# Tutor profile: Shanna M.

## Questions

### Subject: Geology

What are salt deposits and what are the two types of salt basins?

Salt deposits occur as evaporites that form from saline-rich fluid-brines. Pure salt is composed of sodium and chloride, a mineral known as halite. Evaporating seawater can also produce anhydrite and gypsum, which usually occurs interbedded with halite. These minerals are concentrated by the evaporation or freezing of intermittent loads of seawater and accumulate in silled basins. Eustatic sea level changes aid this concentration process, whose continuing recycling of the brine results in the crystallization of evaporites on the basin floor. Evaporite deposits within a basin tend to occur in cycles, with the higher salinity salts occurring closer to the depocenter. Salt basins are generally classified into two different types: halotectonic basins and halokinetic basins. Halotectonic basins are formed as a result of compressive tectonic forces. Halokinetic basins form by isostatic salt movement and are influenced by a number of factors. Halokinesis is triggered by three factors: (1) the development of a density inversion, (2) differential loading, and (3) the existence of a slope at the base of a salt layer

### Subject: Statistics

What is an easy way to test for normality of a data set?

The Q-Q Plot is a more accurate determination of the normality of the distribution. On the Q-Q plot, the data will fall linearly along the plot line, and deviations from this line indicate deviations from normality. It is a graph of the cumulative frequencies plotting a sample order statistic against the Z-statistic of the normal distribution, using the median quantity. The Shapiro-Wilk test can also be calculated, and is a ratio of two estimators of the squared standard deviation. If the distribution is normal, this value will be close to 1. For shape analysis of the distribution, the four moments of the data are calculated and plotted. The first and second moments are the mean and variance. The third and fourth moments are skewness and kurtosis. Skewness measures the symmetry of the distribution around the mean. This value will calculate at 0 for a normal distribution and a negative or positive for distributions that are not normal. Kurtosis measures the peakedness of the distribution. A normal distribution has a standard kurtosis of 3, so the calculation of the kurtosis always subtracts 3 so that a value of 0 is normally distributed.

### Subject: Physical Science

What is stress? What are the types of stress forces?

Stress measures the intensity of a force per unit area. In geology, stress is the intensity of a force applied to a body of rock. There are two types of stress, normal (σ) and shear stresses (τ). We define a force as normal stress if it acts normal to the plane of reference, and shear stress if the force acts parallel to the plane. The entire stress acting on a plane is a vector, a physical quantity that has both direction and magnitude. A vector can be divided into three mutually perpendicular components. Two of these components lie inside the plane, and one acts perpendicular to the plane. Thus the normal component is the normal stress acting on the plane, and the other two components are the shear stresses of the plane. Two types of forces can act on a rock body in a given plane. Body forces do not result from direct contact with another rock body. They act from a distance and on every particle of the rock body. Surface forces can act either on an actual physical surface of the rock or on a single plane at any orientation within the rock body. Surface forces act directly on specific rock body particles on a particular plane. Surface forces acting on a physical plane of a rock body can be either compressive or tensile. Compressive forces effectively squeeze together both sides of the plane. This force is considered positive. Tensile forces stretch either side of a plane and are considered negative.

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