Question

What theory states that plates move around on the asthenosphere? A) continental drift B) seafloor spreading C) subduction D) plate tectonics

Short answer

D) Plate Tectonics

Step-by-step solution

Understand the Exercise

The question is asking us to identify which theory explains the movement of plates on the asthenosphere. We must review the definitions of each provided theory to find the correct one.

Analyze the Options

Look at each option: - A) Continental Drift: This theory, proposed by Alfred Wegener, suggests that continents have moved over geological time but does not specifically address plates or the asthenosphere. - B) Seafloor Spreading: This concept explains the formation of new oceanic crust at mid-ocean ridges but doesn't directly explain plate movement on the asthenosphere. - C) Subduction: This process involves one plate moving under another, linked to the movement but not fully explaining plate movement on the asthenosphere. - D) Plate Tectonics: This theory describes the movement of seven large plates and many smaller ones on the Earth's asthenosphere.

Identify the Correct Answer

Based on the analysis, the theory that clearly describes plates moving on the asthenosphere is Plate Tectonics. This theory combines aspects of continental drift and seafloor spreading to explain how the Earth's surface is made up of plates that float and move on the semi-fluid asthenosphere beneath.

Key concepts

Asthenosphere

The asthenosphere is a vital layer within Earth’s structure that plays a key role in plate tectonics. Situated just beneath the lithosphere, which includes the crust and the uppermost mantle, the asthenosphere extends several hundred kilometers deep. It is characterized by its semi-fluid and plastic-like behavior, which allows it to flow slowly over geological time.

Due to its semi-fluid nature, the asthenosphere enables the movement of tectonic plates that rest atop of it, allowing them to shift, collide, and rearrange. This movement is primarily driven by convection currents within the mantle beneath, caused by heat from the Earth’s core. The heat causes material in the asthenosphere to rise and cool, creating a continuous cycle that propels the plates.

The properties of the asthenosphere, such as its ability to deform like a pliable solid, are why it is fundamental to understanding how and why tectonic plates move.

• Allows movement of the tectonic plates.
• Driven by convection currents.
• Key in plate dynamics.

Continental Drift

Continental drift is a precursor idea to the modern theory of plate tectonics. Proposed by Alfred Wegener in 1912, it suggests that continents have not always been in their current positions but have drifted across the Earth over geological time. This idea was revolutionary at the time because it contradicted the long-held belief that continents were static.

Wegener’s hypothesis was mainly supported by the shape of continents, such as Africa and South America fitting together like pieces of a jigsaw puzzle, and matching geological formations and fossil evidence found on continents now separated by oceans.

However, the concept of continental drift lacked a comprehensive mechanism to explain how continents moved. This gap was later filled by the discovery of seafloor spreading and the understanding of plate tectonics, which showed that continents are part of larger plates interacting over the Earth’s asthenosphere.

• Proposed by Alfred Wegener.
• Continents move over geological time.
• Lacked a mechanism—fill by plate tectonics.

Seafloor Spreading

Seafloor spreading is a process that was discovered in the mid-20th century and plays a crucial role in explaining how new oceanic crust is formed. It occurs at divergent boundaries, typically at mid-ocean ridges, where tectonic plates are moving apart. As the plates separate, magma from the mantle rises, cools, and solidifies to form new crust. This continuous addition of new material pushes the existing oceanic crust away from the ridges, causing the seafloor to gradually spread.

This process is important to understanding plate tectonics because it provides a mechanism for continental drift. The movement of the seafloor is evidence that plates are active and dynamic, reshaping the Earth’s surface continuously. Additionally, it helps to shed light on the aging of oceanic crust as the continuous addition of new crust pushes older crust further away from the spreading centers.

• Occurs at mid-ocean ridges.
• New oceanic crust formation.
• Drives plate movements and continental drift.

Subduction

Subduction is a crucial process within the theory of plate tectonics, involving one tectonic plate being forced beneath another. This typically occurs at convergent boundaries where an oceanic plate collides with a continental plate or another oceanic plate. Because oceanic crust is denser, it is usually the one to sink into the mantle, forming a trench at the boundary.

This process recycles the Earth's crust back into the mantle, where the subducted material melts and eventually may resurface as magma, feeding volcanic activity. Subduction zones are often associated with intense geological activity, including earthquakes and volcanic eruptions, due to the immense pressure and friction involved.

Subduction is essential for balancing the creation of new crust elsewhere through seafloor spreading, highlighting the Earth's intricate system of maintaining crustal equilibrium. Without subduction, the Earth’s surface would grow indefinitely with new crust from seafloor spreading.

• Occurs at convergent boundaries.
• One plate sinks beneath another.
• Important for recycling crust and balancing Earth's surface.