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SOL 5.7 – Earth Patterns, Cycles, and Change

The Earth is very old – approximately 4.6 billion years old.

The planet earth was formed about 4.6 billion years ago, after the collapse of the supermassive gaseous body. As time moved on, the earth cooled down and is still cooling, as of today. As a result of the cooling process, denser materials like iron and sulfur sank to the inside of the earth; whereas, lighter materials like silicates and water floated near the earth's surface.

Four Layers of the Earth: Explained

Our planet earth comprises several layers. Starting from the surface, there are four main layers; namely, the crust, the mantle, the outer core, and the inner core. The pressure and temperature increase tremendously when one goes from the outer layers to the inner layers. Let's take a look at each of them individually.

The CrustThe crust is the outermost layer of the earth made up of silicate rock materials. It makes up only about one percent of the earth and is the thinnest layer in comparison to the remaining three layers. Most earthquakes occur in the crust. The thickness and the composition of the earth's crust vary in the land and the ocean. For example; the continental crust is about 32 kilometers thick and composed of lighter materials like granite, quartz, and feldspar, whereas, the oceanic crust measures about 10 kilometers and is mostly made up of basalt.

The MantleThe mantle, the largest layer of the earth, is made up of iron, aluminum, calcium, magnesium, silicone, and oxygen. In fact, most of the earth's mass (about 80 percent) lies in the mantle. The temperature in this layer is estimated to be about 3,700 °C. It is in this layer that volcano magma is present. The overall thickness of the mantle layer is 2900 kilometers. For simple study and understanding, the mantle layer is further divided into the upper and lower sections. Needless to remind, the upper mantle is much cooler than the lower (deeper) section.

The Outer CoreThe outer core can be regarded as a ball of very hot metals. This layer is liquid and made up of iron and nickel. The recorded density is very high, but less than pure molten iron. Hence, scientists are of the opinion that sulfur and oxygen may be present in the outer core. This is because these two elements dissolve easily in liquid iron. The outer core measures 2200 kilometers in thickness and 4,300 °C in temperature. As the earth rotates, the outer core (consisting of iron) spins over the inner core and generates the earth's magnetic field, which is the factor responsible for functioning of the magnetic compass.

The Inner CoreThe inner core, as the name suggests, is the innermost layer of the earth, and is characterized by extremely high temperature (7,200 °C or higher) and pressure conditions. The temperature of the inner core layer is more than the sun's surface. The intense heat reflected from the inner core mobilizes the materials of the outer core and the mantle. It is due to the high pressure that the inner core materials are unable to move, and hence remain solid. The thickness of the inner core is believed to be about 1250 kilometers.

What Types of Rock Are There?

There are three different types of rock:

Igneous Rock is formed when a magma cools underground and crystallizes or when it erupts unto the surface of the ground, cools and crystallizes. Magma that erupts onto the surface is called lava. When magma cools slowly underground the crystals are large enough to see. When it cools quickly on the surface, the crystals are very small and you would need a magnifier or a microscope to see them. Sometimes, when the magma cools very quickly, it forms a kind of black glass that you cannot see through.

Sedimentary Rock forms from particles, called sediment, that are worn off other rocks. The particles are sand, silt, and clay. Sand has the largest particles while clay has the smallest. If there are a lot of pebbles mixed with the sand, it is called gravel. The sediment gets turned into rock by being buried and compacted by pressure from the weight above it. Another way it becomes rock is from being cemented together by material that has been dissolved in water. Often, both cementing and compaction take place together.

Metamorphic Rock is formed by great heat, or pressure, or both. The pressure can come from being buried very deep in the earth's crust, or from the huge plates of the earth's crust pushing against each other. The deeper below the surface of the earth, the higher the temperature, so deep

burial also means high temperatures. Another way that high temperatures occur is when magma rises through the earth's upper crust. It is very hot and bakes the rock through which it moves.

What is the Rock Cycle?

Rocks, like mountains, do not last forever. The weather, running water, and ice wear them down. All kinds of rocks become sediment. Sediment is sand, silt, or clay. As the sediment is buried it is compressed and material dissolved in water cements it together to make it into sedimentary rock.

If a great amount of pressure is exerted on the sedimentary rock, or it is heated, it may turn into a metamorphic rock.

If rocks are buried deep enough, they melt. When the rock material is molten, it is called magma. If the magma moves upward toward the surface it cools and crystallizes to form igneous rocks. This whole process is called the Rock Cycle.

Using The Rock Key

http://www.rockhounds.com/rockshop/rockkey/#Using%20Key

As you use The Rock Key, you will find a lot of links. They are blue and are underlined. Links let you jump from one place to another in the key. The Yes and no choices are links. "Clicking" on them jumps to the next question that you need to answer. If it is the last question to finding the name of the rock, the link jumps to the description of the rock. The numbers for each step in

the key are not important if you are using the key in your web browser. They are included so, if you wish, paper copies may be used.

You can back up to earlier questions you were asked, by clicking Go Back. Of course, you can go back one step to, where you were, at any time by clicking the Back button at the top of your Browser.

In the Mineral Descriptions there is always a part called, Compare To:, with the names of rocks that look a lot like the one you are reading about. The rock names are links. If you click on one, you jump to the description of that rock.

In order to use The Rock Key there are a few things you need to know:

Crystals: Crystals are what minerals form when they are free to grow in nature; like the quartz crystal in the first drawing. In rocks, crystals grow up against each other. They cannot grow as the quartz crystal did in open space. Crystals in rocks have straight edges and they very often

show flat shiny faces that reflect light like tiny mirrors. They look more like the second drawing.

Grains: Grains that are not crystals in rock do not have flat shiny faces. They are rounded, like grain of sand, or jagged, like a piece of broken rock.

Grain Size: Grain size in rocks can mean the size of crystal grains or of fragments:

Coarse Grained: most of the rock is made of grains as largeas rice, or larger. Medium Grained: the individual grains can be seen without a magnifier, but most of the

rock is made of grains smaller than rice. Fine Grained: the individual grains can not be seen without a magnifier (or microscope).

Layers: Layers in rocks show in different ways.

In some rocks different colored minerals are lined up in ribbons.. Usually there are two colors, often black and white, or green and white, of black and tan or pink.

In sandstones, different sized sand grains sometimes show as different colors. When the grains are sorted by running water or wind, they show different shades of the same color.

The layers in slate are very thin and straight. The top and bottom layers are usually flat and quite smooth.

Ribbon like Layers Mica like Layers Particle Layers Thin Cleavage Layers

in Gneiss in Schist in Sandstone in Slate

Gas Bubbles: Gas bubbles in rock are sort of round or elongated holes. In pumice, the bubbles may be very tiny to the size of a match head. They are a glass froth that may look something like a sponge or gray, glassy soap bubbles. In scoria or vesicular basalt, the bubbles are larger, often as large as peas. They look like small pockets in the rock.

What fossils found in the different regions of Virginia tell us The state of Virginia is home to rich variety of fossils, some of which date back to the

Early Cambrian, a time before animals left the oceans. Virginia's fossils indicate drastic changes in the environment over millions of years. Sea levels fluctuated dramatically and entirely submerged the state underwater for long periods. Some of the earliest fossils were species that existed before the Shenandoah

Paleozoic Era During the early Paleozoic era, more than 500 million years

ago, the land that is now Virginia lay submerged under a shallow sea. Common fossils from this era are stromatolites. Sediment covered these structures, eventually fossilizing them and leaving crucial evidence of some of the earliest life forms on the planet.

Cretaceous marine and terrestrial dwelling animal and plant fossils such as the lobed finned fish called the Coelacanth, snails, bivalves, sharks, rays, early marine mammals, dinosaurs and angiosperm or flowering plants can be found in the gravel, sandstone, siltstone and shale rocks located throughout the entire Appalachian Mountain region.

What causes earthquakes and where do they happen?

The earth has four major layers: the inner core, outer core, mantle and crust. The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle pieces tectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

Convergent Boundaries: Here crust is destroyed and recycled back into the interior of the Earth as one plate dives under another. These are known as Subduction Zones - mountains and volcanoes are often found where plates converge. There are 3 types of convergent boundaries: Oceanic-Continental Convergence; Oceanic-Oceanic Convergence; and Continental-Continental Convergence.

Oceanic-Continental ConvergenceWhen an oceanic plate pushes into and subducts under a continental plate, the overriding continental plate is lifted up and a mountain range is created. Even though the oceanic plate as a whole sinks smoothly and continuously into the subduction trench, the deepest part of the subducting plate breaks into smaller pieces. These smaller pieces become locked in place for long periods of time before moving suddenly and generating large earthquakes. Such earthquakes are often accompanied by uplift of the land by as much as a few meters.

Sliding - Sliding Boundaries are where two plates are sliding horizontally past one another. These are also known as transform boundaries or more commonly as faults. Most transform faults are found on the ocean floor. They commonly offset active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. A few, however, occur on land. The San Andreas fault zone in California is a transform fault. The San Andreas is one of the few transform faults exposed on land. The San Andreas fault zone, which is about 1,300 km long and in places tens of kilometers wide, slices through two thirds of the length of California. Along it, the Pacific Plate has been grinding horizontally past the North

American Plate for 10 million years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (on the Pacific Plate) is moving in a northwesterly direction relative to the land on the east side of the fault zone (on the North American Plate).

There are two two reasons for the occurrence of earthquakes –

1. eruption of the volcanoes and 2. disturbance in the tectonic plates of

the earth.

When the molten magma under the earth's crust releases its pressure, it searches for an opening, thereby, exerting enormous pressure on the crust, thus, erupting a volcano. This, in turn, exerts pressure on the tectonic plates too.

How this changes the earth’s surface:

Primarily, it is the lava contained within the volcano that alters the surface of the earth. After a volcanic eruption has taken place, the extremely hot lava that has been released will flow to the earth’s surface, changing the configuration of the earth’s surface upon its landing. This change occurs, as when the lava cools, it sets upon the ground, consequently producing solid rock. This can make the Earth’s surface appear rough and uneven, but it also makes the land more fertile. During this eruption, many materials are released, such as cinder, pumice and ash. Cinder is the released dark coloured pieces of ash; pumice is the bubbly, frothy rock that becomes hardened, and ash is the fine-grained particles.

These pieces of rock and particles, in addition to the lava flows, significantly affect the surface of the earth, as it is these materials that build up to create cones of volcanic mountains.

Depending upon the magnitude of an eruption, the gases released from a volcano can impact on global warming. In turn, this will reduce the amount of solar radiation reaching the earth’s surface, therefore altering the said surface as a result. Ash and water vapour are normally greatly distributed after an eruption, changing not only the appearance but the texture of the Earth’s surface; affecting the inhabitants of that area, too. Moreover, the heat of the erupted lava has been known to cause fires, which can have disastrous effects upon the surface of the Earth.

Furthermore, there are other ways in which a volcanic eruption can alter the Earth’s surface. Volcanoes can trigger mudflows, avalanches, tsunamis in addition to cracks and fissures in the Earth’s surface.

How to identify rocks

A classification key can be a useful tool to determine rock types. Although after identifying hundreds of rocks, geologists and amateurs alike will be able to name a rock on sight, a key can still provide useful information regarding the diagnostic traits of rocks and their mineral content.

Each type of rock -- igneous, sedimentary and metamorphic -- has its own set of defined parameters. They can be identified using various characteristics such as texture, mineral content, grain type and size, and special features.

Interlocking mineral crystals can be different shapes and sizes.

Define the texture of the rock by referring to its general appearance. This will help determine if a rock is igneous, sedimentary or metamorphic.

1. The most fundamental reference is whether or not the rock is made up of interlocking crystals that have grown or fused together, or if the rock has a granular texture. A granular texture, where individual grains of sand or pebbles are seemingly glued together, is indicative of a sedimentary rock. Rocks with interlocking crystals are identified as igneous. The texture of metamorphic rocks can be fine- or coarse-grained and may display a foliated or banded texture.

2.

Sandstone is classified as a coarse-grained sedimentary rock on a classification key.

Assess the grain size of individual mineral crystals. Grain size is indirectly related to texture. The grain size of a rock is an important quality in identifying a rock. A classification table will list rock types according to grain size and shape.

3.

Quartz is easily identified in rocks because of its hardness -- it easily scratches glass.

Identify the mineral content of the rock. This can sometimes prove a bit problematic because the individual crystals may not be easily manipulated within a rock sample, but minerals, like rocks, can be identified by various characteristics.

Some of the parameters to evaluate when identifying the mineral composition include color, streak, luster, hardness and density. Special traits can also provide information on a mineral's identity, such as a mineral's reaction to hydrochloric acid -- calcitic minerals will fizz, or whether a mineral is magnetic.

4.

Marble shows a smooth texture with veins of a darker mineral running throughout.

Check for preferential mineral banding or foliation. This banding of minerals in a rock, or lack thereof, happens only in metamorphic rocks, and the layering of minerals can be an important diagnostic characteristic.

When heat and directed pressure are applied to shale, minerals are allowed to migrate and move. Minerals tend to navigate to their closest relative neighbors and will form layers, giving those forms a foliated or banded texture. Nonfoliated metamorphic rocks, like marbles, will have a smooth texture, with veins of minerals that have migrated together during the metamorphic processes.

5.

The presence of NaCl, or salt, identifies a sample as rock salt.

Look for other special characteristics that may give a clue to a rock's identification. Most classification keys will provide a list of unique traits that can provide insight. Some may be repetitive of the same traits looked at in mineral composition, simply because it is the chemical composition that gives rocks their uniqueness. Some of these qualities include a rock's reaction to hydrochloric acid or whether gas bubbles, called vesicles, are present. Fossils and organic matter pertain to sedimentary rocks. Some can even have a salty taste, like rock salt.

Test 1 - Close Examination Close grains of medium size, off-white color.

Test 2 - Acid Test Fizzing, Carbon dioxide gas given off.

Test 3 - Scratch/hardness test. Rounded grains can be scratched off with a knife