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461 GPH Exmas


Solid Earth Geophysics part I

Midterm exam. 20 points

1)      Mark each statement true or false, as appropriate.                                          (3/20)



Lithosphere is made of the crust and upper mantle


The speed of the tectonic plate motion is less than one meter per year


The size of the Earth does not expand because the Earth’s crust expands along the oceanic ridges but shrinks and descends into the oceanic trenches



2)      Write in details about the atmosphere layers, and main cause of ozone depletion. (7/20)

3)      Explain in details the three primary types of Tectonic Plate

boundaries. (10/20)


Solid Earth Geophysics part I

Midterm exam. Typical Answers

1)      Mark each statement true or false, as appropriate.                                          (3/20)



Lithosphere is made of the crust and upper mantle


The speed of the tectonic plate motion is less than one meter per year


The size of the Earth does not expand because the Earth’s crust expands along the oceanic ridges but shrinks and descends into the oceanic trenches



2)      Write in details about the atmosphere layers, and main cause of ozone depletion. (7/20)

The Troposphere:

The Troposphere is the lowermost portion of Earth's atmosphere and the one in which most weather phenomena occur. The greenhouse effect also occurs in the troposphere.

The troposphere starts at the earth's surface and extends to an altitude of 16-18 km over tropical regions, decreasing to less than 10 km over the poles. This layer contains approximately 80% of the atmosphere's total mass. Generally, jets fly near the top of this layer. The troposphere is directly below the stratosphere.


The word troposphere stems from the Greek "tropos" for "turning" or "mixing". This region, constantly in motion, is the densest layer of the Earth's atmosphere. Nitrogen and oxygen are the primary gases present in this region.


The lapse rate, which is the change of temperature with respect to height, is larger than in other layers, the temperature decreasing at middle latitudes from approx. +17°C at sea level to approx. -52°C at the beginning of the tropopause. At the poles, the troposphere is thinner and the temperature sinks only to -45 °C, while at the equator the temperature can reach -75 °C.


The reason for the temperature variations in the troposphere is because the temperature is determined by the radiation from the land back into the air. As we move away from the earth's surface, convective heating has a smaller effect and the air cools.


For every 1000 meter increase in altitude, the temperature goes down by approximately 6.4°C. This is because the higher the altitude, the less atmospheric particles there are to trap the heat, therefore resulting in the heat escaping.

The tropopause marks the limit of the troposphere and the beginning of the stratosphere. The temperature above the tropopause increases slowly with height up to about 50 km.


2- The Stratosphere:

The stratosphere is a layer of Earth's atmosphere that is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The stratosphere is situated between about 10 km and 50 km altitude above the surface at moderate latitudes, while at the poles it starts at about 8 km altitude. The stratosphere sits directly above the troposphere and directly below the mesosphere.


The stratosphere is layered in temperature because it is heated from above by absorption of ultraviolet radiation from the Sun. Within this layer, temperature increases as altitude increases; the top of the stratosphere has a temperature of about 270 K, about the same as the ground level temperature. This top is called the stratopause, above which temperature again decreases with height. The vertical stratification, with warmer layers above and cooler layers below, makes the stratosphere dynamically stable: there is no regular convection and associated turbulence in this part of the atmosphere.

The heating is caused by an ozone layer that absorbs solar ultraviolet radiation, heating the upper layers of the stratosphere. The base of the stratosphere occurs where heating by conduction from above and heating by convection from below (through the troposphere) balance out; hence, the stratosphere begins at lower altitudes near the poles due to the lower ground temperature there.

Commercial airliners typically cruise at an altitude near 10 km in temperate latitudes, in the lower reaches of the stratosphere. This is to avoid atmospheric turbulence from the convection in the troposphere. Turbulence experienced in the cruise phase of flight is often caused by convective overshoot from the troposphere below


The stratosphere is a region of intense interactions among radiative, dynamical, and chemical processes, in which horizontal mixing of gaseous components proceeds much more rapidly than vertical mixing. 


Ozone Depletion

The main cause of ozone depletion is the presence of chlorofluorocarbons (aka CFCs - CF2Cl2, CFCl3) in the Earth's stratosphere. Because CFCs are stable, inexpensive, non-toxic, flammable, or corrosive, they are used as propellants دافع, as refrigerants, as solvents, etc. However, it is because of this stability that causes these CFCs to persist within the environment. These molecules eventually find their way to the stratosphere, where they undergo a series of chain reactions which ultimately lead to the destruction of the ozone layer.




The mesosphere starts just above the stratosphere and extends to 85 kilometers (53 miles) high. In this region, the temperatures again fall as low as -93 degrees Celsius as you increase in altitude. The chemicals are in an excited state, as they absorb energy from the Sun. The mesopause separates the mesophere from the thermosphere.

The regions of the stratosphere and the mesosphere, along with the stratopause and mesopause, are called the middle atmosphere by scientists.



The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. (see also: ionosphere)

The thermosphere, named from the Greek (thermos) for heat, begins about 85 km above the Earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation by the small amount of residual oxygen still present. Temperatures are highly dependent on solar activity, and can rise to 2,000°C. Radiation causes the air particles in this layer to become electrically charged (see ionosphere), enabling radio waves to bounce off and be received beyond the horizon. At the exosphere, beginning at 500 to 1,000km above the Earth's surface, the atmosphere blends into space. The few particles of gas here can reach 2,500°C (4500°F) during the day.


The ionosphere is the part of the atmosphere that is ionized by solar radiation. It forms the inner edge of the magnetosphere and has practical importance because it influences high-frequency (HF) (3–30 MHz) radio propagation to distant places on the Earth.


4-The Exosphere:

The exosphere (from the Greek words exo = out(side) and sphaira = ball) is the uppermost layer of the atmosphere. On Earth, its lower boundary at the edge of the thermosphere is estimated to be 500 km to 1000 km above the Earth's surface, and its upper boundary at about 10,000 km. It is only from the exosphere that atmospheric gases, atoms, and molecules can, to any appreciable extent, escape into outer space. The main gases within the exosphere are the lightest gases, mainly hydrogen and helium, with some atomic oxygen near the exobase.

The atmosphere in this layer is sufficiently rarified for satellites to orbit the Earth, although they still receive some atmospheric drag.

Exobase, also called the critical level, the lowest altitude of the exosphere, is defined in one of two ways:

The height above which there are negligible atmospheric collisions between the particles and

The height above which the constituent atoms are on purely ballistic trajectories مسار منحني.

The exact altitude at which the exosphere ends and space begins is not well-defined, and attempting to attach a specific value to it is not particularly useful. 

3)      Explain in details the three primary types of Tectonic Plate boundaries. (10/20)

There are 3 primary types of Tectonic Plate boundaries: Divergent boundaries; Covergent boundaries; and Transform boundaries. As the giant plates move, diverging [pulling apart] or converging [coming together] along their borders, tremendous energies are unleashed resulting in tremors هزات that transform Earth’s surface. While all the plates appear to be moving at different relative speeds and independently of each other, the whole jigsaw puzzle of plates is interconnected. No single plate can move without affecting others, and the activity of one can influence another thousands of miles away. For example, as the Atlantic Ocean grows wider with the spreading of the African Plate away from the South American Plate, the Pacific sea floor is being consumed in deep subduction trenches over ten thousand miles away


Divergent Boundaries:

At divergent boundaries new crust is created as two or more plates pull away from each other. Oceans are born and grow wider where plates diverge or pull apart. As seen below, when a diverging boundary occurs on land a 'rift', or separation will arise and over time that mass of land will break apart into distinct land masses and the surrounding water will fill the space between them.


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 Convergence:

When 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.


Oceanic-Oceanic Convergence:


When two oceanic plates converge one is usually subducted under the other and in the process a deep oceanic trench is formed. The Marianas Trench, for example, is a deep trench created as the result of the Phillipine Plate subducting under the Pacific Plate. Oceanic-oceanic plate convergence also results in the formation of undersea volcanoes. Over millions of years, however, the erupted lava and volcanic debris pile up on the ocean floor until a submarine volcano rises above sea level to form an island volcano. Such volcanoes are typically strung out in chains called island arcs


Continental-Continental Convergence:

When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10 million years


 Transform-Fault Boundaries:

Transform-Fault 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 that connects the East Pacific Rise, a divergent boundary to the south, with the South Gorda -- Juan de Fuca -- Explorer Ridge, another divergent boundary to the north. 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).




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