Review questions and learning assessment

¶ … Earth?

Scientists know what is inside the Earth primarily through firing waves of energy through the planet and measuring how they are affected through ground sensors. Both P. And S. waves move through the Earth at a certain speed and are either reflected or refracted depending on the material they move through. These actions can be tracked by sensors. Most of the information scientists have about the Earth's structure has been learned from observing these travel times, refractions, reflections, and phase transitions of seismic body waves. Earthquakes help in this way. The body waves move through the liquid layers of the Earth, but P-waves are refracted when they move through the transition between the semisolid mantle and the liquid outer core. This results in a P-wave shadow zone which occurs between 104° and 140°. This zone is where the initial P-waves are not able to be registered on sensory equipment like seismometers. However, S-waves do not travel through liquids, instead, they are absorbed.

2. What can one learn about the interior of Earth by measuring the acceleration due to gravity?

The acceleration due to gravity tells scientists that the Earth's structure is made up of different densities. At the surface, this number is usually denoted as "g," or 9.8 meters per second squared. This number depends on the density of the object with gravitational pull. Since the Earth's size and weight are known based upon other astronomical calculations, its density can be figured into the equation. When this was accomplished, scientists realized that the Earth must have a very massive iron core in order to explain the "g" force that is exacted upon the planet's inhabitants. Coupled with seismic measurements, scientists can deduce what exact likely elements and temperatures are present within the Earth's structure.

3. Why do rocks deep in the mantle remain solid, while those in the asthenosphere are close to melting?

Rocks deep in the mantle remain solid because of the huge lithostatic pressures associated with this layer. The rocks are under so much pressure and heat that they cannot become less dense and form a liquid. Those rocks in the asthenosphere are close to melting because they are under less pressure and therefore are very close to becoming a less dense fluid.

4. Describe how the angle at which a seismic wave approaches a discontinuity determines whether or not it is reflected or refracted.

As described in the answer to the first question, the P. wave is not refracted when the refractory angle reaches between 104 degrees and 140 degrees. The wave is dissipated in these zones causing a "shadow zone" familiar to most scientists. So depending on the angle of the wave interception, there may or may not be a refraction or reflection of the wave. So if the wave approaches a discontinuity from this angle range, it will not be reflected.

5. What changes does olivine undergo with depth in the mantle?

Olivine, more commonly referred to in gem quality as peridot, is a very common element in the Earth's mantle. Below a certain depth of about 400 kilometers, the olivine crystal structure becomes unstable due to immense heat and pressure. It undergoes a transition from the olivine structure to the spinel structure. These transitions account for the discontinuous increase of the Earth's mantle as observed by seismic instruments.

6. How do mountains in the ocean basin differ from those on land?

Mountains in the ocean basins are perpetually being "recycled" through subduction zones. Therefore, the rock that they are made of differs in composition to mountains on land because of their younger age and mineral composition. Many of the mountains on land were created through uplifts in the crust, where rock from the crust was pushed upward and folded over time and time again. These types of rock have more metamorphic qualities while the ocean-bottom mountains tend to be more volcanic in nature and are more igneous. Also, the oceanic crust is made of basalt vs. The continental crust made of granite.

7. Why are continents so much higher than ocean basins?

The continents are higher than the ocean basins because of their crust density. The oceanic crust is mainly made up of basalt, which has high iron and nickel content and is therefore thinner and denser. The continental crust is thicker because it is made up of mostly granite which has high levels of silicate and aluminum. The continental crust is of lower density.

8. Why are some mountains belts so much higher than others?

Some mountain belts are much higher than others because of the forces that created them. The Himalayas, Earth's highest mountain range, is a product of plates shifting into one another and begin crushed together. The mountains grow each year because as the plates push together, parts of the crust are lifted up over others. As the wrinkles increase, the mountains are pushed higher and higher. Other mountain ranges are formed through smaller crustal impacts or even through subduction zones. Hawaii is a great example of a mountain range being created near a subduction zone.