Question

Movement along the African Rift Valley is about \(2.1 \mathrm{~cm}\) per year. If plates continue to move apart at this rate, how much larger will the rift be (in meters) in 1,000 years? In 15,500 years? Use the illustration below to answer questions 24 and \(25 .\) Normal magnetic polarity Reversed magnetic polarity Mid-ocean ridge

Short answer

In 1,000 years, the rift will expand by 21 meters; in 15,500 years, by 325.5 meters.

Step-by-step solution

Understand the problem

The movement along the African Rift Valley is given as 2.1 cm per year. We want to calculate how much larger the rift will be in 1,000 years and in 15,500 years at the same rate.

Convert rate to meters per year

Since the rate is given in centimeters, first convert it to meters. We know that 1 meter equals 100 centimeters. Therefore, \( 2.1 \text{ cm/year} = \frac{2.1}{100} \text{ meters/year} = 0.021 \text{ meters/year} \).

Calculate the rift size increase in 1,000 years

To find how much larger the rift will be in 1,000 years, multiply the annual movement by 1,000. \( 0.021 \text{ meters/year} \times 1000 \text{ years} = 21 \text{ meters} \).

Calculate the rift size increase in 15,500 years

Apply the same multiplication for 15,500 years. \( 0.021 \text{ meters/year} \times 15500 \text{ years} = 325.5 \text{ meters} \).

Summarize the calculated expansions

After 1,000 years, the rift will expand by 21 meters; after 15,500 years, it will expand by 325.5 meters.

Key concepts

Plate Tectonics

Plate tectonics is a fundamental theory that explains the large-scale movements of Earth's lithosphere, which is the outermost shell of the planet. This theory helps us understand many geological phenomena. The lithosphere is divided into a number of plates, which float on the semi-fluid asthenosphere beneath them. These plates can move because of the intense heat and pressure from the Earth's core, creating convection currents.

Here are the main types of plate boundaries where various geological activities occur:

• **Divergent boundaries**: Plates move apart from each other. For example, the African Rift Valley is a classic case where plates are moving apart, causing the land to stretch and form a rift.
• **Convergent boundaries**: Plates move towards each other, often leading to one plate being forced below the other, resulting in subduction zones and mountain formation.
• **Transform boundaries**: Plates slide past each other, which can lead to earthquakes.

Understanding plate tectonics is crucial for predicting geological events like earthquakes and volcanic eruptions.

Continental Drift

The concept of continental drift was first proposed by Alfred Wegener in 1912. It suggests that continents are not fixed but move slowly over the Earth's surface. Before the acceptance of plate tectonics, this theory was the primary explanation for the movement of continents.

Key points about continental drift:

• **Evidence from fossils**: Identical fossils have been found on widely separated continents, suggesting they were once connected.
• **Geological similarities**: Mountain ranges and rock formations on different continents match up, as if they were once part of a single landmass.
• **Pangaea**: This term refers to a supercontinent that supposedly existed millions of years ago, when all Earth's landmasses were joined together. Over time, Pangaea broke apart, leading to the continents we see today.

Continental drift laid the groundwork for the modern theory of plate tectonics, enriching our understanding of Earth's dynamic surface.

Geological Time Scale

The geological time scale is a system that describes Earth's history in chronological order, divided into various time periods based on significant geological and paleontological events. It's an essential tool for geologists to contextualize Earth's history and changes over billions of years.

Here's a breakdown of the major divisions:

• **Eons**: The largest time units, such as the Precambrian, which spans from Earth's formation until the emergence of abundant complex life.
• **Eras**: Subdivisions of eons, for example, the Paleozoic, Mesozoic, and Cenozoic eras. Each is known for distinct developments and extinction events.
• **Periods**: Eras are further divided into periods such as the Jurassic and Cretaceous, often known for certain types of flora and fauna.

This time scale helps scientists understand not just when certain events happened but also how they relate to each other, through the study of rock layers and the fossils contained within them.