Grade 7 - Planet Earth
Section 1: Minerals
A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. Minerals are solids-They are not liquids (like water), or gases (like the air around you).
Most minerals are quite rare. Only a few, such as quartz, feldspar, andbmica, are found throughout Earth’s crust (the thin outermost layer ofbEarth). A mineral can be an element (a pure substance) or a compound (two or more elements combined). Quartz, for example, consists of the elements silicon and oxygen. No other mineral has these elements in the same arrangement and proportion. Sulfur, copper, gold, and diamond are each made up of a single element
Properties of Minerals
Color
Most minerals have only one color. A few like Quartz have many colors. The color of a mineral can also be a clue to its identity, however, color alone cannot identify a mineral.
Hardness
Hardness is a measure of how easily a mineral can be scratched. Some minerals are soft and can be scratched with a fingernail. Others are hard and cant even be scratched by a metal. Talc is the softest mineral while diamond is the hardest mineral.
The Mohs Hardness Scale is a set of ten reference minerals (numbered 1 through 10) that are used to determine the relative hardness of minerals and other objects. In this test the hardness of a mineral is defined as its "resistance to being scratched".
Crystal Formation
Crystals are the building blocks of minerals. Crystals occur naturally and have straight edges, flat sides, and regular angles.
Streak
Streak is the color of the powder left when a mineral is rubbed across a white tile. A mineral's streak may or may not be the same as the same minerals color.
Luster
Luster is how light is reflected off a mineral, or the shininess of a mineral. Some minerals are shiny like metal, others are dull. If a mineral shines like a polished metal surface, it is said to have a metallic luster. If a mineral has a duller shine, it has a non-metallic luster.
Cleavage and Fracture
The way a mineral breaks apart can be a clue to its identity. If it breaks along smooth, flat surfaces, or planes, it is said to have cleavage. Minerals that break with rough or jagged edges have fracture.
Section 2: Rocks and the Rock Cycle
Types of Rocks
1. Igneous Rocks
Igneous rock forms when hot magma or lava cools and solidifies. Magma is melted rock found below Earth’s crust, where temperatures and pressures are high. Magma can cool and harden below the surface. The resulting rock is called intrusive rock. Granite is an example of an igneous rock that formed very deep and very slowly in Earth’s crust. When magma breaks through Earth’s surface, in the form of a volcanic eruption, it is called lava. Rock that forms when lava cools on Earth’s surface is called extrusive rock.
The appearance of crystals in igneous rock samples can differ depending on how fast the rocks cooled. Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form.
Physical forms of Igneous Rocks
Felsic: light colored rocks that are rich in elements such as aluminum, potassium, silicon, and sodium.
Mafic: dark colored rocks that are rich in calcium, iron, and magnesium, poor in silicon.
Coarse-grained: takes longer to cool, giving mineral crystals more time to grow.
Fine-grained: cools quickly resulting in no crystals
Sedimentary Rocks
As its name indicates, sedimentary rock is made from sediment — loose material, such as bits of rock, minerals, and plant and animal remains. These sediments become closely packed in layers and cemented together. This arrangement in visible layers is called stratification. Each strata of sediment is squeezed together by the weight of other sediments resulting in compaction. When water soaks into the rock, it forms a natural cement that sticks the larger pieces of sediment together. This is called cementation.
Sedimentary rocks can contain fossils, the remains of plants and animals. Coal is an organic sedimentary rock that forms from the accumulation and preservation of plant materials, usually in a swamp environment. Coal is a combustible rock and along with oil and natural gas it is one of the three most important fossil fuels.
Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3). It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal and fecal debris.
Fossils are formed by soft silts and muds. The soft sediment preserves the fine details in the bones, teeth, and leaves of plants. Sometimes sediments fill an opening in a bone or shell and leave behind a cast of the inside of the living thing. Plants are often fossilized in soft sediments which preserve the structure of the veins in the leaves.
Metamorphic Rocks
The third family of rock is called metamorphic (meaning “changed form”) rock. Metamorphic rock may be formed below Earth’s surface when extremely high pressure and heat cause the original rock, or parent rock, to change form. Changes occur due to heat, pressure, chemical changes and foliation. (Foloiated metamorphic rock contain aligned grains of flat minerals). Gneiss is foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals.
Regional metamorphism is the creation of metamorphic rock from large geographically significant processes like plate tectonics.
Contact metamorphism is the creation of metamorphic rock from the proximity of an existing rock to a heat source provided by a plutonic intrusion.
Metamorphic rock can change so completely that it no longer looks like the parent rock. For example, limestone and marble look different, but both have a hardness value of 3, and both are made of the mineral calcite.
Non-Foliated metamorphic rocks- mineral grains are not arranged in plains or bands. Marble is a nonfoliated metamorphic rock that is produced from the metamorphism of limestone.
The Rock Cycle
The rock components of the crust are slowly but constantly being changed from one form to another. The processes involved are summarized in the rock cycle. The rock cycle is driven by two forces: (1) Earth's internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun.
The rock cycle is active on Earth because the Earth's core is hot enough to keep the mantle moving, the atmosphere is relatively thick, and contains liquid water.
Soil
Soil is a mixture of minerals, weathered rocks, and other things. It has bits of decayed plants and animals called humus. Humus is dark in color. It adds nutrients to soil. Plants use these nutrients for their growth. Humus works like a sponge to soak up rainwater and keep the soil moist. Water, air, and living things are also found in soil.
The making of soil starts with weathering. Weathering causes rocks to break down into smaller and smaller pieces. The tiny bits of weathered rock build up into layers. The top layer is called topsoil. Topsoil is dark and has the most humus and minerals. Below the topsoil is subsoil. This layer is lighter in color and has less humus. Below the subsoil is bedrock, or solid rock.
Properties of Soil
Soil Color: The color of soil depends on the contents. Soil that has high amount of humus appears dark/black colored. Soil that is high in iron appears red. Soils with high calcite look whitish in color.
Soil Texture: Soil texture is a description of the pieces of grain that make up the soil. Sandy soil has larger grains and does not retain water. Silty soils have smaller grains than sandy soils. Clay soil has the smallest grain sizes. Loam soils is a mixture of sand, silt and clay.
Leaching is the removal of soil materials dissolved in water. Water reacts with humus to form an acid layers and carry them through the spaces in the soil to lower layers.
Section 3: Weathering and Erosion
Weathering is the process by which rocks are broken down into sediments slowly over time. There are two major types of weathering: mechanical (or physical) and chemical.
Mechanical weathering breaks down rocks into smaller pieces (eediments) through physical processes. Mechanical weathering changes the shape and size of a rock, but it doesn’t change the rock’s chemical composition. Wind and water are two of the main agents that cause mechanical weathering. Other agents include living things and changing temperatures. For example, winds can pick up small particles and blast them against rock, slowly scraping away at the rock over time. Moving water can weather rocks in a similar way; water often carries larger particles that scrape away at the rock more quickly.
Chemical weathering breaks down rocks through chemical processes that change the rocks’ chemical composition. For example, when carbon dioxide in air dissolves in rain water, carbonic acid is formed. This can dissolve some rocks, including limestone. Many rocks contain minerals that contain iron. Oxygen in the air or dissolved in water can cause the iron in these minerals to rust or oxidize into Iron Oxide, which is a different chemical with different properties.
Some rocks are better able to withstand weathering agents than others. When a region contains many rock types, those that are more resistant to weathering will take longer to break down. This is called differential weathering. Differential weathering can create unique landforms like the one here. Weathering by wind created this rock formation. The less resistant rock weathered away, while the more resistant rock remained.
Weathering breaks rocks down into sediments, and the process of erosion moves these sediments to other locations. Water—liquid and frozen—is an important agent for erosion. Flowing water can carry rocks, sediments, and soil downstream. The faster the water flows, the larger the particles it can carry. These particles may scrape against each other or nearby rocks, causing mechanical weathering at the same time as erosion. Glaciers—large sheets of moving ice—can also cause mechanical weathering, ripping chunks of rock out of the ground as they move across the land. The rocks and sediments caught up in a glacier are carried along the glacier’s path, causing erosion.
Wind is another agent of erosion. Compared to water, winds usually carry smaller sediments. As these sediments scrape against rock in the wind's path, they can cause mechanical weathering at the same time as erosion. Animals are agents of erosion as they burrow into the ground, moving sediments out of their way. Another erosional agent is gravity, which constantly pulls rocks downhill. Many rocks break as they erode downhill, causing additional mechanical weathering.
Because weathering and erosion tend to occur at the same time, rocks that are carried long distances by erosion tend to be more weathered. These rocks tend to be broken into smaller pieces and become more rounded. Rocks that are carried shorter distances, particularly through gravity, tend to have larger pieces with more angular edges.
Deposition
Sediments, rocks, and soil cannot keep moving forever. Eventually, the particles stop moving and settle where the erosional agents have carried them. This process is called deposition. When sediments are eroded by wind, flowing water, ice or gravity, they are deposited in horizontal layers. The oldest layer of sediments is positioned at the bottom, and the more recently deposited layers are at the top.
The Changing Surface of Earth
Glaciers, gravity, wind, and water are agents of erosion. Some changes, such as those caused by glaciers, happen very slowly over many thousands of years. These small changes are called gradual change. Changes such as flash floods, landslides, and rock slides are called sudden change. The most disastrous rock slide in Canadian history was the Frank Slide in 1903 in Alberta’s Crowsnest Pass. Over 80 million tonnes of rock crashed down the side of Turtle Mountain, burying part of the town of Frank. More than 70 people died in the disaster. The slide lasted about 100 seconds.
Water in Motion
Water in motion is one of the most powerful causes of erosion. Sudden changes can occur as rivers erode their banks and fast-moving flood waters carry away large amounts of soil. Heavy rain can disturb the stability of a slope, detaching solid blocks of rock and causing landslides. Oceans, seas, and large lakes erode their shorelines. When waves hit cliffs and shores, rocks are broken down and land is eroded.
Section 4: The Moving Crust
The Earth's Layers
The air around you is the Earth's atmosphere. The atmosphere includes all of the gases around Earth. All of Earth's liquid and solid water, including oceans, lakes, rivers, glaciers, and ice caps, makes up its hydrosphere. The hydrosphere covers approximately 70% of the Earth's surface.
Like an egg, Earth has several layers. The continents and ocean floor make up Earth’s outermost layer, called the crust. The crust is Earth’s thinnest and coolest layer. The layer below the crust is the mantle. Part of the mantle is solid rock. Part is nearly melted rock that is soft and flows. It is a lot like putty. At the center of Earth is the core. The core is the deepest and hottest layer of Earth. The outer core is melted rock. The inner core is solid rock. The biosphere means the parts of Earth where living things are found. Organisms have been found from the atmosphere to the ocean floor.
Continental Drift
In 1915 Alfred Wegener, a German scientist, published his book after noticing that the different large landmasses of the Earth almost fit together like a jigsaw puzzle. The continental shelf of the Americas fits closely to Africa and Europe. Antarctica, Australia, India and Madagascar fit next to the tip of Southern Africa. He presented his Continental Drift hypothesis on 6 January 1912. He analysed both sides of the Atlantic Ocean for rock type, geological structures and fossils. He noticed that there was a significant similarity between matching sides of the continents, especially in fossil plants.
Wegener concluded that all the continents had once been part of a single 'supercontinent.' He called this landmass a Primal continent or Pangaea (a greek term meaning All-Lands or All-Earth. He proposed that the mechanisms causing the drift might be the centrifugal force of the Earth's rotation. This concept became known as Continental drift.
Evidence for Continental Drift
Several obervations were used to develop the continental drift theory. These include:
1. Biological Evidence
In his research, Wegener noticed that several fossils of similar plants and animals had been found on different continents. Mesosaurus lived in freshwater lakes, and its fossils have been found in eastern South America and southern Africa. Providing support for the hypothesis that these areas of the continents were connected.
2. Evidence from Rocks
Wegener examined the observations of other scientists to see if there might be more evidence to support the idea of continental movement. He discovered that geologists had found similarities in rocks on both sides of the Atlantic Ocean. A mountain range, called the Appalachians, in eastern North America was made of the same kind and ages of rock as the mountain range that ran through Britain and Norway.
3. Geological Evidence of Climate
Coal provided further important information about Earth’s history. In order for coal to form, there has to be rich, luxurious plant life in a tropical, swampy environment. The coal beds that exist in North America, Europe, and Antarctica are now in moderate to cold climates.
Plate Tectonics
During Wegener's time, there was widespread rejection of the continental drift theory among geologists until the 1950s when additional evidence was discovered to prove his theory. Scientists developed a model called plate tectonics to explain how the continents and the ocean floor could move. According to this model, the Earth’s surface is broken into pieces, or plates. The plates move over the hot, fluid rock, or magma, in the mantle. The slow movements in the fluid part of the mantle drag the lithosphere and its plates sideways. As the lithosphere moves, so do the ocean floor and continental plates.
As some crustal plates move apart, magma enters the cracks and flows outward. The magma cools, hardens, and builds up into parallel ridges, or raised structures, on the ocean floor. The new rock exerts a sideways force called compression. Magma continues to flow between the plates, forcing them farther apart. This process is called seafloor spreading.
Geologists are still not sure what causes Earth’s plates to move. One explanation is that convection currents in the mantle under Earth’s crust move the plates. A convection current is the flow resulting from the rise of warmer materials and the sinking of cooler materials.
Types of Plate Boundaries
Plates can move in three ways, they can move apart from each other, they can collide or the can slide past each other.
Divergent boundaries (also called constructive boundaries or extensional boundaries) are where two plates slide apart from each other. As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. At zones of continent-to-continent rifting, divergent boundaries may cause new ocean basin to form as the continent splits, spreads, the central rift collapses, and ocean fills the basin. For example the East African Rift.
The image below shows the Arabian and the Nubian plates drifting apart. Similar drift can be observed in the East African Rift.
Convergent boundaries ( also known as destructive boundaries or active margins) occur where two plates slide toward each other to form either a subduction zone (one plate moving underneath the other) or a continental collision. Subduction zones are of two types: ocean-to-continent subduction, where the dense oceanic lithosphere plunges beneath the less dense continent, or ocean-to-ocean subduction where older, cooler, denser oceanic crust slips beneath less dense oceanic crust. At zones of ocean-to-ocean subduction a deep trench to forms in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands e.g. the Aleutian Islands, the Mariana Islands, the Japanese island arc. At zones of ocean-to-continent subduction mountain ranges form, e.g. the Andes, the Cascade Range. At continental collision zones there are two masses of continental lithosphere converging. Since they are of equal density, neither is subducted. The plate edges are compressed, folded, and uplifted forming mountain ranges, e.g. Himalayas and Alps.
Transform boundaries (also called conservative boundaries or strike-slip boundaries) occur where plates are neither created nor destroyed. Instead two plates slide, or perhaps more accurately grind past each other, along transform faults. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
Advances in Technology
Sound Navigation and Ranging, shortened as Sonar is a technology used in nature by bats to navigate around objects in the dark. Sonar works by sending out a sound and then recording the time that the sound takes to bounce back. Scientists can bounce a sound off the ocean floor. By using the time it takes for the sound to bounce back, and the speed at which sound travels, they can calculate the distance to the bottom of the ocean. This technology can be used to map the topography of the ocen floor.
Section 5: Earthquakes
The forces at plate boundaries cause stretching, pushing and bending responses on large sections of rocks. This can build up energy and when the rock breaks, energy is released (the same way an elastic band releases energy when it snaps after being stretched to its limit). When this energy is released, it causes the earth curst to move resulting in earthquakes.
The point below the earth where an earthquake begins is called the focus. The location on the earth directly above the focus is called the epicenter. People living near the epicenter will feel the earthquake first and most intensely. Earthquakes may include many smaller movements called aftershocks, that can last for days or even weeks.
Earthquakes (and volcanic eruptions) create vibrations that travel through the earth resulting in earthquake waves calles seismic waves.
Understanding Seismic Waves
There are two main types of seismic waves; surface waves and body waves. surface waves occur near the surface of the earth. Tehy travel on the surface of the earth slowly like ripples and are the most destructive waves. Body waves travel deep in the interior of the Earth. There are two types of body waves, primary waves (p waves) and secondary waves (s waves). P waves are the fastest seismic waves. They travel through gases, liquids and solids by pushing and pulling against the material they pass through. During movement, they cause the material to compress and stretch through their pushing and pulling forces. P waves move in the same direction as the shaking rock. S waves are slower than P waves and travel only through solids. They cause the marial to move eithe rup and down, or side to side. Therefore S waves vibrate at right angle to the direction of their movement.
Seismic waves can be detected, measured and recorded using a Seismograph.
The measurements of the seismograph can be used to locate the earthquake's epicenter by measuring how much time it takes for the waves to be detected by various earthquake monitoring stations close to the affected area. To find the location, three stations are needed. The distance is charted around each station in a circle. The point where the three circles intersect is the epicenter.
Earthquake Magnitude
The height of the wave on a seismogram indicates the magnitude of the earthquake and is a measure of the energy released during the earthquake.
There are two measures of earthquakes; The Richter scale and The Mercalli scale. The Richter scale measures earthquakes on a scale of 1-10. An increase of 1 on the scale means a tenfold increase in magnitude. The Mercalli scale rates what people feel and observe so it subjective and not reliable.
We have just described three types of tectonic plate movements and how they can influence the geography of the continents. When two plates slide past each other, they can result in shearing forces that work like the blades of a pair of scissors and can cause rocks to break. At divergent boundaires, plates separate from one another and result in tension forces on the rocks. When tension forces exceed the rock's strength, the rock breaks and forms a fault. A Fault is a break or a crack in the rock of te lithosphere along which movements take place. Faults are frequently located along the boundaries of tectonic plates.
There are three kinds of faults:
- Shearing foreces result in strike-slip fault.
- Tension forces produce normal faults. a normal fault occurs when the rocj above the fault moves down.
- Compression forces result in reverse faults, where the rock above the fault moves up.
Mountains form when plates push against each other. Mountains made up mostly of rock layers folded by being squeezed together are called folded mountains. Mountains mad by huge tilted blocks of rock separated from the surrounding rock by faults are called fault-block mountains. A plateau occurs when a large area of the Earth's crust is pushed upwards forming a flat raised area on the land.
Other Effects of Earthquakes
Some earthquakes happen under the sea. The water displaced by an earthquake can become huge waves called tsunamis.
A tsunami (Japanese for harbor wave) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and other underwater explosions. In the open ocean, tsunamis move at speeds of 500 to 1,000 kilometers per hour. However, a tsunami slows down as it approaches a shore. The length of each wave decreases, but the height increases. The water piles up, and it is often pulled away from the coastline as the tsunami approaches land. Finally, the tsunami crashes onto the shore as a giant wall of water.
Section 6: Volcanoes
Volcanoes are formed by powerful forces within Earth. As one crustal plate moves under another, the rock in the mantle and lower crust melts and becomes magma. Melting rock produces gases that mix with magma. Over time, the gas-filled magma rises, because it is less dense than the solid rock around it. Rising magma can build up in a weak part of overlying rock, forming a magma chamber. Magma chambers are the reservoirs from which volcanic materials erupt. When magma reaches the surface, it erupts through a central opening called a vent. Once magma reaches the surface, it is called lava.
Types of Volcanoes
There are three main kinds of landforms produced by volcanic eruptions.
1. Cinder cone volcano is a landform mainly made up of small rock particles, or cinders. As erupting lava shoots into the air, it breaks into small pieces. These fragments cool and harden as they fall back to the ground. The fragments pile around the vent, forming a cone with steep sides.
2. Shield volcano is a landform made up of many layers of rock. As fluid lava flows out to the surface from a vent, it spreads out in all directions, cools, and hardens into rock. Multiple layers of lava rock build up to form a volcano with broad, gently sloping sides.
3. Composite volcano is a landform made up of layers of thick lava flows alternating with layers of ash, cinders, and rocks. These layers form a symmetrical cone with steep sides that are concave, or curving inward.
Other Volcanic Landforms
An island arc is a string of island volcanoes which occurs when one oceanic plate is driven under another. Part of the sinking plate melts, and magma moves up through the crust along a line parallel to where the plates meet.
Rift volcanoes form where plates move apart and volcanoes form at gaps along the plates’ edges.
Dome mountains form when magma rises and pushes against rock layers above it forming large dome shaped mountains. The black hills of South Dakota are good examples.
If magma hardens in vertical cracks across horizontal layers, a dike forms. When magma hardens between horizontal layers of rock, a flat sill is formed.
Section 7: Mountains
Sedimentary rocks that are placed under slow, gradual pressure can either fold or break. Geologists explain that rocks can fold if they are hot enough to act like bendable plastic. The soft rock may bend into curves. Some of the sedimentary rock can be changed to metamorphic rock during the process of folding. The upward or top part of the folded rock is called the anticline. The bottom of the fold is called the syncline. Over time, both of these can erode, but the folded layers still indicate what has happened.
Sometimes the rocks in Earth’s crust are too brittle to fold. When pressure is exerted on them, they break, forming a fault. A fault can be the result of squeezing or stretching of Earth’s crust. When sedimentary rock is squeezed from the sides, it can form into slabs that move up and over each other like shingles on a roof. This process is called thrust faulting. When tectonic forces stretch Earth’s crust, fault blocks can tilt or slide down. The older rock may end up on top of the younger rock. These huge amounts of rock can form mountains called fault block mountains.
Mountains can be formed by the convergence of continental plates and oceanic plates. The continental plate is lighter and rides over the oceanic plate. Melted rock wells up under the edge of the overriding plate, pushing up mountains. The melted rock can break through the surface and erupt as volcanoes. Usually more than one of these processes occurs. A combination of different processes creates complex mountains.
Altitude: Altitude is a measure of of the height of a place above the sea level. The climate at the base of a mountain is usually warmer than at the peak. Warm air rises up the side of the mountain and as the alititude increases, the temperature reduces. Water vapor in the air condenses to form clouds. As the cloud moves up the side of the mountain, the water droplets become heavy and they fall as precipitation. By the time the air passes to the other side of the mountain, it is dry and cannot cause significant precipitation. so the climate on the other side of the mountain tends to be dry. (Picture credit: Eileen Chontos.)
Section 8: Fossils
Fossils are the remains of ancient organisms preserved in soil or rocks. When an organism dies and are covered in soil, sand or other sediments, the sediments harden over and around the organism's remains. Almost all fossils are found in sedimentary rocks. Scientists can study fossils to understand the characteristics of organisms that lived many years ago. These characteristics may also be used to determine the environmental conditions that were present many years ago.
Sometimes the actual organism or part of it may be preserved as a fossil. These are called original remains. Animals and plants have been found preserved in peat bogs, tar pits, and amber. Woolly mammoths have been found in the Yukon preserved in the ice. Trace fossils are evidence of an animal activity. Worm holes, burrows, and footprints can be fossilized. They provide a fascinating glimpse of ancient life on Earth.
Sometimes an organism falls into soft sediment, like mud. As more sediment falls, the original sediment gradually turns into rock. Water and air pass through pores in the rock, reaching the organism. Its hard parts dissolve, leaving a cavity in the rock called a mould. Other sediments or minerals may fill the hole, hardening into rock and producing a cast of the original object
Fossil fuel is a material that formed from the decay of ancient organisms and is used as a source of energy. For example, decayed parts of ocean organisms were buried deep under the ocean. There, a combination of the weight of rock, heat, and the action of bacteria turned the decayed materials into oil and natural gas. Oil and natural gas also are fossil fuels.
Section 9: Geologic Time
The principle of superposition states that in undisturbed layers of rock, the oldest layers are always on the bottom and the youngest layers are on the top. Unless something happens to move these layers, they stay in their original positions. This is the process that results in stratification described eariler in this chapter, and these layers are referred to as strata.
Scientists can estimate the age of fossils by observing the layers of the sediments. Fossils found on the top layers are more recent, were deposited more recently, than fossils found deeper in the sediments. This is called relative age. The word relative, means it has been estimated in comparison to something else. The Absolute age of a fossil is the actual age in years. It can be estimated by estimating the age of the rock layer where the fossil was found. The age of the rocks is estimated by the amounts of various elements.
A fossil used to determine the relative age of the layer of rock is called an index fossil.
The concentration/amount of certain elements reduces at a contant way. For example, lets say it takes 1 million years for half of element A to change into Element B. After 1 million years, the rock contains equal amounts of element A and B. This time point is called an element's half-life. As mentioned, during the first half life, you will observe equal amounts of Element A and B. During the second half-life, more Elemebt A will be converted to Element B, so by the 2 million year mark, there will be 75% of element B and 25% of element A. at the 3 million year mark, there will be 87.5% of element B and 12.5% of element A. Notice that at every subsequent half-life, the concentration of element A is halfed. Different elements different half-lives.
For example, scientists can use radiocarbon dating, a type of radiometric dating, to find out when recent events in Earth’s history occurred. Radiocarbon dating uses carbon-14, a rare form of carbon, as its parent material. This method is used to find the age of fossils, bones, and wood that are up to 50 000 years old. After 50 000 years, the amount of carbon-14 left in the sample is too small to measure. All living things take carbon14 out of the environment to build cells and tissues. When they die, the carbon-14 changes into nitrogen gas in a half-life of 5730 years. The amount of carbon-14 left in the tissue allows scientists to determine the age of the remains.
Using the relative age of fossils and rocks, scientists estimated that the age of the earth is around 4.6 billion years. during those 4.6 billion years, there has been several eras. An era is a unit of time measured in millions of years. Geological periods divide eras into shorter periods of time where specific major geological events occured.
For the first 4 billion years, there is little fossil evidence. This vast expanse of time is called the Precambrian. Scientists think the earliest supercontinent, Rodinia, formed during this time, about 1.1 billion years ago. Rodinia split apart 750 million years ago, forming the ocean basins. There is evidence that simple forms of life, such as bacteria, algae, fungi, and worms, lived on Earth during the Precambrian. Because the bodies of these creatures were soft, they left little fossil evidence. The three eras that are rich in fossil evidence began 570 million years ago: the Paleozoic Era (ancient life), the Mesozoic Era (middle life), and the Cenozoic Era (recent life). Pangaea, the second supercontinent to form, came together during the Paleozoic Era about 350 million years ago, and broke up about 180 million years ago during the Mesozoic Era. The dinosaurs dominated Earth in the Jurassic Period, which was 200 million years ago during the Mesozoic Era. The fossil evidence indicates that Pangaea first split into a northern portion called Laurasia and a southern portion called Gondwanaland.
Section 10: Fossil Fuels
Fossil fuel is a material that formed from the decay of ancient organisms and is used as a source of energy. For example, decayed parts of ocean organisms were buried deep under the ocean. There, a combination of the weight of rock, heat, and the action of bacteria turned the decayed materials into oil and natural gas. Oil and natural gas also are fossil fuels. Limited deposits of oil and natural gas exist in North America, the Middle East, Indonesia, and Venezuela. Once these deposits are used up, they will be gone. Other examples of fossil fuels include petroleum, coal and bitumen.
Fossil fuels and their products have many uses. Power plants use fossil fuels to produce electricity. People heat their homes and other buildings by burning fossil fuels. Gasoline, a product made from petroleum, fuels cars and other kinds of motorized equipment.
One of the main by-products of fossil fuel combustion is carbon dioxide (CO2). The increasing use of fossil fuels in industry, transportation, and construction has added large amounts of CO2 to the atmosphere contributing to global warming.