Wednesday, 20 August 2014

Mass Movement

Mass Movement


Mass wasting does not rely on a carrier material like water, ice or wind. It is due to the force of gravity on an object at rest.
An object at rest stays at rest unless acted upon by a force. Once it is acted upon it will stay in motion until some equal and opposite force stops it.
When an object is placed on a flat surface, the only force acting on it is gravity. As the ground plane is inclined, the force of gravity can be shown to act in two directions. The force which keeps the object from moving is friction. Friction acts like a force opposing any movement.
As the incline increases the force parallel (book calls it tangential) with the surface grows and eventually becomes greater then the frictional force. This force is called the sheer stress. When this happens the object begins to move.
The perpendicular force component acts to keep the body in place. This force decreases as the slope increases. Another force that aids in keeping the body in place is called sheer strength. As long as the sheer strength remains higher than the sheer stress the body will remain motionless.

Capillary Action

Water in small quantities can act like a gluing agent. Through a force called surface tension, water can form a temporary bond between surfaces. If to much water is added the bond is weakened and disappears, and the water then acts more like a lubricant.
When a slope saturated with water receives more water, the added pressure may cause a catastrophic failure.

Tuesday, 19 August 2014

Metamorphic Rocks

Metamorphic Rocks

Greek: "meta" = change , "morph" = form

Metamorphic rocks by definition are formed by a change in another rock. Metamorphism can only occur inside the earth and not at the earth's crust. The changes are caused by heat, and pressure on an already existing rock.
Metamorphic rocks must change without melting, they are solid state transformations. Why? Because if they melted and recrystallized they would be igneous!
Since the changes are done in the solid state, the textures and components in the rock often store historical information about the processes they have undergone. By cataloguing metamorphic rock types and determining the conditions under which they were formed, geologist are attempting to find ancient continental boundaries.

Boundaries Of Metamorphism

Metamorphic changes are defined as changes that happen with temperature over 100 C and over a few 100 Mpa. The upper limit is easily defined as the melting point of the rock. Within the range of possible metamorphic activity are two types of metamorphic change.As we learned in the section on igneous rocks, melt temperature is directly relate to the amount of water (H2O) in the rock. If there is limited water in a rock, then initial melting will be limited to isolated areas in the rock. If a metamorphic rock is found with small areas of partial melting, then it is called a migmatite.
If more material melts, the loss in density and the migration of the melt can produce a basolith or other igneous intrusion. If the original material was metamorphic, then this combination is indicative of a subduction zone or a collision margin. When we find a large intrusive body with surrounding metamorphic rock we know the area was sometime in its history along a plate boundary.

Things that effect the metamorphic process

TemperaturePressureInitial compositionPresence fluidsTime

Sunday, 17 August 2014

Sedimentary Rocks

Sedimentary Rocks

Gravity Rules!The most important thing to know about sedimentation is that "it's a down hill process." Sedimentation" is the deposition of materials in an orderly fashion.Stratification is the arrangement of sediment in distinct layers. As with most things studied in geology they are further subdivided by physical or chemical characteristics.
Multiple layers are referred to as bedding, a single layer is a bed, and the contact points are the bedding planes.

Clastic Sediments                                                

Starting MaterialSedimentary Rock
Loose fragments of rock or mineral that are broken down by mechanical (physical) processes are called detritus. It is also known as clastic sediment. Each individual particle is known as a clast.
These materials may be of any size. From boulders to microscopic clay particles. The table above shows the starting material and the resultant type of sedimentary rock they form.
The first two images to the left show classic sandstones. The close-up image reveals small rounded stones that appear to be glued together. The majority of the stones are transparent to translucent and are likely quartz. Quartz being very resistant to weathering. There may also be some feldspar in the material.
There is a natural process in which the various size grains are classified (separated) into similar size pieces. These then define some of the types of sediment and sedimentary rocks.
Sorting is the measure of the size RANGE of particles in sediment. If the range is narrow the sediment is termed "well sorted."
Sorting is a function of the speed of the transport medium. A fast moving river tends to move larger particles as well as smaller. There are natural places where additional sorting may happen, like a sharp bend in the river. Centrifugal force within fans the debris outward around the corner, and increase the separation by size.
Weight also plays a role in separation. Heavy materials like gold or platinum will sink rapidly and move less. Materials with similar specific gravity will be sorted by size rather than by weight, and hence quartz and feldspars are sorted in this manner.
Enrichment of one mineral over all others is a function of the minerals characteristics and strength. Basically is resistance to weathering. Since quartz is very high on this scale it is often the final survivor. It is "durable".
Particle shape can also be used to help identify some types of strata. There are two similar but different features. First there is "roundness", which describes the wear at a particles corners. If the particle has no sharp corners or crystal structure reaming it is said to be rounded. The overall shape may be thin and long, or it may be roughly equi-dimensional. Rounding is related to the fine structure of the particle and not the overall shape.
If a particle has become more spherical in shape, more equi-dimensional, then you are viewing its sphericity. Sphericity is the measure of the overall shape of the particle.
The farther a particle travels, the more rounded it will become.



The chemical or physical alteration of rock by water, air, or organic matter. Weathering is the process by which rock is converted to regolith. (The irregular blanket of lose material cover the earth.)
Physical weathering is the breaking down of rock by physical means, there is no chemical change, rather there is size reduction while maintaining the same original chemistry.
Chemical weathering, is the reaction of the mineral material and its conversion from one composition to another. The rocks most susceptible to chemical weathering are those that were farthest from the conditions present on the earths surface. (What does that mean? - Rocks formed at high temperature and under high pressure are more susceptible to chemical weathering than rocks formed near or at the surface.)

Physical Weathering .. causes of:
1.) Water (freeze thaw cycle)2.) Salt growth3.) Fire4.) Plant Growth
Water has a rather unique property, when it freezes to a solid it increases in volume rather than decreases. Ice takes up about 9% more volume than the same amount of liquid water.
It's a cyclic process, water enters a crack or joint, freezes and causes the joint to expand. More water can enter the expanded joint, freezing again, and causing further joint expansion.
Enough expansion in a joint will cause the rock to crack, and this is the main process for creating rock debris on higher mountains.

Volcanoes and Magma

Volcanoes And Magma

Magma is molten rock and the other materials contained within: gases, liquids, solids. Magma reaching the earth's surface is expelled through a volcano. It is a vent from lower regions of the earth.
Lava is a river of molten magma. Magma may also be explosively expelled and be in the form of super heated gases and tiny hot particles.
As with most things in geology, magma too can be characterized and compartmentalized. Three distinct types of magma are more common than all others and are described by the following:
Composition and in particular percentage silica.
Dissolved gases in magma are very important in determining the physical characteristics of the magma. Even at low concentrations 0.2 - 3% they have tremendous effect.
The main gases are H2O, and CO2 (98%)gases that sometimes achieve 1% are SO2, HCl, N2 and Ar.

Temperature of magma is hard to measure, first it is not easy to get near molten magma, secondly the magma cools quickly when removed from its heat source, and thirdly the sensor must survive the measurement.
Magmas tend to be in the range of 1000 C - 1400 C
Viscosity is the measure of flow of a magma, most magmas do not flow very fast, but a few have been clocked at speeds in excess of about 10-15 mph. The more viscous a magma is the slower it flows.
A good model for magma coming upward is a bottle of soda pop shaken well then quickly opened. The sudden release of pressure and extra energy placed into the liquid from shaking causes some of the gas in the liquid to come out of solution. This gas being many times less dense than the surrounding liquid rushes to get out of the bottle carrying with it a good deal of the liquid.
We talked about convection in an earlier chapter, and it is convection set up by the heated magma which makes it begin to rise. As a magma gets hotter, it becomes less dense than surrounding cooler rock and begins to rise. If it maintains its heat, it becomes lighter and lighter the further it rises, and as it rises the external pressure also decreases. This makes the escaping gas form bubbles and then the gas expands or builds pressure even faster.

Basaltic magmas tend to move more slowly and have less gas dissolved, so their eruptions tend to be less violent. At the beginning of an eruption there is fast out pouring of gas and fountains are often created for a time. Eventually the magma reaches more of a steady state and issues forth smoothly and without a great deal of energy.

Since it is extremely hot, yet only slightly above the crystallization point of the minerals present within it, it will "skin" rapidly. The outside cools quickly and forms a rock coating. This coating acts like a thermos bottle and maintains the heat inside. This may create a lava tube, a conduit for hot molten lava to travel great distances while maintaining much of its heat.
As it continues to cool and loses more gas, it forms thin tube like structures called "pahoehoe", and eventually slows almost to a halt, it forms rough aa (pronounced ah-ah) flows that barely move. They look a little like black popcorn falling down a very slow moving embankment.

Crust Of The Earth

Crust Of the Earth

Igeology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle. The crusts of Earth, the MoonMercuryVenusMarsIo, and other planetary bodies have been generated largely by igneous processes, and these crusts are richer in incompatible elements than their respective mantles.
The crust of the Earth is composed of a great variety of igneousmetamorphic, and sedimentary rocks. The crust is underlain by the mantle. The upper part of the mantle is composed mostly of peridotite, a rock denser than rocks common in the overlying crust. The boundary between the crust and mantle is conventionally placed at the Mohorovičić discontinuity, a boundary defined by a contrast in seismic velocity. The crust occupies less than 1% of Earth's volume. 
The oceanic crust of the sheet is different from its continental crust. The oceanic crust is 5 km (3 mi) to 10 km (6 mi) thick and is composed primarily of basaltdiabase, and gabbro. The continental crust is typically from 30 km (20 mi) to 50 km (30 mi) thick and is mostly composed of slightly less dense rocks than those of the oceanic crust. Some of these less dense rocks, such as granite, are common in the continental crust but rare to absent in the oceanic crust. Both the continental and oceanic crust "float" on the mantle. Because the continental crust is thicker, it extends both above and below the oceanic crust. The slightly lighter density of felsic continental rock compared to basaltic ocean rock contributes to the higher relative elevation of the top of the continental crust. Because the top of the continental crust is above that of the oceanic, water runs off the continents and collects above the oceanic crust. The continental crust and the oceanic crust are sometimes called sial and sima respectively. Because of the change in velocity of seismic waves it is believed that on continents at a certain depth sial becomes close in its physical properties to sima, and the dividing line is called the Conrad discontinuity.

Outer Core

Outer Core

The outer core of the Earth is a liquid layer about 2,260 km thick composed of iron and nickel which lies above the Earth's solid inner core and below its mantle. Its outer boundary lies approximately 2,890 km (1,800 mi) beneath the Earth's surface]. The transition between the inner core and outer core is located approximately 5,150 km beneath the Earth's surface.
The temperature of the outer core ranges from 4400 °C in the outer regions to 6100 °C near the inner core. Eddy currents in the nickel iron fluid of the outer core are believed to influence the Earth's magnetic field. The outer core is not under enough pressure to be solid, so it is liquid even though it has a composition similar to that of the inner core. Sulfur and oxygen could also be present in the outer core.
Without the outer core, life on Earth would be very different. Scientists believe that convection of liquid metals in the outer core create the Earth's magnetic field. This magnetic field extends outward from the Earth for several thousand kilometers, and creates a protective bubble around the Earth that deflects the Sun's solar wind. Without this field, the solar wind would have blasted away our atmosphere, and Earth would be lifeless like Mars.