Chapter 9

Front 1

· Sketch and explain each of the five principlesof relative dating, providing an example of each principle.

Back 1

The first principle is that most sediments are deposited in horizontal layers. This principal is called original horizontality and it's likely that if they are no longer horizontal, some event must have affected the layers after they formed. Some exceptions are sand dunes or the undersea slopes of a delta.

The second principle is that the younger sedimentary or volcanic unit is deposited on top of older units. This is the principle of superposition. The exceptions to this can be tectonic forces overturning layers, and placing the older rocs on top.

In principle 3, a younger sediment or rock can contain pieces of an older rock. This clearly demonstrates the relative age as it's impossible for a younger rock to be contained in an older rock.

In principle 4, a younger rock or feature can cut across any older rock or feature. This is called cross-cutting relations and occurs by fractures (joints and faults).

Lastly, in principle five, younger rocks and features can cause changes along their contacts with older rocks. Magma comes into contact with preexisting rocks when it erupts onto the surface or solidifies at depth. In either situation, the magma may locally bake the adjacent rocks, or fluids from the magma may chemically alter nearby rocks. These changes, called contact effects, indicate that the magma is younger than the rocks that were altered.

Front 2

· Apply the principles of relative dating to aphotograph or sketch showing geologic relations among several rock units, oramong rock units and structures.

Back 2

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Front 3

· Describe the sequence of events represented in atypical landscape of flat-lying sedimentary rocks.

Back 3

First, the sequence begins with deposition of a new sedimentary unit on top of preexisting metamorphic and igneous rocks. Most sediments, such as a layer of sand, are deposited as nearly horizontal layers. Through time, the depositional environment changes and a series of different sedimentary layers accumulate, with each younger layer being deposited on top. Over time, the layers are lithified. At some point, deposition stops, and all the layers that will be deposited are there. Weathering and erosion can begin. If the region is uplifted or the seas withdraw, the area can begin to be eroded by rivers, streams, glaciers, and the wind. Erosion can more or less uniformly strip the entire land surface, removing the top layers. More likely, erosion will be faster in some areas, like along a river cutting downward in a small canyon. Erosion by a river cuts downward, carving a deeper canyon. The canyon widens as small drainages erode outward from the main river and as the steep canyon walls move downhill in landslides and slower movements. The combination of downcutting, widening, and development of subsidiary drainages, called tributaries, sculps a deeper, wider, and more intricate canyon.

Front 4

· Describe or sketch how you could assess the ageof a landscape surface.

Back 4

The age of a landscape must be younger than any rocks on which it is carved. A landscape surace is older than any rock that is deposited on top of the surface. A lava flow is ideal for dating a surface because it formed during a short time and its age can usually be determined by isotopic dating methods. Sometimes the age of a landscape surface cannot be directly dated, but we can infer its age relative to other features. Many rivers are flanked by raised, gentle surfaces called terraces. A terrace was formed sometime in the past before the river eroded down to its present level. The terrace is older than the modern channel. A landscape surface progressively develops more soil if it remains undisturbed by erosions and deposition. A surface with well-developed soil, such as the uplands shown with thick red clay and white carbonate accumulations must be thousands of years old. Recent sediment along the stream has no soil.

In many climates, especially deserts, rock surfaces develop a dark coating if left undisturbed for hundreds to thousands of years. This coating, called rock varnish or desert varnish, consists of iron-oxide and manganese-oxide materials, which are mostly derived from windblown dust. Rock varnish becomes darker the longer a rock is exposed at the surface, with very dark varnish requiring thousands of years.

In some settings, stones become concentrated on the surface through time, forming a feature called desert pavement. Over time, finer materials wash away, blow away, or move down into the soil, while pebbles and larger clasts remain on the surface or move up from just below the surface. If left undisturbed, the pavement becomes better developed over time, and exposed stones get coated with desert vanish.

Stones on the surface progressively accumulate telltale amounts of certain chemical elements produced when cosmic rays strike the stones. A form of isotopic dating is used to determine how long the stones have been on the surface. Geologists collect samples of the stones and analyze them in the lab.

Front 5

· Sketch an angular unconformity, a nonconformity,and a disconformity, and describe what sequence of events is implied by each.

Back 5

An angular unconformity implies first an unconformity, buried erosion surfaces that represent large intervals of time missing from a rock sequence. Unconformities with underlying rocks that are tilted before formation of the erosion surface are called angular unconformity.

The formation of a nonconformity begins when a granite, or other nonlayered rock, is formed at depth and uplifted. Material that was once on top is eroded away, exposing the granite to weathering and erosion at the surface Weathering can form soils and other weathering products. Subsequently, conditions change, and the erosion surface is buried by sand and cobbles, perhaps derived in part from weathering of the granite. Ultimately, the sediment lithifies into sandstone and conglomerate. The contact between the granite and overlying sedimentary rock is a nonconformity.

The first step in the development of a disconformity is deposition of horizontal layers producing sedimentary rock. Next, the rock is exposed at the surface because he region is uplifted or because sea level drops. Sedimentation stops, and weathering and erosion affect the now-exposed land surface. After some time, sedimentation resumes, and the surface of older rock is eventually buried by a younger layer of sediment, forming a disconformity. This new layer can be deposited by water or can be deposited on land, perhaps as sand dunes in the desert.

Front 6

· Explain how to determine how many half-liveshave passed based on the ratio of parent to daughter atoms.

Back 6

All atoms of any given element must have the same number of protons, but some differ in the number of neutrons they contain. Thus, different varieties of the same element may have different atomic weights; these varieties of the same element are called isotopes. Some isotopes are unstable through time, changing into a new element or isotope by the process of radioactive decay.

The starting atons are called the parent atoms or parent isotopes. Over time some of the parent isotope will decay into a different element called the daughter product or daughter atom. At a later time, half of the parent atoms will have decayed into the daughter product. The amount of time it takes for this to occur is called the half-life. After one half-life, there are an equal number of parent and daughter atoms. After a time equal to another half-life has passed, half of the remaining parent atoms have decayed into daughter atoms. That is, after two half-lives, 3/4 of the parent atoms have decayed and 1/4 remains.

Front 7

· Describe the different ways that isotopic datingis used for dating geologic events.

Back 7

Isotopic dating can be used to date volcanic units by using K-Ar. Volcanic rocks form on the surface and cool rapidly, so an age of the rock is typically the age of eruption.

Hot plutons lose certain isotopes, so we determine the age of such bodies using only those minerals that retain isotopes and provide the age of crystallization. Today, we mostly use U-Pb dating of the mineral zircon.

Some minerals, such as biotite mica in plutons, tell us when a rock cooled through a specific temperature, as when it was being uplifted to the surface.

Black pieces of charcoal incorporated into recent sediment can be dated with carbon-14, which provides an age for deposition of the sediment.

Dates from individual boulders, cobbles or even sand-sized grains in a sedimentary rock help us to infer the age of the source rocks from which the sediment was eroded. The oldest age ever measured, more than 4.3 billion years old, are for individual grains in sedimentary rocks from Australia.

We investigate the age of a metamorphic event, like baking next to the pluton using minerals that formed during metamorphism or minerals that record certain metamorphic temperatures.

Front 8

· Describe the different ways in which a plant oranimal can be preserved as a fossil.

Back 8

A fossil can be preserved hard parts, or parts that have been replaced by hard minerals of marine organisms, like shellfish and coral. Vertebrate animals have hard parts, most significantly bones, that can be preserved. Most bones are found as fragments instead of complete skeletons because of the destruction and disperal caused by scavengers. Some fossils are preserved because the original organic material is replaced by silica, pyrite, or some other mineral. One example is wood from trees that is replaced by fine grained silica, forming petrified wood. Another type of fossil forms when an animal is buried and decays. This leaves a cavity in the rock that mimics the animals shape. The cavity is a mold if unfilled, and is a cast if it is later filled by materials. After burial, some carbon-rich plants and animals become thin films of carbon or other materials that preserve the original shape of the plant or animal. FIsh and other soft creatures can be preserved as impressions, especially when the remains come to rest in quiet waters of a lake or deep sea. Such fossils can preserve amazing details, including fins and scales. Animals and plants can become fossils in other ways, such as becoming trapped in tree sap, which through time hardens, into golden-brown amber. Such preservation can preserve fragile features of the animal, like wings, legs, and antennae on these insects. Some fossils do not preserve the actual organism, but instead represent something that the organism constructed.

Front 9

· Describe the two main factors that influencewhether a creature is preserved as a fossil.

Back 9

Preservation as a fossil is much more likely if a creature has a shell, bones, teeth, or some other hard part. Only 30% of modern animals have hard parts, but such animals are over-represented in the fossil record compared to animals like insects or jellyfish that lack hard parts. Soft parts of creatures can be eaten by scavengers, crushed during sediment compacting, dissolved by chemical reactions, or otherwise destroyed. Some ancient creatures with only soft parts may nowhere have been preserved.

A fossil cannot be preserved unless it is buried. If a creature's remains are left on the surface, whether on land or in the sea, they can decompose due to exposure to the atmosphere and water, or can be scavenged by other creatures. Rapid burial means less opportunity for destruction. Preservation is easier beneath the sea than on land b more rapid, and because burial is generally more rapid, and because a lower content of oxygen in the deep sea slows decay.

Front 10

· Describe the four chapters of Earth history andhow the boundaries are defined.

Back 10

Early geologists recognized that fossils change upward from older layers of sedimentary rock to younger layers. This systematic change of fossils with age, called faunal succession, helped geologists identify time periods defined by major changes in life on Earth. Using the principles of relative dating and faunal succession, geologists subdivided geologic time into four major chapters, each with subdivisions. Later, results from isotopic dating provided numeric ages, in millions of years before present, for when each chapter started and ended.

The Cenozoic Era, meaning recent life, spans the last 65 million years. It is called the age of mammals because mammals, such as mammoths, became a dominant type of life on Earth.

The Mesozoic Era (middle life) is known as the age of the dinosaurs, because dinosaurs rose to dominance during this era. The end of the Mesozoic Era, at 65 Ma, is marked by the extinction of dinosaurs.

The Paleozoic Era (ancient life) was dominated by several major groups of marine animals, including coral, creatures like clams that had ahrd shells and various types of fish. Plants, insects, and amphibians also colonized the land during this era. The end of the Paleozoic Era is marked by a major time of extinction called the Great Dying. This extinction killed off many species of animals in the seas and on land.

The Precambrian (before the Cambrian period) comprises nearly 90% of geologic time. For most of this time, only simple life forms existed, such as bacteria and algae that formed stromatolites.

Front 11

· Describe how fossils can change through a section of rocks. Provide examples of using index fossils, abrupt boundaries between fossils, and fossil overlaps to precisely infer an age of a rock layer.

Back 11

A fossil can change through a section of rocks as different varieties of fossils are found. Index fossils are really useful for dating because they're widespread over a very specific period of time, allowing a specific date. Abrupt boundaries are also useful for time markers. Fossil overlaps are useful because they generally only overlap for a narrow range, allowing you to narrow down the age inference.

Front 12

· Describe or sketch the ways we use fossils and rock types to correlate two rock sequences.

Back 12

Fossils and rocks are used when comparing two rock sequences together to see how similar the sequences are in order to fill unconformity's.

Front 13

· Briefly summarize how the geologic timescale was developed.

Back 13

After it was established that fossil assemblages change upward through sections of rock, geologists recognized that two different sites that have matching fossil assemblages were the same age. They recognized sequences of related layers across Europe and in North America and named different geologic time periods after places where rocks of that age are well exposed.Reynolds, Stephen; Julia Johnson; Paul Morin; Chuck Carter (2015-01-09). Exploring Geology (Page 252). McGraw-Hill Higher Education. Kindle Edition.

Front 14

· Explain or sketch how numeric ages are assigned to the timescale and how the timescale is used to assign numeric ages to fossil-bearing rocks.

Back 14

Geologists have assigned numeric ages to geologic periods and their subdivisions by studying localities where isotopically dated igneous rocks have a clear relationship to fossil-bearing layers.

Once the age of the periods and shorter units of geologic time were constrained, these ages could be used to estimate numeric ages of fossil-bearing units that lack datable igneous rocks.

Front 15

· Describe evidence that suggests Earth has a long history, including isotopic ages on basement rocks in North America.

Back 15

Tree rings, which record annual growth cycles, have textures that vary between seasons and widths that vary from dry years to wet ones. Thickness patterns in bristlecone pine rings from the American West can be correlated from living trees to dead ones to form a continuous record back to 9,000 years ago. This approach is independent of, but strongly supported by, many carbon-14 ages.

Ice cores, cylinders of ice sheets in Greenland and Antarctica, contain thin layers produced by yearly cycles of the seasons. Some ice-core records are thousands of meters long, representing tens of thousands of years to over 100,000 years. Determining the age is done by counting the rings, along with determining isotopic ages for layers of volcanic ash within the ice.

Varves are alternating light and dark sediment layers that form in lakes because of seasonal variations of sedimentation and biologic activity. Lighter layers represent increased accumulations of sand and silt during the summer, whereas darker layers record the slower deposition of mud during the winter. There are more than 4,800 varves in glacial lakes that existed in New England around 10,000 to 14,000 years ago, and the Green River Formation in Wyoming has millions of varves that represent at least several million years.

Current rates of plate motion can be measured precisely to be on the order of centimeters per year. At these rates, it would have taken the Atlantic Ocean more than 100 million years to form as Africa and the Americas moved apart. Similar rates of motion explain the ages of the Hawaiian Islands and displacement on the San Andreas fault.

Front 16

· Describe how meteorites and Moon rocks are used to interpret the age of Earth.

Back 16

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Front 17

Describe or sketch the principles by which two sequences of rocks can be correlated.

Back 17

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Front 18

· Describe or sketch why layers can change from one sequence to another.

Back 18

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Front 19

· Reconstruct the sequence of events from a cross section or block diagram.

Back 19

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