بنك الاسئلة EMR 2010 2ND SEMESTER 421

Model Answer

1- The following two component phase diagram represents the diopside-anorthite eutectic crystallization

1- Label all phases present on the diagram

2- Define the following:

(a) liquidus:

The line separating the field of all liquid from that of liquid plus crystals

(b) Solidus:

The line separating the field of all solid from that of liquid plus crystals

3- Label and define the eutectic point

The point on a phase diagram, where the maximum number of allowable phases are in equilibrium. When this point is reached, the temperature must remain constant until one of the phases disappears.

4- Illustrate with labeled drawing type of textures you might expect at the right side of the diagram?

Ophitic texture

5- Trace the crystallization path of the point (a).

The Path abfh on the diagram

2- Explain the role of temperature, viscosity, and gas content of the magma in producing each of the following features

(a) Lava flow:

If the liquid part of the magma has a low viscosity and high temperature, then the gas can expand relatively easily. When the magma reaches the surface, the gas will easily expand and a non-explosive eruption will occur, usually as a lava flow (Lava is the name we give to magma on the surface of the Earth).

(b) Volcanic dome

Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent

3- Compare between the following:

4- Compare between the following:

5- Describe the different ways to melt a rock, Illustrate your answer by labeled drawings and give an example for each.

1- Decompression: Decompression melting occurs at mid-ocean ridges

2- Addition of Fluxes: Rocks melt at a lower temperature in the presence of volatiles, such as water and carbon dioxide. How do you get water underneath a volcano? The most common way to do it is to send it down a subduction zone.

3- Rising temperature: The third way to melt a rock is by conduction. Conduction is the simplest way to transfer heat.

6- Name 5 processes that might cause the chemical composition of magma to change.

1- Partial melting (to produce different magmas)

2- Crystal fractionation

3- Liquid immiscibility

4- Magma mixing

5- Assimilation

7- What kind of evidence one look for to determine if crustal assimilation has operated to change the composition of magma.

Criteria for recognition of assimilation:

1-     The occurrence of xenoliths in the igneous rocks, which are of similar composition to the intruded country rocks.

2-     Disequilibrium mineral assemblage and Resorbed xenocrysts.

3-     On variation diagrams, the composition of the igneous rock after assimilation lies on a mixing (straight) line between its composition prior to assimilation and the composition of the assimilated rock.

4-     “Higher” ⁸⁷Sr/⁸⁶Sr and ¹⁸O values (as will be discussed later).

8- The following diagram represents the REE patterns for 4 melts (a, b, c, and d) derived by different degrees of partial melting of a mantle source. Which melt represent the lower degree of partial melting and why?

Pattern d represent the lower degree of partial melting

This is because the LREE are incompatible elements (Elements prefer to be present in the melt) and their contents will become less as the degree of partial melting increase.

9- The following Table shows the partition coefficient (K_D) values for Ni in the listed minerals.

a- Define partition coefficient (K_D)

The partition coefficient is a ratio of concentrations of an element between crystalline solids and coexisting liquid phases. Kd= Conc. of element in solid/conc. Of element in liquid

b- Calculate the bulk partition coefficient (D) for Ni in garnet lherzolite containing 60% olivine, 25% orthopyroxene, 10% clinopyroxene and 5% garnet.

Bulk Kd_Ni=(0.6*14)+(0.25*5)+(0.1*7)+(0.0995*0.05)= 10.35

10- Complete the following Table for pyroclastic classification.

11- Compare between the different types of magmas according to the following table:

12- Complete the following Table as the given example: