Environmental and Safety Engineering

The Earth's structureEarths inside structure is quite different to its hard, crusty shell. We sometimes get a glimpse of Earths interior through the action of active volcanoes. Earths rocky crust is by no means stationary and we regularly see evidence of crust movement in the form of earthquakes. Earthquakes in ocean regions produce destructive ocean waves called tsunamis. The universal acceptance of plate tectonic theory is recognized as a major milestone in the earth sciences. It is comparable to the revolution caused byDarwins theory of evolution or Einsteins theories about motion and gravity. Plate tectonics provide a framework for interpreting the composition, structure and internal processes of Earth on a global scale.The Earth has a layered structure made up of the core, mantle and crust. The lithosphere - the crust and upper part of the mantle - is broken into large pieces called tectonic plates. These move slowly over the mantle.

The Earth is almost a sphere. These are its main layers, starting with the outermost:1. Crust- The Earths crust is the outermost layer, consisting mainly of the chemical elements silicon and aluminum. The crust has two types: a continental crust that varies in thickness between 20 km and 90 km, and an oceanic crust that varies in thickness between 5 km and 10 km. The oceanic crust is denser than the continental crust.

2. Mantle-The mantle is the thickest of Earths layers and takes up 83% of Earths volume. It extends down to about 2900 km from the crust to Earths core and is largely composed of a dark, dense, igneous rock called peridotite, containing iron and magnesium. The mantle has three distinct layers: a lower, solid layer; the asthenosphere, which behaves plastically and flows slowly; and a solid upper layer. Partial melting within the asthenosphere generates magma (molten material), some of which rises to the surface because it is less dense than the surrounding material. The upper mantle and the crust make up the lithosphere, which is broken up into pieces called plates, which move over the asthenosphere. The interaction of these plates is responsible for earthquakes, volcanic eruptions and the formation of mountain ranges and ocean basins. The section on plate tectonic theory later in this topic explains the occurrence of these events further.

3. outer core- made from liquid nickel and iron4. inner core- made from solid nickel and ironThelithosphereconsists of the crust and outer part of the mantle. It is the relatively cold outer part of the Earths structure.

Movement of Earths crust

Plate tectonic theoryPlate tectonic theory is a theory developed in the 1910s by a German meteorologist, Alfred Wegener, who amassed a tremendous amount of geological, paleontological and climatological data that indicated continents moved through time. He proposed the hypothesis of continental drift to explain his data. However, Wegeners theory was not accepted at the time because it could not account for a mechanism by which the huge continental masses move; evidence of a possible mechanism was not found until the 1950s and 1960s. Plate tectonic theory is now universally accepted. The significance of this theory is enormous when you consider that it can account for many seemingly unrelated geological features and events. According to plate tectonic theory, the lithosphere is divided into about a dozen rigid sections, called plates, which move over the asthenosphere, the part of the mantle that behaves plastically and flows slowly (imagine bricks moving over freshly laid mortar).

Firstly, there are three types of plate boundary, each related to the movement seen along the boundary. Divergent boundaries are where plates move away from each other Convergent boundaries are where the plates move towards each other Transform boundaries are where the plates slide past each other.

Divergent Boundaries

Divergent boundary is a linear feature that exists between two tectonic plates that are moving away from each other. These areas can form in the middle of continents or on the ocean floor. As the plates pull apart, hot molten material can rise up this newly formed pathway to the surface - causing volcanic activity.

Iceland is located right on top of a divergent boundary. In fact, the island exists because of this feature. As the North American and Eurasian plates were pulled apart (see map) volcanic activity occurred along the cracks and fissures (see photographs). With many eruptions over time the island grew out of the sea! Question: Why dont we have islands like Iceland where ever we get an Ocean Ridge? Answer: Scientists believe that there is a large mantle plume (an upwelling of hot mantle material) located right underneath where Iceland has formed. This would mean that more material would be erupted in the Iceland area compared with if there was just the divergent boundary without the plume underneath it.

Convergent Boundaries One plate sinks beneath another plate (called subduction) along a subduction zone. The leading edges of the colliding plates may both be oceanic, or one plate may be oceanic (and will be the sinking plate) and the other continental, or both plates may be continental. Where oceanic plates collide, deep trenches in the ocean occur. As the plate descends it melts to generate magma. As this magma rises, it may erupt at Earths surface, forming a chain of volcanoes. Where continental plates collide, mountain ranges such as the Himalayas arise. Mountain chains are also formed where one of the plates is continental and the other is oceanic.

Example: India used to be an island, but about 15 million years ago it crashed into Asia (see map). As continental crust was pushing against continental crust the Himalayan mountain belt was pushed up. Mountains were also pushed down into the mantle as the normally 35 km thick crust is approximately 70 km thick in this region. Mt Everest is the highest altitude mountain on our planet standing 8,840 meters high. This means that below the surface at the foot of the mountain the crust is a further 61 km deep!!

At a convergent boundary where continental crust pushes against oceanic crust, the oceanic crust which is thinner and denser than the continental crust, sinks below the continental crust. This is called a Subduction Zone. The oceanic crust descends into the mantle at a rate of centimeters per year. This oceanic crust is called the Subducting Slab (see diagram). When the subducting slab reaches a depth of around 100 kilometres, it dehydrates and releases water into the overlying mantle wedge (Presenter: explain all of this using the diagram). The addition of water into the mantle wedge changes the melting point of the molten material there forming new melt which rises up into the overlying continental crust forming volcanoes. Subduction is a way of recycling the oceanic crust. Eventually the subducting slab sinks down into the mantle to be recycled. It is for this reason that the oceanic crust is much younger than the continental crust which is not recycled.

Transform Boundaries

Plates slide sideways past each other. The San Andreas Fault in California is a transform plate boundary separating the Pacific plate and North American plate. Sliding plates build up pressure in certain places, causing the sudden movement of plates to release the pressure. The sudden movements of plates are earthquake.

Recoverable or Renewable Resources

Renewable resources are natural resources that can be replenished in a short period of time.

Solar Geothermal Wind Biomass Water

1. Solar energyis radiant light and heat from the sunharnessed using a range of ever-evolving technologies such assolar heating, photovoltaics,solarthermalenergy,solar architecture and artificial photosynthesis. It is an important source ofrenewable energyand its technologies are broadly characterized as eitherpassive solaroractive solardepending on the way they capture and distribute solar energy or convert it intosolar power.

2. Geothermal energy is the heat from the Earth. It's clean and sustainable. Resources of geothermal energy range from the shallow ground to hot water and hot rock found a few miles beneath the Earth's surface, and down even deeper to the extremely high temperatures of molten rock called magma.Palinpinon Geothermal Plant,Negros Philippines.

3. Wind energy or power is extracted fromair flowusingwind turbinesorsailsto produce mechanical orelectrical power.Windmills are used for their mechanical power,wind pumpsforwater pumping, andsailsto propelships. Wind power as an alternative tofossil fuels, is plentiful,renewable, widely distributed,clean, produces nogreenhouse gasemissions during operation, and uses little land.Bangui Wind Farm,Ilocos Norte Philippines.

4. Hydroelectric power, or hydroelectricity, is generated by the force of falling water. (Hydrocomes from the Greek word for water.) Its one of the cleanest sources of energy, and its also the most reliable and costs the least.