Unit 3: The Changing Earth

Lesson 2: Paleontology, the scientific study of ancient life, uses rocks & fossils as the primary source of data.

Introduction: In Lesson 2, you will examine the rock cycle, fossils and geological time lines

Objective 1.21: The Rock Cycle (Textbook: p. 53-76)

WHAT IS THE DIFFERENCE BETWEEN MINERALS & ROCKS? A mineral is a pure natural substance, while a rock is a natural substance that contains one or more minerals held together.

WHAT ARE THE MAIN TYPES OF ROCKS?

Types of RocksType Characteristics Examples

Igneous rocks

Igneous rocks are formed when magma from Earth's core rises, cools, and solidifies. Magma that moves out of the crust and cools quickly is called extrusive igneous rock (pumice). These igneous rock have small crystals and is susceptible to erosion and weathering. Magma that never reaches the surface and cools slowly is called intrusive igneous rock (obsidian). These igneous rock have large crystals.

Composition of Igneous Rocks Igneous rocks are divided into three compositional groups: 1) felsic - composed mostly of the minerals quartz and potassium feldspar which are generally white to pink in color. 2) intermediate - composed mostly of the minerals amphibole and sodium-rich plagioclase feldspar, with some quartz and pyroxene possible. Intermediate igneous rocks are usually light to dark gray in color. 3) mafic - composed mostly of the minerals pyroxene, calcium-rich plagioclase feldspar and olivine. Mafic rocks are usually black to dark green

in color.

1) Andesite

2) Granite

3) Basalt

Obsidian-intrusive

Pumice-extrusive

OTHERS:Diorite

Rhyolite

Sedimentary rocks

Sedimentary rocks are made up of sediments eroded from igneous, metamorphic, other sedimentary rocks, and even the remains of dead plants and animals. These materials are deposited in layers, or strata, and then are squeezed and compressed into rock (called lithification). Most fossils are found in sedimentary rocks.

Sandstone

Shale

Conglomerate

Limestone

Chert

Coal

Gypsum

Metamorphic rocks

Metamorphic rocks are produced when sedimentary or igneous rocks are transformed by heat and/or pressure. The word "metamorphic" comes from the Greek language, which means "to change form."

Marble

Slate

Quartzite

Schist

Gneiss

WHAT DOES THE ROCK CYCLE LOOK LIKE? (See Plate 2.2)

Objective 2: Fossils ( pg 41-44) HOW DO SCIENTISTS INTEPRET ROCK LAYERS?

The study of rock layers or strata is called stratigraphy. Scientists use five principles to help them interpret rock strata.

1) The Principle of Uniformitarianism: What is happening now, happened in the past.

2) The Principle of original horizontality: A new layer is placed horizontally over an older layer. Folds or tilts indicate a geological event.

3) The Principle of Superposition: The layers become younger as you move to the top.

4) The Principle of Cross-cutting: Disruptions (like intrusions or faults) are younger than the layers they disrupt or cut through.

5) The Principle of Faunal succession: Layers of rock with similar fossils or rocks are the same relative ages – the age in terms of older or younger than. Well known fossils that lived a short time are good for age of rock and are called index fossils.

See what you can interpret from the strata on the next page, using these five principles.

WHAT DO FOSSILS TELL SCIENTISTS?

To tell the age of most layered rocks, scientists study the fossils these rocks contain. Fossils provide important evidence to help determine what happened in Earth history and when it happened. Paleontologist (Palaios = ancient) are scientists who study living things of the past.

There are two processes that form fossils. 1) Petrification occurs when organic remains turn to stone. Petrified wood & bones are examples of petrification, where the wood & bones are completely replaced by agate rock. 2) Carbonization occurs when pressure causes a thin carbon film to form around the organism. Preservation of soft bodied organisms like jellyfish is an example of carbonization.

Paleontologist classify fossils into four types: 1) Remains of a life form (dinosaur bones); 2) imprint or mould (leaves); 3) casts (cavity is filled with sediment); 4) Traces (tacks or burrows). Fossils are found in limestone, shale or sandstone.

Fossil fuels like coal, oil and natural gas came from living photosynthesizing swamp plants that got buried millions of years ago. Over time, heat and pressure transformed these plants into fossil fuels. Natural gas & oil (less dense than water) moved through the permeable limestone, until they reached dense, non-porous rock where the gas and oil was trapped. Oil companies use this information to help them determine where gas and oil might be located.

Three concepts are important in the study and use of fossils: (1) Fossils represent the remains of once-living

organisms. (2) Most fossils are the remains of extinct organisms; that is, they belong to species that are no longer living anywhere on Earth. (3) The kinds of fossils found in rocks of different ages differ because life on Earth has changed through time.

Objective 3: Geological Time ( pg 41-44)

Geological time of earth is separated into three units:

ERAS: Large units of time determined by major global environmental or geological changes. There are four eras – Precambrian, Paleozoic, Mesozoic & Cenozoic.

PERIODS: are units of time within each era symbolized by regional changes.

EPOCHS: are the small units of time within the Cenozoic Era which are based upon the rise and fall of sea levels.

The Proterozoic and Archean ages are also known as the Precambrian era.

What's in your backyard?

Go to the following website and read the article. When you are finished, take time to browse the rest of the Royal Tyrell Museum of Paleontology website.

http://www.tyrrellmuseum.com/events/index2.php?strSection=3

The Four Eras

The Cenozoic Era (65 Million Years to the Present)

The Cenozoic is the most recent of the three major subdivisions of animal history. The other two are the Paleozoic and Mesozoic. The Cenozoic spans only about 65 million years, from the end of the Cretaceous and the extinction of non-avian dinosaurs to the present. The Cenozoic is sometimes called the Age of Mammals, because the largest land animals have been mammals during that time. This is a misnomer for several reasons. First, the history of mammals began long before the Cenozoic began. Second, the diversity

of life during the Cenozoic is far wider than mammals. The Cenozoic could have been called the "Age of Flowering Plants" or the "Age of Insects" or the "Age of Teleost Fish" or the "Age of Birds" just as accurately.

The Cenozoic is divided into two main sub-divisions: the Tertiary and the Quaternary. Most of the Cenozoic is the Tertiary, from 65 million years ago to 1.8 million years ago. The Quaternary includes only the last 1.8 million years.

The Mesozoic Era

The Mesozoic is divided into three time periods: the Triassic (245-208 Million Years Ago), the Jurassic (208-146 Million Years Ago), and the Cretaceous (146-65 Million Years Ago).

Mesozoic means "middle animals", and is the time during which the world fauna changed drastically from that which had been seen in the Paleozoic. Dinosaurs, which are perhaps the most popular organisms of the Mesozoic, evolved in the Triassic, but were not very diverse until the Jurassic. Except for birds, dinosaurs became extinct at the end of the Cretaceous. Some of the last dinosaurs to have lived are found in the late Cretaceous deposits of Montana in the United States.

The Mesozoic was also a time of great change in the terrestrial vegetation. The early Mesozoic was dominated by ferns, cycads, ginkgophytes, bennettitaleans, and other unusual plants. Modern gymnosperms, such as conifers, first appeared in their current recognizable forms in the early Triassic. By the middle of the Cretaceous, the earliest angiosperms had appeared and began to diversify, largely taking over from the other plant groups.

The Paleozoic Era

The Paleozoic is bracketed by two of the most important events in the history of animal life. At its beginning, multicelled animals underwent a dramatic "explosion" in diversity, and almost all living animal phyla appeared within a few millions of years. At the other end of the Paleozoic, the largest mass extinction in history wiped out approximately 90% of all marine animal species. The causes of both these events are still not fully understood and the subject of much research and controversy. Roughly halfway in between, animals, fungi, and plants alike colonized the land, the insects took to the air, and the limestone shown in this picture was deposited near Burlington, Missouri.

The Paleozoic took up approximately 300 million years. During the Paleozoic there were six major continental land masses; each of these consisted of different parts of the modern continents. For instance, at the beginning of the Paleozoic, today's western coast of North America ran east-west along the equator, while Africa was at the South Pole. These Paleozoic continents experienced tremendous mountain building along their margins, and numerous incursions and retreats of shallow seas across their interiors. Large limestone outcrops, like the one shown above, are evidence of these periodic incursions of continental seas.

Many Paleozoic rocks are economically important. For example, much of the limestone quarried for building and industrial purposes, as well as the coal deposits of western Europe and the eastern United States, were formed during the Paleozoic.

The Precambrian Era

4.5 billion years ago, the Earth was born. Comprehending that vastness in time is no easy task. John McPhee, in his book Basin and Range, recounts a nice illustration of what this sort of time means. Stand with your arms held out to each side and let the extent of the earth's history be represented by the distance from the tips of your fingers on your left hand to the tips of the fingers on your right. Now, if someone were to run a file across the fingernail of your right middle finger, then the time that humans have been on the earth would be erased.

Nearly 4 thousand million years passed after the Earth's inception before the first animals left their traces. This stretch of time is called the Precambrian. To speak of "the Precambrian" as a single unified time period is misleading, for it makes up roughly seven-eighths of the Earth's history. During the Precambrian, the most important events in biological history took place. Consider that the Earth formed, life arose, the first tectonic plates arose and began to move, eukaryotic cells evolved, the atmosphere became enriched in oxygen -- and just before the end of the Precambrian, complex multicellular organisms, including the first animals, evolved.

Objective 4: Radioactive Decay and Half Life (Textbook: p. 44-52)

WHAT IS RADIOACTIVE DATING?

Scientists use several different methods of dating fossils.  One of these is radiometric dating. This is also called radioactive dating. Naturally-occurring radioactive materials break down into other materials at known rates. This is known as radioactive decay. Radioactive parent elements decay to stable daughter elements. Radioactive dating using radioactive isotopes is used to determine absolute age – the exact age -- of rocks and fossils.

Radioactivity was discovered in 1896 by Henri Becquerel. In 1905, Rutherford and Boltwood used the principle of radioactive decay to measure the age of rocks and minerals (using Uranium decaying to produce Helium). In 1907, Boltwood dated a sample of urnanite based on uranium/lead ratios. Amazingly, this was all done before isotopes were known, and before the decay rates were known accurately.

The invention of the MASS SPECTROMETER after World War I (post-1918) led to the discovery of more than 200 isotopes.

Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. Once this rate is known, geologists can estimate the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements.

Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product.

For example:

Sodium 24 (Na 24) has a half life of 15 hours. The following table shows the decay of a 100 gram sample.

grams of Na 24 Hours Passed100 050 1525 30

12.5 456.25 60

3.125 75

Half Lives for Radioactive ElementsRadioactive Parent Stable Daughter Half life

Potassium 40 Argon 40 1.25 billion yrsRubidium 87 Strontium 87 48.8 billion yrsThorium 232 Lead 208 14 billion yearsUranium 235 Lead 207 704 million yearsUranium 238 Lead 206 4.47 billion yearsCarbon 14 Nitrogen 14 5730 years

How does Carbon-14 dating work?

1. Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide.

2. Living things are in equilibrium with the atmosphere, and the radioactive carbon dioxide is absorbed and used by plants. The radioactive carbon dioxide gets into the food chain and the carbon cycle.

3. All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion).

4. At death, Carbon 14 exchange ceases and any Carbon 14 in the tissues of the organism begins to decay to Nitrogen 14, and is not replenished by new C-14.

5. The change in the Carbon 14 to Carbon 12 ratio is the basis for dating.

6. The half-life is so short (5730 years) that this method can only be used on materials less than 70,000 years old. Archaeological dating uses this method.) Also useful for dating the Pleistocene Epoch (Ice Ages).

7. Assumes that the rate of Carbon 14 production (and hence the amount of cosmic rays striking the Earth) has been constant (through the past 70,000 years).

Looking at this information, why would you not be able to determine the age of a dinosaur bone using Carbon dating?