The Virtual Textbook

Table of Contents
Plate Tectonics
Materials and Processes that Shape a Planet

EvolutionKey words
selective breedingtraitspecies
evolutionnatural selectionvariation

Selective Breeding
Humans have had an affect on organisms for thousands of years by selecting and helping the species that are found to be the most useful. Small differences between parents and offspring can accumulate from generation to generation so that descendants are very different from their ancestors. For example, farmers often choose the best animals to breed and the best plants from which to collect seeds. Therefore they will have the best traits passed on from generation to generation.
Selection for specific traits imposed by humans, either deliberately or otherwise, upon wild or domesticated plants and animals is called selective breeding. A trait is a characteristic that an organism can pass along to its offspring through its genes.

Darwin’s Observations
Charles Darwin was a British Naturalist and Clergyman who lived from 1809–1882. In 1831, Darwin left from England on a ship (called The Beagle) that made many stops along the coast of South America. Darwin’s job was to learn about the living things he saw. Darwin’s important observations included the diversity of living things and the remains of ancient organisms.
Darwin saw many new species of plants and animals that he had never seen before in his native England. A species is a group of similar living things that can mate with each other and produce offspring. Darwin also made observations of fossils in South America. A fossil is the preserved remains of an organism that lived long ago.

Galápagos Organisms
Darwin’s important observations included the characteristics of organisms on the Galápagos Islands, a group of islands off the West coast of South America. He saw that the plants and animals on the Galápagos Islands, though similar to those on the mainland, had some very important differences. For example, Galápagos iguanas had larger claws than mainland iguanas. This led Darwin to the hypothesis that some plants and animals came to the Galápagos Islands from the mainland. He reasoned that eventually, the offspring of these plants and animals became different from their mainland relatives.
Darwin also saw that tortoises and finches were different from one Galápagos island to the next. Finches, for example, had different beak shapes. The finches of the different islands had beaks that helped them gather food in the unique terrains and ecosystems. Beak shape is an example of an adaptation. An adaptation is a trait that helps an organism survive and reproduce.

Darwin reasoned that plants or animals that arrived on the Galápagos Islands faced conditions that were different from those on the mainland. Darwin hypothesized that the species gradually changed over many generations and became better adapted to the new conditions. The gradual change in a species over time is called evolution. Darwin concluded that the living things that came to the Galápagos Islands from the mainland had changed over time. The living things changed so that they could live better in the island environment. Darwin’s ideas are often called the theory of evolution. A scientific theory is a well-tested idea.

Natural Selection
Darwin proposed that, over a long time, natural processes could lead to change. Helpful variations may gradually accumulate (build up) in a species, while unfavorable ones may disappear. Darwin suggested that evolution happens because of natural selection. In natural selection, individuals that are better adapted to their environment are more likely to survive and reproduce. Factors that affect natural selection are overproduction, competition, and variation.

  • Overproduction
    • Most species produce far more offspring than can possibly survive. Overproduction makes it more likely that some offspring will survive.
  • Competition
    • Food and other resources are limited. Members of a species must compete with each other for these resources. Some members of a species may not find enough to eat, so they do not survive.
  • Variation
    • Any difference between individuals of the same species is called a variation. Some variations make individuals better adapted to their environment. Individuals that are better adapted are more likely to live and produce more offspring. Their offspring may inherit these helpful variations. After many generations, more members of the species will have the helpful variations.

Charles Darwin published his findings in a book entitled “On The Origin of the Species” in November of 1859. One aspect of the theory of Evolution is the idea that several species could evolve over thousands of years from a common ancestor. This means that similar species, such as humans and apes could have evolved from a distant ancestor that has similar characteristics. This theory was opposed by certain religious leaders with an alternate view of where humans came from. Though there still is some debate as to how species have come to be on our planet, most scientists subscribe to Darwin’s theory of Evolution.

Plate Tectonics
Table of ContentsA Continental PuzzleSea Floor SpreadingThe Layers of the EarthEarthquakes, Volcanoes, and Tsunamis

A Continental Puzzle

Drifting Continents

In 1910, a meteorologist from Germany named Alfred Wegener (Vay-guh-ner) developed a theory based on many observations. Even though meteorologists study the weather, his theory was about another branch of science called Geology. Geology is the study of the Earth, including the rocks, landforms and the processes that work to change them.
Wegener’s work as a meteorologist included the study of weather maps. Like many scientists before him, he made the observation that the continents of the earth looked a lot like pieces in a giant puzzle. He noticed that the coastline of Eastern South America matched up very well with the coastline of Western Africa.
As he began to further investigate this discovery he found that scientists had found identical fossils of extinct land organisms on both coastlines. He also noticed mountain chains that began on one continent and seemed to continue on other continents. He began to wonder how this could be when they were, in many cases, separated by oceans such as the Atlantic.
Wegener developed a theory that he called Continental Drift. Wegener’s hypothesis was that the continents were once joined together in a single landmass. According to the age of the fossils, he calculated that the continents were together 300,000,000 years ago. He called this landmass
Pangaea (pan-jee-uh) which means all lands. His theory also stated that the continents broke up into many large pieces called plates. He believed that they are still moving to this day and will continue to move until they meet up again to form another giant landmass.
Unfortunately, Wegener had a hard time getting the scientists of the world to accept his theory. Some pointed out the fact that he was trained as a meteorologist, not a geologist. More importantly, and most discouraging for Wegener, he could not explain what was causing the continents to move. While he had theories about that as well, they were not supported with as much evidence.
Alfred Wegener spent the rest of his life trying to prove his theory of Continental Drift. He died in 1930 while on a scientific expedition to Greenland. It wasn’t until many years after his death that scientists found the evidence to prove his theory to be correct.

Sea-Floor Spreading

The Mid Atlantic Ridge

While serving in the Navy during World War II, a navy officer and scientist named Harry Hess sought to prove Wegener right. After studying Wegener’s work he thought he might be able to find the proof for Continental Drift using a new technology used on the Navy ships.
While mapping the ocean floor during the 1950s, Hess made an interesting discovery that helped prove Wegener’s theory of Continental drift. Using sonar, a device that sends and receives sound waves under water, he discovered the existence of undersea mountain ranges that exist in many of the Earth's oceans. The largest is known as the Mid Atlantic Ridge and stretches for thousands of kilometers down the center of the Atlantic Ocean.
As Hess studied the ridges and trenches he began developing his own theory about the continents. His hypothesis was that molten lava is constantly oozing up from the interior of the earth along places like the Mid Atlantic Ridge, creating new sea floor that slowly spreads away. This new seafloor is called oceanic crust. The seafloor then sinks down into deep sea trenches. When the oceanic crust sinks back down to be melted again it is called subduction.

The Theory of Plate Tectonics

Harry Hess’s investigation of the Mid Atlantic Ridge led to a better understanding of how the Earth’s surface, both above and below the water is in a constant state of change. The Theory of Plate Tectonics was developed to explain how the plates are formed, move, and are destroyed over time.

Boundaries Between the Plates

Scientists have identified three different ways that the continental plates interact with each other. They are called divergent boundaries, convergent boundaries, and transform boundaries.
Divergent Boundaries are places where two plates move away from each other. This happens at mid ocean ridges where sea floor spreading is happening. At divergent boundaries new crust is being formed. Divergent boundaries also occur on land where they are called rift valleys. An example of this is the Great Rift Valley in Africa.

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Convergent Boundaries are where two plates are sliding toward each other. Where they collide with each other, several things can happen depending on certain traits of the rock. Because oceanic crust is more dense than continental crust, when the two collide, oceanic crust sinks beneath continental crust in a process called subduction. When two plates with continental crust meet, they both rise up forming huge mountains.

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Transform Boundaries are places where two plates slide next to each other, each going a different direction. This is where earthquakes often occur. There is a large transform boundary in California called the San Andreas Fault. A fault is a break in the Earth’s crust.

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The Layers of the Earth

Why are the Plates Moving?

An understanding of how the plates are moving leads to the question of why the plates are moving. To understand that, you need to understand what is inside the earth. Unfortunately, this is a difficult question to answer.
To understand the interior of the earth better, many geologists spend time in caves. While some caves seem to be very deep in the earth - the deepest cave in the world is the Krubera Cave in Eastern Europe at a depth of over 2,000 meters - they hardly scratch the surface of our earth.
There are two main ways that geologists study the interior of the earth: rock samples from the Earth’s surface and indirect evidence from seismic (earthquake) waves.

Rock samples can be obtained in several ways. One way is to observe rocks that were brought up from beneath the earth through natural processes such as volcanic eruptions. Another way is through drilling deep holes and bringing up samples. However, the deepest hole ever dug (the Kola Superdeep Borehole, in Russia) only goes down 12 kilometers. This may seem like a great depth but is pretty shallow when compared to how far the center of the earth is.

The Layers of the Earth

Geologists have discovered that the Earth is divided into four distinct layers. We have learned about the different layers through observations of material ejected from volcanoes and through seismic waves from earthquakes. The layers of the the Earth are the crust, mantle, outer core, and inner core.
The Crust is where we live. It is the surface of the Earth. The temperature of the crust is cool compared to the other layers. It is 5-70 kilometers thick, depending on where you are on the Earth. The crust is made up of more than a dozen plates of solid, mostly basaltic rock. The plates are constantly moving around and interacting with each other. The crust and the upper part of the mantle is called the lithosphere.
The Mantle is below the Crust. It is very hot, with an average temperature of 1,200 degrees Celsius. The mantle is 2,867 km thick and is divided into upper and lower parts. The upper mantle is made up of two parts. The part closest to the crust is called the lithosphere. It is similar to crust except much hotter. Below that is the asthenosphere. It is under a lot of pressure and has a consistency much like plastic. The lower mantle is very hot and mostly solid all the way to the core.
The Core is the center of the Earth. It is made up of two parts; the outer and inner core. The core is made up mostly of nickle and iron. It is the hottest layer with an average temperature of 3,200 degrees Celsius. The outer core is made up of molten metal that flows around the inner core. It is 2,266 km thick. The inner core is made up of solid, dense, metal that is under extreme pressure. It is 1,216 km thick .

Convection in the Mantle

The mantle is not just one temperature. It is cooler in the upper mantle than it is in the lower mantle, which is hotter because of the extreme heat from the core. Cooler materials are more dense than hotter materials, therefore they have a tendency to sink down toward the core, pulled by the force of gravity. The hotter, less dense material rises to the top of the mantle where it eventually becomes cooler and begins sinking down as well. This creates a cycle know as convection currents.
Convection currents in the mantle are the cause of the movement of the continents. This has been going on for 4 billion years inside the earth! Convection also occurs in the outer core as well. Convection in the outer core results in the magnetic field that surrounds the earth!

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Earthquakes, Volcanoes, and Tsunamis


The earth seems pretty solid beneath your feet most days. For most of us, that is true almost all of the time. However, if you live in certain regions of the world you may have the misfortune of feeling the ground beneath your feet behaving like a trampoline. This is called an earthquake.
An earthquake is caused by a sudden shifting of the crust on a fault line. This results in the release of energy into the crust that travels in the form of seismic waves. The seismic waves travel out in all directions and may cause the ground to noticeably shake. The strength of an earthquake depends upon how deep beneath the surface the shifting occurs and how large the fault is. Earthquake strength is measured using the Richter scale, which was developed in 1935 by Charles Richter.


The earth rumbles, a colossal explosion fills the air, and day seems to be turned to night. A volcano just erupted! A volcano is an opening in the earth’s crust that allows magma from the mantle to reach the surface. Once magma escapes, it releases gases and becomes lava.
Volcanoes are responsible for much of the creation of new land on our planet. The gasses released from an eruption also contribute to our atmosphere. Along with lava and gasses, volcanic ash can be launched many kilometers into the air, sometimes enough to block the sunlight!

There are different types of volcanoes that have different kinds of eruptions. For example, Mt. St. Helens in Washington State is a composite volcano that has had very violent, explosive eruptions. Its last major eruption, on May 18, 1980, resulted in the death of 57 people and the destruction of 250 homes as well as 185 miles of highway.
A shield volcano, on the other hand, has mostly quiet eruptions. The lava from shield volcano eruptions travel many more miles than more explosive eruptions. The Hawaiian islands are the largest and most well-know shield volcanoes on Earth.

Where Are They Found?

Most of the world’s earthquakes and volcanoes occur or are found on or near continental plate boundaries. This is where the crust is thinnest and weakest. The largest accumulation of earthquakes and volcanoes can be found around the edges of the Pacific Ocean. This is often referred to as the Ring of Fire. States such as California and Alaska as well as countries such as Peru and Japan are accustomed to frequent earthquakes and volcanoes because of their proximity to the Ring of Fire.


Water is one of the most destructive forces in the world. When an earthquake happens under water the energy released by the movement of the crust may result in a large wave, called a tsunami, which can travel for thousands of miles and cause catastrophic damage to seaside areas. The word tsunami means “harbor wave” in Japanese. Japan has had frequent tsunamis throughout history, the most notable of which occurred on March 11, 2011. Damages from the earthquake and tsunami are still being calculated.
A tsunami can be caused by volcanic eruptions and landslides, but it most often caused by undersea earthquakes. There are several stages to a tsunami:

  • Initiation begins with a disturbance (earthquake) on the ocean floor which results in a large column of water being lifted and dropped down.
  • The waves travel in each direction out from the center of the disturbance.
  • In a process called amplification, as the wave travels over the continental slope toward the shore its height (amplitude) increases and its length (wavelength) decreases.
  • As the tsunami wave travels from the deep-water, continental slope region to the near-shore region, tsunami run up occurs.

For more information on earthquakes, volcanoes, and tsunamis, you can visit the website for the United States Geological Survey, a U.S. government organization that collects data about earthquakes from around the world.

A Puzzle Solved?

Scientists have been studying the earth and its processes for thousands of years. Each new discovery brings us closer to a deeper understanding of the many changes that occur on a daily basis all around the world. Though he never lived to see it, the contributions of Alfred Wegener led to a revolution in our understanding of the earth and its changing surface. Though we have come a long way there are still mysteries and puzzles to solve for future generations of geologists!

Prentice Hall Science Explorer, Inside Earth Copyright 2007, Pearson Education

Materials and Processes
Shape a Planet

Table of Contents
Types of Rocks
The Rock Cycle

We learned in the Plate Tectonics unit that the Earth is in a constant state of change due to processes that are taking place beneath the surface. It is difficult to see the short term effects of these processes because they occur over such a long period of time. In the Materials and Processes that Shape a Planet unit, you will be looking at that as well as shorter-term changes in the earth’s landscape.

Types of Rocks
Have you ever stopped to consider the rocks in a driveway? Most people probably do not. Take a look some time. You may be surprised to see what a wide variety of rocks there are in one square meter of driveway.

A close examination will reveal different grains or particles that make up the various rocks. Rocks can often be identified by the types of minerals they are made of. There are 3,000 known minerals but only 20 are very common. Some examples of rock forming minerals are mica and horneblend.
You may also notice that the rocks have different textures. Texture is the look and feel of a rock. Some may be rough while others may be smooth and glassy. These are just a few of the ways that geologists study rocks.
There are three types of rocks. They are Igneous, Metamorphic, and Sedimentary. You can identify them by the way they were formed.

Igneous Rocks

When the earth formed, close to 4.6 billion years ago (4,600,000,000) it did not resemble the earth of today. Temperatures were so hot that molten lava flowed across the surface. As the Earth cooled the lava hardened into igneous (ig-nee-us) rock. This was the first rock to form on the earth.
Igneous rocks are formed from magma or lava. The word igneous comes from the Latin word “ignis” which means “fire.” There are two groups of igneous rocks; intrusive and extrusive. Intrusive igneous rocks are formed deep underground from magma. The most common intrusive igneous rock is granite. Extrusive igneous rocks form from lava that hardens on the surface of the earth. Basalt is the most common extrusive igneous rock

Sedimentary Rocks

In the driveway that you inspected earlier, you might have noticed small pieces of rock such as sand, or pebbles. These pieces are known as sediment. They are small, solid pieces of material which may have come from larger rocks or even living things. Some examples of sediment are rocks, shells, leaves, and even bones. Sedimentary rock is formed from sediment.

From Sediment to Sedimentary

Larger rocks of different kinds are constantly being broken down in a process called weathering. Weathering can be a physical or chemical process. When wind, water, or other forces wear down a rock into smaller pieces of the same kind of rock, it is called mechanical weathering. Ice can cause mechanical weathering when it expands in cracks in rocks.
Another way that rock is broken down is through chemical weathering. This is when rock is dissolved, loosened up, or decomposed through chemical processes. When a high level of acid is in the rain it can cause increased weathering. Chemical weathering results in a new type of rock being formed. Most caves are formed from chemical weathering. The stalactites and stalagmites are formed from water dripping through cracks in the cave and depositing minerals over a long period of time.

Movement of Sediment

After rocks are broken down into sediment by weathering they don’t just sit around; they begin a journey called erosion. Erosion is when sediment is carried away by wind, water, and gravity. One place you can observe erosion of sediment is in a stream or river. The current can move small particles of rock for many kilometers.
The journey of sediment ends when whatever is carrying it slows down and deposits it in layers. When it is being carried by water, it usually ends up in the bottom of a lake or an ocean. When sediments settle out of the wind or water that is carrying it, it is called deposition.

Compaction and Cementation

Eventually, the sediments will pile up and compact under the pressure of the layers above it. This compaction presses the sediments together. As the rocks are compacted, the minerals that are in the rock dissolve and cement the sediments together in a process called cementation. This results in a new rock being formed; sedimentary. You can often see the layers of sediment in a sedimentary rock.
Some examples of sedimentary rocks are sandstone, shale and limestone. Sedimentary rocks have been used for many years for building and to make tools. For example, flint arrowheads are made from sedimentary rocks.

Metamorphic Rocks

One thing that you learn about studying the earth for any amount of time is that it is constantly changing. A great example of this is the formation of metamorphic rocks. In Greek, “meta” means change, and “morphosis” means form.
Metamorphic rocks come from other rocks that have been subjected to heat and pressure. The heat comes from deep underground, near the mantle. The pressure is from all of the rocks that are above it, pressing down. Rocks are forced down toward the mantle during the processes of Plate Tectonics.
The type of metamorphic rock that forms depends on the type of rock it starts as, the amount of heat it is subjected to, and the amount of pressure it undergoes. Some examples of metamorphic rocks are gneiss (neese) whick is formed from granite , quartzite which is formed from sandstone, and slate which is formed from shale.
Many sculptures are made from metamorphic rock. Quite a few of the historical sites around Washington, D.C. have artwork created from marble, a type of metamorphic rock formed from limestone.

The Rock Cycle

A study of the rocks that are on our planet reveal one important fact: They do not remain the same! Rocks are in a constant state of change. When magma or lava cools and hardens it becomes igneous rock but after weathering, erosion, and deposition it will be on its way to becoming a sedimentary rock. If that new sedimentary rock gets subducted to the mantle it could become a metamorphic rock. And, if that new metamorphic rock gets pushed down close enough to the mantle, it could melt back into the magma from which it originally came.
This process is known as The Rock Cycle. The rock cycle is the process that changes one form of rock into another. Scientists have developed a diagram of the rock cycle that helps explain all of the possibilities for the future of a rock.




copyright © 2011 Rob Honer