belts (elongated mountainous regions parallel to the east and west coasts).

Question 50

Discuss the three major depositional environments that exist on the earth now and that existed in the geological past.

Answer 50

1. Marine: forms around reefs on the shallow marine shelf or on the deep ocean floor.
2. Continental: deposited above seal level and away from the sea’s influence might be deposited in a lake, stream, alluvial fan, or desert.
3. Transitional: deltas, beaches, tidal flats, barrier islands, lagoons.
4. Epieric sea: These seas formed at times when much of the North American continent was flooded with shallow oceans. Since sea level ins relatively low at the present time, no broadly flooded areas are currently available for study.

Question 51

Outline the significance of color, texture, and sedimentary structures in determining the environment of deposition for any given sedimentary rock.

Answer 51

• Color: usually a reflection of sediment interacting with oxygen. The results of the oxygen on the color can help determine where the rocks can be find.
• Texture: determined by the size, sorting, and shape of the sedimentary particles that compromise the rock. Wentworth Scale determines the sedimentary grain size.
• Sedimentary structures: large, outcrop-scale features that develop during deposition of a sedimentary layer. The most common of these are layering (stratification), mud cracks (indicative of intermittently wet and dry conditions), raindrop imprints, ripple marks, cross-bedding (useful in determining direction and vigor of sediment transport), and graded bedding (typical of undersea avalanches called turbidity currents).
• Texture and sedimentary structures indicate conditions within the basin of deposit.

Question 52

Indicate the major differences between clastic sedimentary rocks (sandstone, shale, conglomerate) and carbonate sedimentary rocks (limestone).

Answer 52

• Clastic: From when weathered material is brought from a sediment source area to a basin of deposition by streams, glaciers or wind. The Clastic rock’s composition reflects the nature of the sediment source area, which may be located hundreds of miles from the final resting place of the sediment. They are classified by texture and composition.
• Limestone: comprised of biologically produced carbonate grains in the form of shells and carbonate muds that accumulate in clear, warm, shallow seawater. Unlike sandstone, those comprising carbonate rocks are produced in the basin of deposition in close proximity to their final resting place by organic activity.

Question 53

Understand the concept of lithostratigraphy (subdivision of the rock record into formations, members, and so on).

Answer 53

The basic unit of lithostratigraphy is the formation, a layer or rock body distinguished from overlying and underlying units on the basis of rock type

Question 54

Explain the concept of sedimentary facies.

Answer 54

Sedimentary facies refers to the features of a rock that indicate its environment of deposition.

Question 55

Summarize the origin and significance of sea-level changes in the sedimentary rock archive.

Answer 55

1. PRESENT SEA LEVEL IS LOWER THAN MOST OF PHANEROZOIC TIME
2. SEA LEVEL RISES AND FALLS CONTINUALLY
3. SEA LEVEL ARE MORE GRADUAL THAN SEA LEVEL DROPS
4. MAJOR TRANSGRESSIONS AND REGRESSIONS OCCUR OVER TENS OF MILLION OF YEARS

Question 56

Nonconformity

Answer 56

Surfaces where stratified rocks rest on older INTRUSIVE or metamorphic rocks. A nonconformity suggests that a period of long‐term uplift, weathering, and erosion occurred to expose the older, deeper rock at the surface before it was finally buried by the younger rocks above it. A nonconformity is the old erosional surface on the underlying rock.

Question 57

Angular unconformity:

Answer 57

The contact that separates a younger, gently dipping rock unit from older underlying rocks that are tilted or deformed layered rock. The contact is more obvious than a disconformity because the rock units are not parallel and at first appear cross‐cutting. Angular unconformities generally represent a longer time hiatus than do disconformities because the underlying rock had usually been metamorphosed, uplifted, and eroded before the upper rock unit was deposited.

Question 58

Disconformity:

Answer 58

Usually a result of EROSION but can occasionally represent periods of nondeposition. A gap in the fossil record indicates a gap in the depositional record, and the length of time the disconformity represents can be calculated.

Question 59

Think of the prevailing processes and the resulting sedimentary features.

Answer 59

A. A shallow marine setting is well-oxygenated and subject to wave and storm energy. The abundance of light would result in an active plant and animal community. If this seafloor were adjacent to a delta or other source of silicate sand and gravel, the resulting deposits would be light-colored (tan or greenish-gray), moderately well-sorted, cross-bedded sandstone with abundant trace fossils. If the seafloor was protected from influx of clay and sand, the dominant sediment source of would be the remains of plants and animals living on the seafloor. As a result, a light gray, fossilrich limestone would be deposited.
B. Deep marine rocks of the continental rise will reflect deposition by turbidity currents in an oxygen-starved setting. Graded bedding and dark coloration would be the two biggest clues to this setting.
C. Deltas are characterized by river and wave energy and an influx of sand, gravel, and mud. Swamps often form on the top of the delta plain. Interbedded sandstones, coals, and clays would result.
D. Wind is the dominant means of sediment transport in a desert. Desert sediments are readily distinguished by the high degree of sorting. Coarse sand and gravel cannot be moved, and silt and clay are blown into the atmosphere, leaving behind the fine – to medium-sized quartz particles. These grains are bounced along the ground through a process known as saltation. As the grains bounce, the corners are knocked off, leaving the grains rounded. The surface is scratched and pitted giving a frosted appearance under the microscope. Wind shapes the sand into large dunes with internal cross-beds that might be tens of feet high. Hence, sedimentary rocks deposited under desert conditions will be well-rounded, quartzose, well-sorted, light colored (tan to reddish-tan), and crossbedded.

Question 60

UNCONFORMITY

Answer 60

BREAKS IN THE STRATIGRAPHIC RECORD

Question 61

Why are sandstones and siltstones of desert environments rarely black or gray in color?

Answer 61

Black and dark gray coloration in sedimentary rocks results from the presence of compounds rich in organic carbon and iron. Hence, black sediments represent deposition in an environment where
A. organisms were abundant and
B. where oxygen levels are low enough to prevent destruction of the organic matter by oxidation or bacterial action.
• The desert environment lacks both the many organisms and the anoxic conditions necessary to preserve organic matter

Question 62

How does matrix in a rock differ from cement? What are the most common kinds of cements found in sandstones? Which of these is most durable?

Answer 62

Matrix is fine-grained material deposited with sand grains, pebbles, or boulders at the time of sediment deposition. Cement is a post-depositional component of a sedimentary rock. As groundwater percolates through tiny pores in the unconsolidated sediment, tiny crystals precipitate on the walls of the pores much like stalactites and stalagmites form on the walls of a cave. These crystals (cement) eventually fill up the porosity and glue the particles together. Common cements in sandstones are silica and calcite. Of these, the former is much more durable.

Question 63

What would be the probable origin of a poorly sorted sandstone composed of angular grains in a 30 percent matrix of mud?

Answer 63

When sediment is rapidly deposited without being selectively separated into sizes by currents.

Question 64

What difference in texture and composition serve to distinguish between a mature and an immature sandstone?

Answer 64

An immature sandstone is characterized by poor sorting, angular grains, a high percentage of clay and silt in the matrix, and a mineral composition that includes unstable minerals, such as feldspar and ferromagnesian minerals. A mature sandstone is comprised chiefly of quartz grains that are well sorted and well-rounded.
In a columnar section of sedimentary rocks, a limestone is overlain by a shale, which in turn in overlain by sandstone.

Question 65

What might this coarsening upward sequence indicate with regards to the advance or retreat of a shoreline?

Answer 65

The limestone represents deposition furthest from shore, the shale represents deposition closer to shore, and the sandstone represents deposition at the shoreline. The vertical succession reflects an upward transition from offshore limestone to nearshore sandstone; thus, the water was becoming increasingly shallow as the rock column was deposited. This would reflect the retreat of the shoreline towards the ocean (regression).

Question 66

What conditions in the Bahama Banks carbonate platform result in the high production of calcium carbonate sediment?

Answer 66

The main condition required for the development of a Bahama-like carbonate platform is clear, warm, shallow seawater. Carbonate production is so prolific in the Bahamas because there are no rivers dumping clay and sand into the region. As a result, the waters are quite clear. The top of the carbonate platform is covered with nutrient-rich water that is no deeper that thirty or forty feet, well within the photic zone. Since it is located at low latitudes, the surface water is warm. The warm, clear water promotes production of marine plants and algae, which in turn support a prolific community of carbonate secreting invertebrates that, upon death, donate their shells to the growing layer of seafloor sediment.

Question 67

What origin might you infer for a well-sorted sandstone that contains fossils of marine clams in composed almost entirely of quartz, and has well-developed ripple marks?

Answer 67

Tidal-zone

Question 68

Paleontology:

Answer 68

The science that seeks to understand life of the past and it’s relationship to the modern biosphere. The data base for this science consists of fossils.

Question 69

Fossils:

Answer 69

The remains or traces of past life preserved in sedimentary rock strata.

Question 70

Recrystallization:

Answer 70

Fossilization process where in the original crystals comprising a shell have been enlarged while retaining their original mineralogy (transition from small calcite crystals to larger calcite crystals); this process often results in obliterating the original internal structure of a shell (such as layering).

Question 71

Replacement:

Answer 71

Fossilization process wherein the original shell of skeletal substance is replaced by a different mineral (e.g., replacement of calcite by pyrite or silica).

Question 72

Carbonization:

Answer 72

Process of fossilization wherein original soft parts are flattened and reduced to a film of carbon.

Question 73

Per mineralization:

Answer 73

Process of infilling of pores within bones and shells with additional mineral material.

Question 74

Discuss the completeness of the fossil record.

Answer 74

A. Sedimentation not continuous
B. Preservation is rare
C. Calculated that under 1% of species are fossilized.
D. Plants and animals are not evenly distributed
E. Land plants/animals are less likely to have fossil remains
F. Geologic processes destroy
a. biological materials are delicate and susceptible to microscopic decomposers and macroscopic scavenger
b.Hard parts, unless rapidly buried become disarticulated and battered by waves and currents.

Question 75

Indicate the role of fossils in establishing the age equivalence of sedimentary strata.

Answer 75

Index Fossils=fossils that indicate specific or restricted periods of time
o	short-ranging species
o	abundant, 
o	geographically widespread, and 
o	environmentally robust

Question 76

Explain how fossils are used to reconstruct ancient ecosystems, climates, and geographies.

Answer 76

The study of fossils as members of ancient communities and ecosystems is called paleoecology. Paleoecologists assume that past biological communities had many of the same physical and biological attributes and requirements of their modern counterparts. The biosphere has been likened to a Shakespearean play wherein the roles and stage requirements remain fixed while different actors reprise the same roles just as different species have evolved to fill ecological niches through geologic time.

Question 77

What factors determine whether or not a particular fossil will be valuable as an indicator of the age and correlation of a stratum?

Answer 77

INDEX FOSSIL–those that existed for a short time(short geological range)

• -widespread geographical distribution
• -easily identified and abundant

Question 78

Fossil A occurs in rocks of Cambrian and Ordovician age. Fossil B occurs in rocks that range in age from early Ordovician through Permian. Fossil C is found in Mississippian through Permian strata. (a) What is the maximum possible range of age for a stratum containing only fossil B? (b) What is the maximum possible range for a stratum containing both A and B? (c) Which is the better guide fossil, A or C?

Answer 78

A: Silurian and Devonian
B: Ordovician
C: This is difficult to evaluate given the scanty data. A would be better guide fossil than C for Cambrian and Ordovician rocks and vice versa.

Question 79

A chronostratigraphic (time rock) unit contains a different fossil assemblage at one locality than at another located 200 n away. Suggest a possible cause for the dissimilarity.

Answer 79

Although that the two fossil assemblages are TIME equivalent(ie: chronstratigraphic), the difference must be because of ecological differences. For example, a shallow marine location would have different fossils than a deep water location two hundred kilometers.

Question 80

In drilling for oil, geologist recover Devonian conodonts in a stratum known to be Permian in age. Explain how this may have occurred.

Answer 80

Conodonts are made of the durable mineral-apatite. The conodont was eroded from a Devonian layer along w/quartz sand grains, then transported by wind and water, and deposited into a layer of sediment during the Permian. This is known as REWORKED fossil

Question 81

Using fossils for age correlation is dependent on a priori knowledge of their age ranges. How has this knowledge been obtained?

Answer 81

This knowledge was obtained through carefully observing fossil occurrences in rock layers during the last 200 years. For example, dinosaur bones have been found in Mesozoic rocks, but to date, no dinosaurs have been found in Paleozoic rocks. Similarly conodonts have been found in late-Cambrian, Ordovician, Silurian, Devo. Miss. Penn, Perm, and Triassic strata. They have not been found in Jurassic or younger formations, in spite of efforts to find them there.