Question

What evidence in rocks supports the theory of seafloor spreading? A) plate movement B) magnetic reversals C) subduction D) convergence

Short answer

B) Magnetic reversals provide evidence of seafloor spreading.

Step-by-step solution

Understanding the Question

Begin by examining the four options given: plate movement, magnetic reversals, subduction, and convergence. Identify which of these options directly relates to changes observed in the ocean floor rocks that align with seafloor spreading.

Analyzing Each Option

Assess each option in the context of seafloor spreading: - Plate movement relates to the movement of Earth's lithospheric plates but is too broad to specifically indicate seafloor spreading. - Magnetic reversals refer to changes in Earth's magnetic field recorded in oceanic crust. - Subduction involves one plate sinking under another, which is more of a consequence elsewhere than evidence. - Convergence describes plates moving towards each other, typically leading to mountain formation or subduction.

Identifying the Correct Evidence

The primary evidence for seafloor spreading lies in the symmetrical pattern of magnetic reversals on either side of mid-ocean ridges. These reversals are recorded in the rocks and match Earth's historical magnetic field changes. This magnetic alignment serves as a perfect historical record of spreading and new crust formation at ridges.

Selecting the Correct Answer

Conclude that magnetic reversals (option B) provide clear, direct evidence of seafloor spreading, as they demonstrate the periodic creation and symmetrical outward movement of new oceanic crust.

Key concepts

Magnetic Reversals

Magnetic reversals are fascinating events that involve changes in Earth's magnetic field direction. These shifts are recorded in rocks, particularly those on the ocean floor, offering profound insights into how the seafloor spreads over time.

When volcanic material at mid-ocean ridges cools and solidifies, it captures the Earth's magnetic field direction at that moment. Over geological time, Earth's magnetic field flips, causing the magnetic north and south poles to switch. These flips leave behind a pattern of magnetic stripes on the ocean floor rocks, each stripe representing a period when Earth's magnetic field was aligned in a particular direction.

• The pattern of these magnetic stripes is symmetrical on either side of mid-ocean ridges, illustrating the spread of new crust from these underwater mountain ranges.
• This evidence is crucial as it validates the idea that the ocean floor is continuously forming and moving outward from the ridges.

Ocean Floor Rocks

Ocean floor rocks hold the key to understanding the process of seafloor spreading. Made mostly of basalt, a type of volcanic rock, they are formed as magma rises from beneath the Earth's crust at mid-ocean ridges.

These rocks record crucial information about the Earth's magnetic history. As they form, they align with the Earth’s magnetic field at that time, effectively acting as a magnetic tape recording of the direction and intensity of the field. This characteristic allows scientists to trace back and understand changes in Earth's magnetic field over millions of years.

• The age and composition of these rocks change as you move away from the mid-ocean ridges towards the continents.
• Newer rocks are found closer to the ridge, whereas older rocks are located farther away, suggesting that new crust is continuously formed and then pushed outward.
• This consistent pattern across different oceanic locations reinforces the theory of seafloor spreading.

Mid-Ocean Ridges

Mid-ocean ridges are vast underwater mountain systems that play a central role in the process of seafloor spreading. They form at divergent plate boundaries where tectonic plates are moving apart.

At these ridges, magma from the mantle rises through cracks in the Earth's crust, cools, and solidifies to create new oceanic crust. This process continually pushes older crust away from the ridge.

• The active and dynamic nature of mid-ocean ridges is the engine of seafloor spreading.
• They are characterized by high seismic activity and volcanic eruptions as new materials are constantly added to the ocean floor.
• The symmetry of magnetic reversals found on either side of the ridges strongly supports the theory that they are critical sites of crust formation.

Plate Tectonics

Plate tectonics is a unifying theory in geology that explains the movement of Earth's lithosphere which includes the crust and part of the mantle. The lithosphere is divided into tectonic plates that float on the semi-fluid asthenosphere beneath them.

The interaction of these plates at their boundaries is responsible for many geological phenomena, including earthquakes, volcanic activity, and the creation of mid-ocean ridges. Seafloor spreading is a direct consequence of the divergent boundary action, where two plates move away from each other.

• At divergent boundaries like mid-ocean ridges, the upwelling of magma creates new crust that pushes plates apart.
• This movement explains the presence of symmetrical magnetic reversals and the creation of ocean floor rocks.
• Plate tectonics provides the framework for understanding not only seafloor spreading but also other processes like subduction and plate convergence.