(89).Early Earth

Vocabulary

differentiation

outgassing

paleontologist

Introduction

Historical geologists study Earth’s past to understand what happened and when it happened. Paleontologists do the same thing, but with an emphasis on the history of life, especially as it is understood from fossils. Despite having very little material from those days, scientists have many ways of learning about the early Earth.

Formation of Earth

Earth came together (accreted) from the cloud of dust and gas known as the solar nebula nearly 4.6 billion years ago, the same time the Sun and the rest of the solar system formed. Gravity caused small bodies of rock and metal orbiting the proto-Sun to smash together to create larger bodies. Over time, the planetoids got larger and larger until they became planets. More information about planet formation is in the chapter about the solar system.

There is little hard evidence for scientists to study from Earth’s earliest days. Much of what scientists know about the early Earth come from three sources: (1) zircon crystals, the oldest materials found on Earth, which show that the age of the earliest crust formed at least 4.4 billion years ago; (2) meteorites that date from the beginning of the solar system, to nearly 4.6 billion years ago (Figurebelow); and (3) lunar rocks, which represent the early days of the Earth-Moon system as far back as 4.5 billion years ago.

[Figure 1]

The Allende Meteorite is a carbonaceous chondrite that struck Earth in 1969. The calcium-aluminum-rich inclusions are fragments of the earliest solar system.

Molten Earth

When Earth first came together it was really hot, hot enough to melt the metal elements that it contained. Why was the early Earth so hot?

Gravitational contraction: As small bodies of rock and metal accreted, the planet grew larger and more massive. Gravity within such an enormous body squeezes the material in its interior so hard that the pressure swells. As Earth’s internal pressure grew, its temperature also rose.

Radioactive decay: Radioactive decay releases heat, and early in the planet’s history there were many radioactive elements with short half lives. These elements long ago decayed into stable materials, but they were responsible for the release of enormous amounts of heat in the beginning.

Bombardment: Ancient impact craters found on the Moon and inner planets indicate that asteroid impacts were common in the early solar system. Earth was struck so much in its first 500 million years that the heat was intense. Very few large objects have struck the planet in the past many hundreds of millions of year.

Differentiation

When Earth was entirely molten, gravity drew denser elements to the center and lighter elements rose to the surface. The separation of Earth into layers based on density is known as differentiation. The densest material moved to the center to create the planet’s dense metallic core. Materials that are intermediate in density became part of the mantle (Figurebelow).

[Figure 2]

Earth’s interior: Inner core, outer core, mantle, and crust.

Lighter materials accumulated at the surface of the mantle to become the earliest crust. The first crust was probably basaltic, like the oceanic crust is today. Intense heat from the early core drove rapid and vigorous mantle convection so that crust quickly recycled into the mantle. The recycling of basaltic crust was so effective that no remnants of it are found today.

How the Moon Formed

One of the most unique features of planet Earth is its large Moon. Unlike the only other natural satellites orbiting an inner planet, those of Mars, the Moon is not a captured asteroid. Understanding the Moon’s birth and early history reveals a great deal about Earth’s early days.

To determine how the Moon formed, scientists had to account for several lines of evidence:

The Moon is large; not much smaller than the smallest planet, Mercury.

Earth and Moon are very similar in composition.

Moon’s surface is 4.5 billion years old, about the same as the age of the solar system.

For a body its size and distance from the Sun, the Moon has very little core; Earth has a fairly large core.

The oxygen isotope ratios of Earth and Moon indicate that they originated in the same part of the solar system.

Earth has a faster spin than it should have for a planet of its size and distance from the Sun.

Can you devise a “birth story” for the Moon that takes all of these bits of data into account?

Astronomers have carried out computer simulations that are consistent with these facts and have detailed a birth story for the Moon. A little more than 4.5 billion years ago, roughly 70 million years after Earth formed, planetary bodies were being pummeled by asteroids and planetoids of all kinds. Earth was struck by a Mars-sized asteroid (Figure below).

[Figure 3]

An artist’s depiction of the impact that produced the Moon.

The tremendous energy from the impact melted both bodies. The molten material mixed up. The dense metals remained on Earth but some of the molten, rocky material was flung into an orbit around Earth. It eventually accreted into a single body, the Moon. Since both planetary bodies were molten, material could differentiate out of the magma ocean into core, mantle, and crust as they cooled. Earth’s fast spin is from energy imparted to it by the impact.

[Figure 4]

The Genesis Rock was brought back to Earth by the Apollo 15 astronauts.

Lunar rocks reveal an enormous amount about Earth’s early days. The Genesis Rock, with a date of 4.5 billion years, is only about 100 million years younger than the solar system (Figure above). The rock is a piece of the Moon’s anorthosite crust, which was the original crust. Why do you think Moon rocks contain information that is not available from Earth’s own materials?

Can you find how all of the evidence presented in the bullet points above is present in the Moon’s birth story?

Earth's Early Atmosphere and Oceans

At first, Earth did not have an atmosphere or free water since the planet was too hot for gases and water to collect. The atmosphere and oceans that we see today evolved over time.

Earth’s First Atmosphere

Earth’s first atmosphere was made of hydrogen and helium, the gases that were common in this region of the solar system as it was forming. Most of these gases were drawn into the center of the solar nebula to form the Sun. When Earth was new and very small, the solar wind blew off atmospheric gases that collected. If gases did collect, they were vaporized by impacts, especially from the impact that brought about the formation of the Moon.

Eventually things started to settle down and gases began to collect. High heat in Earth’s early days meant that there were constant volcanic eruptions, which released gases from the mantle into the atmosphere (Figure below). Just as today, volcanic outgassing was a source of water vapor, carbon dioxide, small amounts of nitrogen, and other gases.

[Figure 5]

Nearly constant volcanic eruptions supplied gases for Earth’s early atmosphere.

Scientists have calculated that the amount of gas that collected to form the early atmosphere could not have come entirely from volcanic eruptions. Frequent impacts by asteroids and comets brought in gases and ices, including water, carbon dioxide, methane, ammonia, nitrogen, and other volatiles from elsewhere in the solar system (Figure below).

[Figure 6]

The gases that create a comet’s tail can become part of the atmosphere of a planet.

Calculations also show that asteroids and comets cannot be responsible for all of the gases of the early atmosphere, so both impacts and outgassing were needed.

Earth’s Second Atmosphere

The second atmosphere, which was the first to stay with the planet, formed from volcanic outgassing and comet ices. This atmosphere had lots of water vapor, carbon dioxide, nitrogen, and methane but almost no oxygen. Why was there so little oxygen? Plants produce oxygen when they photosynthesize but life had not yet begun or had not yet developed photosynthesis. In the early atmosphere, oxygen only appeared when sunlight split water molecules into hydrogen and oxygen and the oxygen accumulated in the atmosphere.

Without oxygen, life was restricted to tiny simple organisms. Why is oxygen essential for most life on Earth?

1. Oxygen is needed to make ozone, a molecule made of three oxygen ions, O₃. Ozone collects in the atmospheric ozone layer and blocks harmful ultraviolet radiation from the Sun. Without an ozone layer, life in the early Earth was almost impossible.

2. Animals need oxygen to breathe. No animals would have been able to breathe in Earth’s early atmosphere.

Early Oceans

The early atmosphere was rich in water vapor from volcanic eruptions and comets. When Earth was cool enough, water vapor condensed and rain began to fall. The water cycle began. Over millions of years enough precipitation collected that the first oceans could have formed as early as 4.2 to 4.4 billion years ago. Dissolved minerals carried by stream runoff made the early oceans salty. What geological evidence could there be for the presence of an early ocean? Marine sedimentary rocks can be dated back about 4 billion years. By the Archean, the planet was covered with oceans and the atmosphere was full of water vapor, carbon dioxide, nitrogen, and smaller amounts of other gases.

Lesson Summary

Earth and the other planets in the solar system formed about 4.6 billion years ago.

The early Earth was frequently hit with asteroids and comets. There were also frequent volcanic eruptions. Both were sources of water and gases for the atmosphere

The early Earth had no ozone layer, no free oxygen, and was very hot.

The oceans originally formed as water vapor released by volcanic outgassing and comet impacts cooled and condensed.

Earth was struck by a giant impactor, which flung material out into orbit around the planet. This material accreted into Earth’s only natural satellite, the Moon.

(88).Earth's history.

Earth looks very different today than it did when it first formed more than 4.5 billion years ago. Rare parts of the planet may retain a bit of the feel of the ancient environment, such as the Grand Prismatic Spring in Yellowstone National Park. Earth’s internal heat creates hot springs that are home to extremophiles, organisms that thrive in extreme environments. The orange, spongy materials in this photo are mats of thermophilic bacteria, organisms that thrive in extremely hot environments. Since the Earth’s environment was undoubtedly more extreme in the early days, it seems likely that the most ancient life forms were forms of extremophiles.

Life on Earth has changed tremendously since those early days. Creatures have become multicellular; they have gained the ability to make their own food energy by photosynthesis; they have adapted to living in water, on land and in the air; and they've even evolved intelligence. The geology of the planet has also changed. The Earth's crust has hardened, mountains have risen, oceans have grown, and erosion has reduced features to flat plains. All of this has happened over an extremely long period of time, and humans have been around for only a tiny part.

(87).Absolut ages of rocks.

Vocabulary

absolute age

daughter product

half-life

ice core

parent isotope

radioactive isotope

radioactivity

radiometric dating

tree ring

Introduction

What was missing from the early geologic time scale? While the order of events was given, the dates at which the events happened were not. With the discovery of radioactivity in the late 1800s, scientists were able to measure the absolute age, or the exact age of some rocks in years. Absolute dating allows scientists to assign numbers to the breaks in the geologic time scale. Radiometric dating and other forms of absolute age dating allowed scientists to get an absolute age from a rock or fossil.

Tree Ring Dating

In locations where summers are warm and winters are cool, trees have a distinctive growth pattern. Tree trunks display alternating bands of light-colored, low density summer growth and dark, high density winter growth. Each light-dark band represents one year. By counting tree rings it is possible to find the number of years the tree lived (Figure below).

[Figure 1]

Cross-section showing growth rings.

The width of these growth rings varies with the conditions present that year. A summer drought may make the tree grow more slowly than normal and so its light band will be relatively small. These tree-ring variations appear in all trees in a region. The same distinctive pattern can be found in all the trees in an area for the same time period.

Scientists have created continuous records of tree rings going back over the past 2,000 years. Wood fragments from old buildings and ancient ruins can be age dated by matching up the pattern of tree rings in the wood fragment in question and the scale created by scientists. The outermost ring indicates when the tree stopped growing; that is, when it died. The tree-ring record is extremely useful for finding the age of ancient structures.

An example of how tree-ring dating is used to date houses in the United Kingdom is found in this article: http://www.periodproperty.co.uk/ppuk_discovering_article_013.shtml.

Ice Cores and Varves

Other processes create distinct yearly layers that can be used for dating. On a glacier, snow falls in winter but in summer dust accumulates. This leads to a snow-dust annual pattern that goes down into the ice (Figure below). Scientists drill deep into ice sheets, producing ice cores hundreds of meters long. The information scientists gather allows them to determine how the environment has changed as the glacier has stayed in its position. Analyses of the ice tell how concentrations of atmospheric gases changed, which can yield clues about climate. The longest cores allow scientists to create a record of polar climate stretching back hundreds of thousands of years.

[Figure 2]

Ice core section showing annual layers.

Lake sediments, especially in lakes that are located at the end of glaciers, also have an annual pattern. In the summer, the glacier melts rapidly, producing a thick deposit of sediment. These alternate with thin, clay-rich layers deposited in the winter. The resulting layers, called varves, give scientists clues about past climate conditions (Figurebelow). A warm summer might result in a very thick sediment layer while a cooler summer might yield a thinner layer.

[Figure 3]

Ancient varve sediments in a rock outcrop.

Age of Earth

During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. What methods can you think of for doing this? One example is that by measuring how much sediment a stream deposited in a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer. Not surprisingly, these methods resulted in wildly different estimates. A relatively good estimate was produced by the British geologist Charles Lyell, who thought that 240 million years had passed since the appearance of the first animals with shells. Today scientists know that this event occurred about 530 million years ago.

In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old (Figure below). He did this systematically assuming that the planet started off as a molten ball and calculating the time it would take for it to cool to its current temperature. This estimate was a blow to geologists and supporters of Charles Darwin’s theory of evolution, which required an older Earth to provide time for geological and evolutionary processes to take place.

[Figure 4]

Lord Kelvin.

Thomson’s calculations were soon shown to be flawed when radioactivitywas discovered in 1896. Radioactivity is the tendency of certain atoms to decay into lighter atoms, a process that emits energy. Radioactive decay of elements inside Earth’s interior provides a steady source of heat, which meant that Thomson had grossly underestimated Earth’s age.

Radioactive Decay

Radioactivity also provides a way to find the absolute age of a rock. To begin, go back to the Earth's Minerals chapter and review the material about atoms.

Some isotopes are radioactive; radioactive isotopes are unstable and spontaneously change by gaining or losing particles. Two types of radioactive decay are relevant to dating Earth materials (Table below):

Types of Radioactive DecayParticleCompositionEffect on NucleusAlpha2 protons, 2 neutronsThe nucleus contains two fewer protons and two fewer neutrons.Beta1 electronOne neutron decays to form a proton and an electron. The electron is emitted.

The radioactive decay of a parent isotope (the original element) leads to the formation of stable daughter product, also known as daughter isotope. As time passes, the number of parent isotopes decreases and the number of daughter isotopes increases (Figure below).

Radioactive materials decay at known rates, measured as a unit called half-life. The half-life of a radioactive substance is the amount of time it takes for half of the parent atoms to decay. This is how the material decays over time.

Pretend you find a rock with 3.125% parent atoms and 96.875% daughter atoms. How many half lives have passed? If the half-life of the parent isotope is 1 year, then how old is the rock? The decay of radioactive materials can be shown with a graph (Figurebelow).

[Figure 6]

Decay of an imaginary radioactive substance with a half-life of one year.

Notice how it doesn’t take too many half lives before there is very little parent remaining and most of the isotopes are daughter isotopes. This limits how many half lives can pass before a radioactive element is no longer useful for dating materials. 

Radiometric Dating of Rocks

Different isotopes are used to date materials of different ages. Using more than one isotope helps scientists to check the accuracy of the ages that they calculate.

Radiocarbon Dating

Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range is especially useful for determining ages of human fossils and habitation sites (Figure below).

[Figure 7]

Carbon isotopes from the black material in these cave paintings places their creating at about 26,000 to 27,000 years BP (before present).

The atmosphere contains three isotopes of carbon: carbon-12, carbon-13 and carbon-14. Only carbon-14 is radioactive; it has a half-life of 5,730 years. The amount of carbon-14 in the atmosphere is tiny and has been relatively stable through time.

Plants remove all three isotopes of carbon from the atmosphere during photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. After the organism’s death, the carbon-14 decays to stable nitrogen-14 by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon-14 that has decayed can be estimated by measuring the proportion of radioactive carbon-14 to stable carbon-12. As time passes, the amount of carbon-14 decreases relative to the amount of carbon-12

Potassium-Argon Dating

Potassium-40 decays to argon-40 with a half-life of 1.26 billion years. Argon is a gas so it can escape from molten magma, meaning that any argon that is found in an igneous crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of potassium-40 to argon-40 yields a good estimate of the age of that crystal.

Potassium is common in many minerals, such as feldspar, mica, and amphibole. With its half-life, the technique is used to date rocks from 100,000 years to over a billion years old. The technique has been useful for dating fairly young geological materials and deposits containing the bones of human ancestors.

Uranium-Lead Dating

Two uranium isotopes are used for radiometric dating.

Uranium-238 decays to lead-206 with a half-life of 4.47 billion years.

Uranium-235 decays to form lead-207 with a half-life of 704 million years.

Uranium-lead dating is usually performed on zircon crystals (Figure below). When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. If any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium.

[Figure 8]

Zircon crystal.

Uranium-lead dating is useful for dating igneous rocks from 1 million years to around 4.6 billion years old. Zircon crystals from Australia are 4.4 billion years old, among the oldest rocks on the planet.

Limitations of Radiometric Dating

Radiometric dating, or the process of using the concentrations of radioactive substances and daughter products to estimate the age of a material, is a very useful tool for dating geological materials but it does have limits:

1. The material being dated must have measurable amounts of the parent and/or the daughter isotopes. Ideally, different radiometric techniques are used to date the same sample; if the calculated ages agree, they are thought to be accurate.

2. Radiometric dating is not very useful for determining the age of sedimentary rocks. To estimate the age of a sedimentary rock, geologists find nearby igneous rocks that can be dated and use relative dating to constrain the age of the sedimentary rock.

Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossil ages have become increasingly accurate.

All of this evidence comes together to pinpoint the age of Earth at 4.6 billion years.

Lesson Summary

Earth is very old, and the study of Earth’s past requires us to think about times that were millions or even billions of years ago.

Techniques such as superposition and index fossils can tell you the relative age of objects, which objects are older and which are younger.

Geologists use a variety of techniques to establish absolute age, including radiometric dating, tree rings, ice cores, and annual sedimentary deposits called varves.

The concentrations of several radioactive isotopes (e.g. carbon-14, potassium-40, uranium-235 and -238) and their daughter products are used to accurately determine the age of rocks and organic remains.

(85).Possil.

Vocabulary

amber

body fossil

cast

fossilization

index fossil

microfossil

mold

permineralization

trace fossil

Introduction

Throughout human history, people have discovered fossils and wondered what they are and what they represent. In ancient times, fossils inspired legends of monsters and other strange creatures. The Chinese writer, Chang Qu, 2,000 years ago reported the discovery of “dragon bones,” which were probably dinosaur fossils. Look at the two photos in the Figure below and try to trace the origin of the creature on the left.

[Figure 1]

The griffin, a mythical creature with a lion’s body and an eagle’s head and wings (left), was probably based on skeletons of Protoceratops (right) that were discovered by nomads in Central Asia.

[Figure 2]

Ammonites (left) and elephant skull (right).

Ancient Greeks named ammonites after the ram god Ammon since they look like the coiled horns of a ram. Legends of the Cyclops may be based on fossilized elephant skulls found in Crete and other Mediterranean islands (Figure above). Can you see why?

Many of the real creatures whose bones became fossilized were no less marvelous than the mythical creatures they inspired (Figure below).

[Figure 3]

(a) The giant pterosaur Quetzalcoatlus had a wingspan of up to 12 meters (39 feet). (b) Argentinosaurus had an estimated weight of 80,000 kg, equal to the weight of seven elephants! Other fossils, such as the trilobite Kolihapeltis ch (c) impress us with their bizarre forms. These suture marks on an ammonite fossil (d) display a delicate beauty.

How Fossils Form

A fossil is any remains or traces of an ancient organism. Fossils include body fossils, left behind when the soft parts have decayed away, and trace fossils, such as burrows, tracks, or fossilized coprolites (feces) (Figure below). Collections of fossils that are found together are known as fossil assemblages.

[Figure 4]

Coprolite from a meat-eating dinosaur.

The process of a once-living organism becoming a fossil is called fossilization. Fossilization is very rare: Only a tiny percentage of the organisms that have ever lived become fossils.

Why do you think only a tiny percentage of living organisms become fossils after death? Think about an antelope that dies on the African plain (Figure below).

[Figure 5]

Hyenas eating an antelope. Will the antelope in this photo become a fossil?

Most of its body is eaten by hyenas and other scavengers and the remaining flesh is devoured by insects and bacteria. Only bones are left behind. As the years go by, the bones are scattered and fragmented into small pieces, eventually turning into dust. The remaining nutrients return to the soil. This antelope will not be preserved as a fossil.

Is it more likely that a marine organism will become a fossil? When clams, oysters, and other shellfish die, the soft parts quickly decay, and the shells are scattered. In shallow water, wave action grinds them into sand-sized pieces. The shells are also attacked by worms, sponges, and other animals (Figurebelow).

[Figure 6]

Shell that has been attacked by a boring sponge.

How about a soft bodied organism? Will a creature without hard shells or bones become a fossil? There is virtually no fossil record of soft bodied organisms such as jellyfish, worms, or slugs. Insects, which are by far the most common land animals, are only rarely found as fossils (Figure below).

[Figure 7]

A rare insect fossil.

Despite these problems, there is a rich fossil record. How does an organism become fossilized?

Usually it’s only the hard parts that are fossilized. The fossil record consists almost entirely of the shells, bones, or other hard parts of animals. Mammal teeth are much more resistant than other bones, so a large portion of the mammal fossil record consists of teeth. The shells of marine creatures are common also.

Quick burial is essential because most decay and fragmentation occurs at the surface. Marine animals that die near a river delta may be rapidly buried by river sediments. A storm at sea may shift sediment on the ocean floor, covering a body and helping to preserve its skeletal remains (Figure below).

[Figure 8]

This fish was quickly buried in sediment to become a fossil.

Quick burial is rare on land, so fossils of land animals and plants are less common than marine fossils. Land organisms can be buried by mudslides, volcanic ash, or covered by sand in a sandstorm (Figurebelow). Skeletons can be covered by mud in lakes, swamps, or bogs.

[Figure 9]

People buried by the extremely hot eruption of ash and gases at Mt. Vesuvius in 79 AD.

Unusual circumstances may lead to the preservation of a variety of fossils, as at the La Brea Tar Pits in Los Angeles, California (Figure below).

[Figure 10]

Although the animals trapped in the Ta Brea Tar Pits probably suffered a slow, miserable death, their bones were preserved perfectly by the sticky tar.

In spite of the difficulties of preservation, billions of fossils have been discovered, examined, and identified by thousands of scientists. The fossil record is our best clue to the history of life on Earth, and an important indicator of past climates and geological conditions as well.

Exceptional Preservation

Some rock beds contain exceptional fossils or fossil assemblages. Two of the most famous examples of soft organism preservation are from the 505 million-year-old Burgess Shale in Canada (Figure below). The 145 million-year-old Solnhofen Limestone in Germany has fossils of soft body parts that are not normally preserved (Figure below).

[Figure 11]

(a) The Burgess shale contains soft-bodied fossils. (b) Anomalocaris, meaning “abnormal shrimp” is now extinct. The image is of a fossil. (c) The famous Archeopteryx fossil from the Solnhofen Limestone has distinct feathers and was one of the earliest birds.

Types of Fossilization

Most fossils are preserved by one of five processes outlined in the Figure below.

[Figure 12]

Five types of fossils: (a) Insect preserved in amber, (b) petrified wood (permineralization), (c) cast and mold of a clam shell, (d) pyritized ammonite, and (e) compression fossil of a fern.

Preserved Remains

Most uncommon is the preservation of soft-tissue original material. Insects have been preserved perfectly in amber, which is ancient tree sap. Mammoths and a Neanderthal hunter were frozen in glaciers, allowing scientists the rare opportunity to examine their skin, hair, and organs. Scientists collect DNA from these remains and compare the DNA sequences to those of modern counterparts.

Permineralization

The most common method of fossilization is permineralization. After a bone, wood fragment, or shell is buried in sediment, mineral-rich water moves through the sediment. This water deposits minerals into empty spaces and produces a fossil. Fossil dinosaur bones, petrified wood, and many marine fossils were formed by permineralization.

Molds and Casts

When the original bone or shell dissolves and leaves behind an empty space in the shape of the material, the depression is called a mold. The space is later filled with other sediments to form a matching cast within the mold that is the shape of the original organism or part. Many mollusks (clams, snails, octopi, and squid) are found as molds and casts because their shells dissolve easily.

Replacement

The original shell or bone dissolves and is replaced by a different mineral. For example, calcite shells may be replaced by dolomite, quartz, or pyrite. If a fossil that has been replace by quartz is surrounded by a calcite matrix, mildly acidic water may dissolve the calcite and leave behind an exquisitely preserved quartz fossil.

Compression

Some fossils form when their remains are compressed by high pressure, leaving behind a dark imprint. Compression is most common for fossils of leaves and ferns, but can occur with other organisms.

Clues from Fossils

Fossils are our best form of evidence about Earth history, including the history of life. Along with other geological evidence from rocks and structures, fossils even give us clues about past climates, the motions of plates, and other major geological events.

History of Life on Earth

That life on Earth has changed over time is well illustrated by the fossil record. Fossils in relatively young rocks resemble animals and plants that are living today. In general, fossils in older rocks are less similar to modern organisms. The history of life will be discussed in the "Earth's History" chapter.

Environment of Deposition

By knowing something about the type of organism the fossil was, geologists can determine whether the region was terrestrial (on land) or marine (underwater) or even if the water was shallow or deep. The rock may give clues to whether the rate of sedimentation was slow or rapid. The amount of wear and fragmentation of a fossil allows scientists to learn about what happened to the region after the organism died; for example, whether it was exposed to wave action.

Geologic History

The presence of marine organisms in a rock indicates that the region where the rock was deposited was once marine. Sometimes fossils of marine organisms are found on tall mountains indicating that rocks that formed on the seabed were uplifted (Figure below).

[Figure 13]

The summit of Mt. Everest, the world’s tallest mountain, is limestone that formed in an ancient sea.

Climate

By knowing something about the climate a type of organism lives in now, geologists can use fossils to decipher the climate at the time the fossil was deposited. For example, coal beds form in tropical environments but ancient coal beds are found in Antarctica. Geologists know that at that time the climate on the Antarctic continent was much warmer. Recall from the chapter about plate tectonics that Wegener used the presence of coal beds in Antarctica as one of the lines of evidence for continental drift.

Index Fossils

An index fossil can be used to identify a specific period of time. Organisms that make good index fossils are distinctive, widespread, and lived briefly. Their presence in a rock layer can be used to identify that period of time over a large area.

KQED: Science on the SPOT: Lupe the Mammoth Comes to Life

The fossil of a juvenile mammoth found near downtown San Jose California reveals an enormous amount about these majestic creatures: what they looked like, how they lived, and what the environment of the Bay Area was like so long ago. Learn more in the video below:

 

 

Lesson Summary

Fossils are the remains of ancient life. Body fossils are the remains of the organism itself; trace fossils are burrows, tracks, feces, or other evidence of activity.

Fossilization is a very rare process. The chances of becoming a fossil are enhanced by quick burial and the presence of hard parts, such as bones or shells.

Fossils form in five ways: by preservation of the remains, permineralization, molds and casts, replacement, and compression.

Types of organisms that make good index fossils are widespread but only existed for a short period of time. Index fossils help scientists to determine the approximate age of a rock layer and to match that layer up with other rock layers.

Fossils give clues about the history of life on Earth, environments, climate, geologic history, and other events of geological importance.

(84).Evidence about earth's past

Identifying locations where abundant and interesting fossils are found is a paleontologist’s first step in unraveling Earth history. First, rocks of the right age need to be identified. Desert areas are better for fossil hunting because the rocks are better exposed and weathering processes have not degraded the rocks or their fossils. But there are a lot of desert areas in the world and it is not possible to search them all on foot. Paleontologists now use satellites to locate good fossil sites. This Landsat image of Mongolia’s Gobi Desert allowed researchers to locate exposed sedimentary rocks. While the true-color image gave a broad look at the area of interest, the false-color image shown here elucidates so much more. Different colors highlight vegetation and individual rock types. By studying the different colors scientists can single out the area they think is most likely to produce fossils. In this chapter, you will learn about some of the ways that scientists study the history of Earth and how they use clues from rocks and fossils to piece together pictures of how the Earth has changed over billions of years.

(83).(Erosion and deposition by gravity.

Vocabulary

avalanche

creep

landslide

mass movement

mass wasting

mudflow

slump

talus slope

Introduction

Gravity shapes the Earth’s surface by moving weathered material from a higher place to a lower one. This occurs in a variety of ways and at a variety of rates including sudden, dramatic events as well as slow steady movements that happen over long periods of time. The force of gravity is constant and it is changing the Earth’s surface right now.

Types of Movement Caused by Gravity

Movements caused by gravity are together referred to as mass wasting or mass movement. Weathered material may fall away from a cliff because there is nothing to keep it in place. Rocks that fall to the base of a cliff make a talus slope (Figure below). Sometimes as one rock falls, it hits another rock, which hits another rock, and begins a landslide.

[Figure 1]

Pieces of rock regularly fall to the base of cliffs to form talus slopes.

Landslides and Avalanches

Landslides and avalanches are the most dramatic, sudden, and dangerous examples of earth materials moved by gravity. Landslides are sudden falls of rock, whereas avalanches are sudden falls of snow.

When large amounts of rock suddenly break loose from a cliff or mountainside, they move quickly and with tremendous force (Figure below). Air trapped under the falling rocks acts as a cushion that keeps the rock from slowing down. Landslides and avalanches can move as fast as 200 to 300 km/hour.

[Figure 2]

(a) Landslides are called rock slides by geologists. (b) A snow avalanche moves quickly down slope, burying everything in its path.

Landslides are exceptionally destructive. Homes may be destroyed as hillsides collapse. Landslides can even bury entire villages. Landslides may create lakes when the rocky material dams a stream. If a landslide flows into a lake or bay, they can trigger a tsunami (Figurebelow).

[Figure 3]

The 1958 landslide into Lituya Bay, Alaska, created a 524m tsunami that knocked down trees at elevations higher than the Empire State Building (light gray).

Landslides often occur on steep slopes in dry or semi-arid climates. The California coastline, with its steep cliffs and years of drought punctuated by seasons of abundant rainfall, is prone to landslides. At-risk communities have developed landslide warning systems. Around San Francisco Bay, the National Weather Service and the U.S. Geological Survey use rain gauges to monitor soil moisture. If soil becomes saturated, the weather service issues a warning. Earthquakes, which may occur on California’s abundant faults, can also trigger landslides.

Mudflows and Lahars

Added water creates natural hazards produced by gravity (Figure below). On hillsides with soils rich in clay, little rain, and not much vegetation to hold the soil in place, a time of high precipitation will create a mudflow. Mudflows follow river channels, washing out bridges, trees, and homes that are in their path.

[Figure 4]

The white areas on green hillsides mark scars from numerous mudflows.

A lahar is mudflow that flows down a composite volcano (Figure below). Ash mixes with snow and ice melted by the eruption to produce hot, fast-moving flows. The lahar caused by the eruption of Nevado del Ruiz in Columbia in 1985 killed more than 23,000 people.

[Figure 5]

A lahar is a mudflow that forms from volcanic ash and debris.

Slump and Creep

Less dramatic types of downslope movement move earth materials slowly down a hillside. Slump moves materials as a large block along a curved surface (Figure below). Slumps often happen when a slope is undercut, with no support for the overlying materials, or when too much weight is added to an unstable slope.

[Figure 6]

Slump material moves as a whole unit, leaving behind a crescent shaped scar.

Creep is the extremely gradual movement of soil downhill. Curves in tree trunks indicate creep because the base of the tree is moving downslope while the top is trying to grow straight up (Figure below). Tilted telephone or power company poles are also signs of creep.

[Figure 7]

Trees with curved trunks are often signs that the hillside is slowly creeping downhill.

Contributing Factors

There are several factors that increase the chance that a landslide will occur. Some of these we can prevent and some we cannot.

Water

A little bit of water helps to hold grains of sand or soil together. For example, you can build a larger sand castle with slightly wet sand than with dry sand. However too much water causes the sand to flow quickly away. Rapid snow melt or rainfall adds extra water to the soil, which increases the weight of the slope and makes sediment grains lose contact with each other, allowing flow.

Rock Type

Layers of weak rock, such as clay, also allow more landslides. Wet clay is very slippery, which provides an easy surface for materials to slide over.

Undercutting

If people dig into the base of a slope to create a road or a homesite, the slope may become unstable and move downhill. This is particularly dangerous when the underlying rock layers slope towards the area (Figure below).

[Figure 8]

The slope of underlying materials must be considered when making road cuts.

When construction workers cut into slopes for homes or roads, they must stabilize the slope to help prevent a landslide (Figure below). Trees roots or even grasses can bind soil together. It is also a good idea to provide drainage so that the slope does not become saturated with water.

[Figure 9]

A rock wall stabilizes a slope that has been cut away to make a road.

Ground shaking

An earthquake, volcanic eruption, or even just a truck going by can shake unstable ground loose and cause a slide. Skiers and hikers may disturb the snow they travel over and set off an avalanche

Landslides cause $1 billion to $2 billion damage in the United States each year and are responsible for traumatic and sudden loss of life and homes in many areas of the world. To be safe from landslides:

Be aware of your surroundings and notice changes in the natural world.

Look for cracks or bulges in hillsides, tilting of decks or patios, or leaning poles or fences when rainfall is heavy. Sticking windows and doors can indicate ground movement as soil pushes slowly against a house and knocks windows and doors out of alignment.

Look for landslide scars because landslides are most likely to happen where they have occurred before.

Plant vegetation and trees on the hillside around your home to help hold soil in place.

Help to keep a slope stable by building retaining walls. Installing good drainage in a hillside may keep the soil from getting saturated.

Lesson Summary

Gravity moves earth materials from higher elevations to lower elevations.

Landslides, avalanches, and mudflows are examples of dangerous erosion by gravity.

Slump and creep move material slowly downslope.

Plants, retaining walls, and good drainage are ways to help prevent landslides.

(82).Glacial erosion and deposition

Vocabulary

alpine (valley) glacier

continental glacier

end moraine

glacial erratic

glacial striations

glacial till

glaciers

ground moraine

hanging valley

lateral moraine

medial moraine

moraine

plucking

terminal moraine

varve

Introduction

Glaciers cover about 10% of the land surface near Earth’s poles and they are also found in high mountains. During the Ice Ages, glaciers covered as much as 30% of Earth. Around 600 to 800 million years ago, geologists think that almost all of the Earth was covered in snow and ice. Scientists use the evidence of erosion and deposition left by glaciers to do a kind of detective work to figure out where the ice once was.

Formation and Movement of Glaciers

Glaciers are solid ice that move extremely slowly along the land surface (Figure below). Glacial ice erodes and shapes the underlying rocks. Glaciers also deposit sediments in characteristic landforms. The two types of glaciers are:

Continental glaciers are large ice sheets that cover relatively flat ground. These glaciers flow outward from where the greatest amount of snow and ice accumulate.

Alpine or valley glaciers flow downhill through mountains along existing valleys.

[Figure 1]

A satellite image of glaciers in the Himalaya with some features labeled.

Glacial Erosion

Glaciers erode the underlying rock by abrasion and plucking. Glacial meltwater seeps into cracks of the underlying rock, the water freezes and pushes pieces of rock outward. The rock is then plucked out and carried away by the flowing ice of the moving glacier (Figure below). With the weight of the ice over them, these rocks can scratch deeply into the underlying bedrock making long, parallel grooves in the bedrock, called glacial striations.

[Figure 2]

Glacial striations point the direction a glacier has gone.

Mountain glaciers leave behind unique erosional features. When a glacier cuts through a ‘V’ shaped river valley, the glacier pucks rocks from the sides and bottom. This widens the valley and steepens the walls, making a ‘U’ shaped valley (Figure below).

[Figure 3]

A U shaped valley in Glacier National Park.

Smaller tributary glaciers, like tributary streams, flow into the main glacier in their own shallower ‘U’ shaped valleys. A hanging valley forms where the main glacier cuts off a tributary glacier and creates a cliff. Streams plunge over the cliff to create waterfalls (Figure below).

[Figure 4]

Yosemite Valley is known for waterfalls that plunge from hanging valleys.

Up high on a mountain, where a glacier originates, rocks are pulled away from valley walls. Some of the resulting erosional features are shown: Figurebelow and Figure below.

[Figure 5]

(a) A bowl-shaped cirque in Glacier National Park was carved by glaciers. (b) A high altitude lake, called a tarn, forms from meltwater trapped in the cirque. (c) Several cirques from glaciers flowing in different directions from a mountain peak, leave behind a sharp sided horn, like the Matterhorn in Switzerland. (d) When glaciers move down opposite sides of a mountain, a sharp edged ridge, called an arête, forms between them.

[Figure 6]

A roche moutonée forms where a glacier smooths the uphill side of the bedrock and plucks away rock from the downslope side.

Depositional Features of Glaciers

As glaciers flow, mechanical weathering loosens rock on the valley walls, which falls as debris on the glacier. Glaciers can carry rock of any size, from giant boulders to silt (Figure below). These rocks can be carried for many kilometers for many years. These rocks with a different rock type or origin from the surrounding bedrock are glacial erratics. Melting glaciers deposit all the big and small bits of rocky material they are carrying in a pile. These unsorted deposits of rock are called glacial till.

[Figure 7]

A large boulder dropped by a glacier is a glacial erratic.

Glacial till is found in different types of deposits. Linear rock deposits are called moraines. Geologists study moraines to figure out how far glaciers extended and how long it took them to melt away. Moraines are named by their location relative to the glacier:

Lateral moraines form at the edges of the glacier as material drops onto the glacier from erosion of the valley walls.

Medial moraines form where the lateral moraines of two tributary glaciers join together in the middle of a larger glacier (Figure below).

[Figure 8]

The long, dark lines are medial and lateral moraines.

Sediment from underneath the glacier becomes a ground moraine after the glacier melts. Ground moraine contributes to the fertile transported soils in many regions.

Terminal moraines are long ridges of till left at the furthest point the glacier reached.

End moraines are deposited where the glacier stopped for a long enough period to create a rocky ridge as it retreated. Long Island in New York is formed by two end moraines.

[Figure 9]

(a) An esker is a winding ridge of sand and gravel deposited under a glacier by a stream of meltwater. (b) A drumlin is an asymmetrical hill made of sediments that points in the direction the ice moved. Usually drumlins are found in groups called drumlin fields.

While glaciers dump unsorted sediments, glacial meltwater can sort and re-transport the sediments (Figure above). As water moves through unsorted glacial till, it leaves behind the larger particles and takes away the smaller bits of sand and silt (Figure below).

[Figure 10]

(a) A sorted deposit of sand and smaller particles is stratified drift. A broad area of stratified drift from meltwater over broad region is an outwash plain. (b) Kettle lakes form as blocks of ice in glacial till m

Several types of stratified deposits form in glacial regions but are not formed directly by the ice. Varves form where lakes are covered by ice in the winter. Dark, fine-grained clays sink to the bottom in winter but melting ice in spring brings running water that deposits lighter colored sands. Each alternating dark/light layer represents one year of deposits. If during a year, a glacier accumulates more ice than melts away, the glacier advances downhill. If a glacier melts more than it accumulates over a year, it is retreating (Figure below).

[Figure 11]

Grinnell Glacier in Glacier National Park has been retreating over the past 70 years.

Lesson Summary

The movement of ice in the form of glaciers has transformed our mountainous land surfaces with its tremendous power of erosion.

U-shaped valleys, hanging valleys, cirques, horns, and aretes are features sculpted by ice.

The eroded material is later deposited as large glacial erratics, in moraines, stratified drift, outwash plains, and drumlins.

Varves are a very useful yearly deposit that forms in glacial lakes.