Solved Lab 2: Plate Tectonics Exercises Group Members: One | Chegg.com - Free Printable

Problem Analysis:

The task involves analyzing a plate tectonic map of the circum-Pacific region and answering questions related to different plate boundaries, the types of rocks formed, topography, and plate movements. Below is a detailed explanation and solution for each part of the problem.

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Exercise 1: East Pacific Rise

#### (a) What type of plate boundary is this, and which plates are at this boundary?
- Answer: The East Pacific Rise is a divergent plate boundary. At this boundary, two tectonic plates are moving away from each other.
- Plates involved: The East Pacific Rise separates the Pacific Plate on one side and the Nazca Plate or Cocos Plate on the other side, depending on the specific segment of the rise.

#### (b) What type of igneous rock (formed from magma) is produced here?
- Answer: At divergent boundaries like the East Pacific Rise, magma rises to fill the gap created by the separating plates. This magma cools and solidifies to form basalt, an extrusive igneous rock.

#### (c) Is the topography higher or lower at this plate margin compared to the surrounding ocean basin?
- Answer: The topography at the East Pacific Rise is higher than the surrounding ocean basin. This is because the rising magma at the divergent boundary causes the crust to be thicker and elevated, forming a mid-ocean ridge.

#### (d) Observe the nearly symmetrical 'stripes' oriented parallel to the mid-ocean ridge. Do the presence of these stripes reinforce the idea that the plates are moving apart? How do these 'stripes' form? (Hint: think about the discussion of paleomagnetism in lecture.)
- Answer:
- Yes, the symmetrical 'stripes' reinforce the idea that the plates are moving apart. These stripes are magnetic anomalies caused by reversals in Earth's magnetic field over time.
- Formation of stripes: As magma rises at the divergent boundary and cools, it records the Earth's magnetic field at the time of its formation. Over millions of years, Earth's magnetic field reverses periodically (normal to reversed polarity). When new magma erupts and solidifies, it locks in the current magnetic orientation. As the plates move apart, these alternating bands of normal and reversed magnetism create symmetrical patterns on either side of the ridge. This pattern provides evidence for seafloor spreading and plate tectonics.

#### (e) What could be a possible explanation for the asymmetry (i.e., stripes do not have the same spacing on each side of the spreading ridge) in these stripes?
- Answer: Asymmetry in the magnetic stripes can occur due to several factors:
1. Unequal spreading rates: If the rate of seafloor spreading differs on either side of the ridge, the spacing between the stripes will vary.
2. Irregularities in the mantle: Variations in the mantle's temperature and composition can affect the rate of magma production and cooling, leading to uneven stripe spacing.
3. Past changes in spreading direction: Historical changes in the direction or rate of plate movement can also cause asymmetry.
4. Subduction zones nearby: If one side of the ridge is closer to a subduction zone, it may experience faster recycling of material, affecting stripe patterns.

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Exercise 2: West Coast of South America

#### (a) What type of plate boundary is this, and which plates are at this boundary?
- Answer: The west coast of South America is a convergent plate boundary. Here, the Nazca Plate (oceanic) is subducting beneath the South American Plate (continental).

#### (b) What topographic feature is produced on the continent by this plate motion?
- Answer: Convergent boundaries where an oceanic plate subducts beneath a continental plate typically produce mountain ranges. On the west coast of South America, this process has formed the Andes Mountains.

#### (c) What type of igneous rock (formed from magma) is produced here?
- Answer: At convergent boundaries, the subducting oceanic plate melts as it descends into the mantle, generating magma. This magma rises and forms granite (intrusive igneous rock) or andesite/dacite (extrusive igneous rocks) when it reaches the surface. Volcanoes along the Andes are examples of such igneous activity.

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Exercise 3: Juan de Fuca Plate off the coast of Washington State

#### (a) How fast is the Juan de Fuca plate moving, and in what direction?
- Answer: The Juan de Fuca Plate is moving northeastward at a rate of approximately 2–5 cm per year. This movement is relative to the North American Plate.

#### (b) How fast is the North American plate moving, and in what direction?
- Answer: The North American Plate is moving southwestward at a rate of approximately 1–2 cm per year. This movement is relative to the Juan de Fuca Plate.

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Final Answer:

\[
\boxed{
\begin{array}{l}
\text{1. (a) Divergent plate boundary; Pacific Plate and Nazca/Cocos Plates} \\
\text{(b) Basalt} \\
\text{(c) Higher} \\
\text{(d) Symmetrical stripes confirm plate movement; formed by magnetic reversals} \\
\text{(e) Asymmetry due to unequal spreading rates, mantle irregularities, or past changes} \\
\text{2. (a) Convergent plate boundary; Nazca Plate and South American Plate} \\
\text{(b) Andes Mountains} \\
\text{(c) Granite/Andesite/Dacite} \\
\text{3. (a) 2–5 cm/year, northeastward} \\
\text{(b) 1–2 cm/year, southwestward} \\
\end{array}
}
\]