This model was prepared by my students of Don Bosco School as part of their seminar last year. They presented their seminar through powerpoint presentation and models. Well Done!
Saturday, December 27, 2008
Sunday, December 21, 2008
Tonga, South Pacific Ocean, March 18, 2009--An undersea volcano erupts, sending towering columns of volcanic gas and steam into the air (watch video of the volcanic eruption and read full story).The explosion started early Monday morning following a series of earthquakes, and continues to spew Thursday.Tonga's coastal villages, which are located about six miles (ten kilometers) from the blast zone, are thought to be safe, the Associated Press reported
—Photograph by Lothar Slabon/New Zealand Herald/AP
Saturday, December 20, 2008
So, what is an earthquake?
An earthquake (also known as a tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event — whether a natural phenomenon or an event caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments. An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.
•Earthquakes are a result of motion within the earth.
•This only occurs where the earth is solid and therefore can only occur within about 100 miles of the surface.
•Earthquakes provide the best evidence regarding the interior structure of the Earth.
Causes of earthquakes
Earth's crust is composed of many huge, rocky plates known as tectonic plates. These plates constantly move slowly across the surface of Earth, bumping into each other, overrunning each other, and pulling away from each other. When the strain produced by these movements increases beyond a certain level, the pent-up energy ruptures the crust and creates a fracture known as a fault. The released pressure also causes the ground-shaking vibrations associated with an earthquake.
So, my dear students, tell me, what is an earthquake? You have seen the video clippings. Do you know what is it?
Earthquakes are the shaking, rolling or sudden shock of the earth’s surface. They are the Earth's natural means of releasing stress. More than a million earthquakes rattle the world each year. The West Coast is most at risk of having an earthquake, but earthquakes can happen in the Midwest and along the East Coast. Earthquakes can be felt over large areas although they usually last less than one minute. Earthquakes cannot be predicted - although scientists are working on it!
LET US RECALL THE POINTS:
There are about 20 plates along the surface of the earth that move continuously and slowly past each other. When the plates squeeze or stretch, huge rocks form at their edges and the rocks shift with great force, causing an earthquake. Think of it this way: Imagine holding a pencil horizontally. If you were to apply a force to both ends of the pencil by pushing down on them, you would see the pencil bend. After enough force was applied, the pencil would break in the middle, releasing the stress you have put on it. The Earth's crust acts in the same way. As the plates move they put forces on themselves and each other. When the force is large enough, the crust is forced to break. When the break occurs, the stress is released as energy which moves through the Earth in the form of waves, which we feel and call an earthquake.
Ok, let's go through the lesson again!
•Large continents begin to crack and split apart
•The gaps fill with water
•Small seas become oceans
•The mid ocean ridge continues to produce new crust
Why is the Atlantic still getting wider?
•The plates are pulled apart by convection currents in the mantle below .
•Caused by heat released from natural radioactive processes
•At the mid Atlantic ridge molten rock from below rises up to fill the gap with new basaltic rock
•More evidence has been found for Wegeners theory
•The Earths North and South pole have flipped many times
•These leaves magnetic ‘stripes’ in rock containing iron minerals
•The friction melts rock .
•Eventually when plates move together the continental crust collides
•The heat and pressure make metamorphic rock
•It also pushes and folds the rocks into high mountains
•The Himalayas rise to 8848m and are still growing today
What is an earthquake and what causes them to happen?
Ans: An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake. An EQ occurs when plates grind and scrape against each other. In California there are two plates the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. These primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone. The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great EQs when it finally releases.
Q: Can we cause earthquakes? Is there any way to prevent earthquakes?
Ans: Earthquakes induced by human activity have been documented in a few locations in the United States, Japan, and Canada. The cause was injection of fluids into deep wells for waste disposal and secondary recovery of oil, and the use of reservoirs for water supplies. Most of these earthquakes were minor. The largest and most widely known resulted from fluid injection at the Rocky Mountain Arsenal near Denver, Colorado. In 1967, an earthquake of magnitude 5.5 followed a series of smaller earthquakes. Injection had been discontinued at the site in the previous year once the link between the fluid injection and the earlier series of earthquakes was established.
Other human activities, even nuclear detonations, have not been linked to earthquake activity. Energy from nuclear blasts dissipates quickly along the Earth's surface. Earthquakes are part of a global tectonic process that generally occurs well beyond the influence or control of humans. The focus (point of origin) of earthquakes is typically tens to hundreds of miles underground. The scale and force necessary to produce earthquakes are well beyond our daily lives. We cannot prevent earthquakes; however, we can significantly mitigate their effects by identifying hazards, building safer structures, and providing education on earthquake safety.
Q: What are plate tectonics?
Ans: Plate tectonics is the continual slow movement of the tectonic plates, the outermost part of the earth. This motion is what causes earthquakes and volcanoes and has created most of the spectacular scenery around the world.
Q: What is a fault and what are the different types?
Ans: A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse, depending on their motion. Faults that move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults, which show both dip-slip and strike-slip motion are known as oblique-slip faults.
The following definitions are adapted from The Earth by Press and Siever.
Normal fault- a dip-slip fault in which the block above the fault has moved downward relative to the block below. This type of faulting occurs in response to extension and is often observed in the Western United States Basin and Range Province and along oceanic ridge systems.
Thrust fault- a dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block. This type of faulting is common in areas of compression, such as regions where one plate is being sub ducted under another as in Japan. When the dip angle is shallow, a reverse fault is often described as a thrust fault.
Strike-slip fault - a fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.
A left-lateral strike-slip fault is one on which the displacement of the far block is to the left when viewed from either side.
A right-lateral strike-slip fault is one on which the displacement of the far block is to the right when viewed from either side.
Q: At what depth do earthquakes occur?
Ans: Earthquakes occur in the crust or upper mantle, which ranges from the earth's surface to about 800 kilometers deep (about 500 miles).
Q: What is "surface rupture" in an earthquake?
Ans: Surface rupture occurs when movement on a fault deep within the earth breaks through to the surface. NOT ALL earthquakes result in surface rupture.
Q: What is the relationship between faults and earthquakes? What happens to a fault when an earthquake occurs?
Ans: Earthquakes occur on faults - strike-slip earthquakes occur on strike-slip faults, normal earthquakes occur on normal faults, and thrust earthquakes occur on thrust or reverse faults. When an earthquake occurs on one of these faults, the rock on one side of the fault slips with respect to the other. The fault surface can be vertical, horizontal, or at some angle to the surface of the earth. The slip direction can also be at any angle.
Q: How do we know a fault exists?
Ans: if the earthquake left surface evidence, such as surface ruptures or fault scarps (cliffs made by EQs).
if a large EQ has broken the fault since we began instrumental recordings in 1932.
if the faults produces small earthquakes that we can record with the denser seismographic network established in the 1970s.
Q: Where can I go to see the/a fault?
Ans: The closest fault depends on where you live. Some earthquakes produce spectacular fault scarps, and others are completely buried beneath the surface. Sometimes you may not even know that you are looking at a fault scarp.
Q: What does an earthquake feel like?
Ans: Generally, during an earthquake you first will feel a swaying or small jerking motion, then a slight pause, followed by a more intense rolling or jerking motion. The duration of the shaking you feel depends on the earthquake's magnitude, your distance from the epicenter, and the geology of the ground under your feet. Shaking at a site with soft sediments, for example, can last 3 times as long as shaking at a stable bedrock site such as one composed of granite. If the site is in a building, then the height of the building and type of material it is constructed from are also factors. For minor earthquakes, ground shaking usually lasts only a few seconds. Strong shaking from a major earthquake usually lasts less than one minute.
Q: Foreshocks, aftershocks - what is the difference?
Ans: "Foreshock" and "aftershock" are relative terms. Foreshocks are earthquakes, which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes, which occur in the same general area during the days to years following a larger event or "mainshock", defined as within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. As a general rule, aftershocks represent minor readjustments along the portion of a fault that slipped at the time of the main shock. The frequency of these aftershocks decreases with time. Historically, deep earthquakes (>30km) are much less likely to be followed by aftershocks than shallow earthquakes.
Q: Two earthquakes occurred on the same day. Are they related?
Ans: Often, people wonder if an earthquake in Alaska may have triggered an earthquake in California; or if an earthquake in Chile is related to an earthquake that occurred a week later in Mexico. Over these distances, the answer is no. Even the Earth's rocky crust is not rigid enough to transfer stress fields efficiently over thousands of miles.
Words to Know
Epicenter: The location where the seismic waves of an earthquake first appear on the surface, usually almost directly above the focus.
Fault: A crack running through rock that is the result of tectonic forces.
Focus: The underground location of the seismic event that causes an earthquake.
Modified Mercalli scale: A scale used to compare earthquakes based on the effects they cause.
Richter scale: A scale used to compare earthquakes based on the energy released by the earthquake.
Seismic waves: Classified as body waves or surface waves, vibrations in rock and soil that transfer the force of the earthquake from the focus into the surrounding area.
Seismic waves traveling along Earth's surface are called surface waves or L waves (long). The two main types, Rayleigh waves and Love waves, are named after two prominent seismologists (scientists who study earthquakes). Although surface waves move slower than body waves—less than 2 miles (3.2 kilometers) per second—they cause greater damage. Rayleigh waves cause the ground surface in their path to ripple with little waves. Love waves move in a zigzag along the ground. Both Rayleigh and Love waves set off avalanches, landslides, and other earthquake damage.
Tuesday, December 16, 2008
Naturally-occurring mixtures of minerals, mineraloids, glass or organic matter.
- The crust of the Earth is made up of rocks of various types.
Term ‘rock’ refers not only to hard materials like granite but also to soft and loose particles like sand, silt and clay .
-Rocks are made up of minerals in different combinations
- Minerals are naturally occurring solid inorganic substances having definite chemical composition and physical properties.
- Minerals are generally crystalline in appearance.
- Are homogeneous in form while rocks are heterogeneous in their composition.
- Are made up of chemical elements.
- About 2,000 minerals are known to exist in the crust
- These combine in different proportions to form various kinds of rocks.
A rock is a naturally occurring, solid aggregate of minerals. It consist of all the materials that make up the Earth’s surface, whether they are solid granite boulders, soft clay, solid gravel or combustible coal.
-Major part of earth’s crust – formed by rocks and minerals
-Minerals have definite chemical composition by which they can be identified.
-ROCK – mixture of various minerals
- An aggregate of minerals that forms a more or less definite unit of the lithosphere.
-Rock is a consolidated and compact mass of more than two minerals
- Rocks differ from each other in colour, texture or origin.
- On the basis of their mode of formation, rocks can be classified as:
1. Igneous Rocks (made by “fire”) - Solidified from molten rock (i.e., magma).
2.Sedimentary Rocks - Deposited and buried at Earth’s surface.
3. Metamorphic Rocks (“changed form”) - Transformed from preexisting rocks under high pressure and temperature.
Mineralogy - Constituent minerals and their relative proportions.
Texture - Sizes, shapes, and arrangements of minerals within the rock, e.g.,
–Foliated (planar fabric)
All are clues to a rock’s origin and history.
What are They?
* Fire Rocks
* Formed underground by trapped, cooled magma
* Formed above ground when volcanoes erupt and magma cools
- “Ignis” means fire in Latin
Agni - Sanskrit
- Rocks that are formed from cystallization of magma
- Magma is molten rock
–Lava is magma that is on the Earth’s Surface
So, Igneous rocks are :-
- Formed from the cooling of either magma or lava
The most abundant type of rock
Classified according to their origin and composition.
What is an igneous rock?
An igneous rock is a rock that is created by a volcano. Some rocks cool quickly and are very shiny.
-Other rocks cool as they fly through the air and release lots of air, causing lots of holes.
IGNEOUS ROCKS ARE:
-Are of thermal origin and are formed by the process of solidification of molten rock material called magma.
- Such magma erupts during volcanic eruptions and on reaching the Earth’s surface, gets solidified by cooling.
- So known as primary rocks
- Magma may get solidified below the surface of the earth.
- COOLING and SOLIDIFICATION are 2 processes involved in the formation of igneous rocks.
- Form by solidification (crystallization) of melted minerals
- At the surface, LAVA hardens to form EXTRUSIVE rocks with tiny (FINE-GRAINED) crystals or GLASSY (no crystal) TEXTURES
- Beneath the surface, MAGMA hardens to form INTRUSIVE rocks with easily visible (COARSE-GRAINED) crystal texture.
ORIGIN— Where rocks are formed?
- Below ground = from magma (intrusive igneous rock)
- Usually have LARGE crystal grains (they cooled slowly)
Where are igneous rocks formed?
Igneous rocks form when magma pours out of a volcano as lava and cool.
TYPES OF IGNEOUS ROCKS
Minerals crystallize from melt, derived from deep within Earth’s crust or mantle
–High temperatures, up to 700° C or more!!
–Crystal size depends on cooling rate.
uIntrusive rocks cool slowly within deep magma chambers:
–Course, interlocking crystals
uExtrusive rocks cool rapidly at (or near) the surface of the earth:
–Fine-grained, often “glassy”
1. INTRUSIVE IGNEOUS ROCKS:
- When molten materials of earth’s interior (magma) do not reach earth’s surface, they cool and solidify below the surface – are called intrusive igneous rocks
Eg. Granite, diorite, gabbro
- Called Plutonic rocks
- Have larger crystals, compact, resisten and glassy in appearance
- E.g. Quartz, batholith
Rate of cooling is slower.
Been formed at great depths..
A. SILLS: some of the lava may push its way through passages in the form of sills or dykes.
uWhen magma is being forced upwards, some of the lava may push its way in between two layers of rock and get solidified there.
uThe intrusive rock formed – called sill – almost horizontal
B. DYKES: Magma also forces its way before reaching the main fissure, trying to go up but gets solidified before reaching the surface.
- These intrusive rocks lying in a slanting direction as an off shoot are called dykes.
2. EXTRUSIVE IGNEOUS ROCKS:
- When lava pours out on Earth’s surface through volcanic eruptions or fissures, it cols and soldifies to form extrusive igneous rocks.
- Rate of cooling is more rapid than in the interior. So crystals are finer.
According to chemical composition:
1.ACIDIC IGNEOUS ROCKS: acidic rocks are light coloured , less dense. Granite , crystals, feldspar, mica
2.BASIC IGNEOUS ROCKS: contain lower percentage of silica, higher percentage of oxides of denser elements – iron, aluminium, or magnesium. Basic rocks are denser, darker in colour than acidic rocks.
3.ULTRA BASIC ROCKS: consists of ferre, magnesium
–Does a rock melt like an ice cube, all at the same time?
Rocks melt according to their melting points.
–Example: Ice cube with wax
Which would melt first ice or wax?
This example is known as partial melting.
–Partial Melting: some minerals melt at lower temperatures and other minerals remain solid
–Think of “stew”
Derived from Latin word – sedimentum=settling down
Denudation is the disintegration & decomposition of rocks, as well as the wearing away of the surface of the land.
SEDIMENTARY ROCKS: are the secondary rocks which are formed from the loose fragments detrital or clastic sediments produced by weathering of older rocks.
•Almost 90% of earth crust is made up of igneous rocks
•75% of land surface on the earth is covered by thin veneer of sediments or sedimentary rocks.
•These sediments are transported and deposited by river water, wind or by movement of glacial ice. Transportation is either in suspension or in solution.
•When settle down on the beds of ocean, river and lakes undergo compaction/cementation for millions of years to form SEDIMENTARY ROCKS
•These consolidated layered strata are known as stratified rocks.
•The strata varies in thickness and may be tilted or horizontal.
•Sediments consist of loose particles or gravel, sand, silt and clay in various proportions – hence – porous and permeable.
•Loose particles get consolidated or compacted into hard rocks by presence of cementing substances like lime or the presence of overlying deposits.They are separated by bedding planes.
•Lithification is the cementation, compaction and hardening of sediments into sedimentary rocks
•Called secondary rocks – they are derived by denudation of other pre-existing primary or parent rocks.
•Called stratified rocks – as sediments are deposited in waterbodies, they get sorted out according to their size.
•Sediments accumulate in different layers or stata arranged one above the other
•Each layer or stratum has particles of a given size.
•Presence of remains of plants and animals between layers of sediments
•These remains get preserved between strata of sedimentary rocks.
•These organic remains or their skeletal impressions are called fossils.
•Fossils help us in fixing the relative ages of rocks.
IMPORTANCE OF SEDIMENTARY ROCK:
“Present is the key to the past”
•Helps in knowing depositional environment viz. marine (ocean deposits), fluvial (river deposits), aeolian (wind deposits), glacial, estuarine, Lacustrine (lake deposits) etc.
•Helps in knowing the provenance (i.e. source area of the sediments); change in climatic conditions i.e. in knowing and understanding old climate=paleoclimate.
•Sedimentary rocks are subject-divided on the basis of the nature of sediments, origin, composition and mode of formation.
Sedimentary rocks may be made of rock fragments—sediments—or by chemical reactions. The classification of sediments is shown below.
Wind and water break down the earth
* Bits of earth settle in lakes and rivers
* Layers are formed and build up
* Pressure & time turn the layers to rock
Formed from sediments (rock fragments, mineral grains, animal & plant remains) that are pressed or cemented together or when sediments precipitate out of a solution.
These sediments are moved by wind, water, ice or gravity.
Sedimentary rocks represent 7% of the Earth’s crust, but they cover 70% of the Earth’s surface.
Sedimentary rocks are fossil-carrying rocks.
What turns sediments into solid rock?
Water or wind breaks down and deposits sediment (erosion & deposition)
The heavy sediments press down on the layers beneath (compaction)
Dissolved minerals flow between the particles and cement them together (cementation)
How can sedimentary layers help us understand the age of fossils?
As sedimentary rocks are deposited, they form horizontal layers
Scientists know that the layers on top (and the fossils in the top layer) are YOUNGER than the fossils in lower layers.
Limestone made when calcite mineral precipitates from sea water
Rock Salt—made from evaporation of sea waters
Minerals precipitate from dissolved chemicals in water
–Chemical & Biochemical Sediments
All are the products of Weathering - that breaks up and decays rocks, and Erosion - that transports from source to point of deposition
What is a sedimentary rock?
A sedimentary rock is a rock that is made of layers of sediment (sand, clay, mud) close to the earth’s surface.
Where are sedimentary rocks formed?
Sedimentary rocks are formed under rivers, lakes, oceans, or streams.
Where are sedimentary rocks formed?
Sedimentary rocks are formed under rivers, lakes, oceans, or streams.
Biologic sedimentary rocks come from the remains of organic matter.
The most important of these is coal. Anthracite coal results from the greatest pressure and releases the most energy when burned. Other varieties are bituminous and lignite. “Petrified” (permineralized) wood is another organic rock.
Rocks that have changed
They were once igneous or sedimentary
Pressure and heat changed the rocks
Rocks that have changed due to intense temperature and pressure
“Meta” means “change” and morphosis means “form” in Greek
Igneous, sedimentary and other metamorphic rocks can change to become metamorphic rocks
High temperatures and pressures at depth cause changes in mineralogy, texture, and composition
–Changes take place in Solid State by recrystallization and chemical reactions
–Temperatures greater than 250°, less than 700°
Regional Metamorphism - High pressures and temperatures derive from regional collision, deformation and mountain building.
Contact Metamorphism - Locally high temperatures, adjacent to intrusions.
Where magma intrudes relatively cool rock
Near colliding plates (near mountain ranges)
Places that are covered miles thick with other rock causing pressure
When hot water intrudes rock
Where a meteorite strikes Earth (rare)
Where lightning bolts strike rocks (rare)
A metamorphic rock is any sedimentary or igneous rock that has been changed, or morphed, because of pressure and heat.
These rocks are made deep inside the earth where heat and pressure change the rocks .
What occurs in the Earth to change these rocks?
* Pressure from overlying rock layers
* High heat, but not enough to melt the rock
* Rocks may be flattened or bent or atoms may be exchanged to form new minerals.
*You can think of metamorphic rocks as a squished peanut butter & jelly sandwich in your lunch.
How can Starbursts represent the Rock Cycle? Which rock form does your stacked Starbursts represent?
Now press your sedimentary rock in the palm of your hands for at least 2 minutes (do not twist)
Some have large & small crystals (called porphyritic)
* Above ground = from lava (extrusive igneous rock)
* Usually have SMALL or NO crystals (they cooled too quickly)
Basalt Igneous Rocks —made from lava/magma that is low in silica, rich in iron and magnesium. Rocks are dark-colored.
Granite Igneous Rocks—made from magma/lava high in silica and oxygen. Rocks are light-colored.
Andesitic Igneous Rocks—have a composition between basaltic and granitic.
Practice Classifying Igneous Rocks according to their composition:
Once a rock is formed, does it stay the same rock forever?
- Rocks are continually changed by many processes, such as weathering, erosion, compaction, cementation, melting, and cooling
- Rocks can change to and from the three types
What is the process through which rocks change?
The Rock Cycle—earth materials change back and forth among the different types of rocks
The parent material of all rocks are igneous rocks. As soon as the igneous rocks come out of the surface of the Earth, they are eroded by different agents of eroison. The rock material changes into sediments. The sediments deposit in layers at some place to sorm sedimentary rocks. These sedimentary rodks are again buried into the Earth due to forces of the Earth. If these sedimentary rocks go very deep in the Earth they melt, change into lava and again come out as igneous rocks. If these sedimentary rocks donot go very deep, they may change into metamorphic rocks or igneous rocks which are again converted into sediments whenever they come out of the surface of the Earth. In this way, a cyclic process of changing the form of rocks is formed. This is called Rock Cycle.