Which of the following observations best supports the dynamo theory for the origin of the Earth magnetic field?

Introduction / Context:
The dynamo theory explains the Earth magnetic field as the result of electric currents generated by the motion of conducting fluids within the planet. Understanding what kind of interior structure is needed for this process is an important topic in geophysics. This question asks which observation provides the strongest support for the dynamo theory as the origin of the Earth global magnetic field.

Given Data / Assumptions:
- The Earth has several layers: crust, mantle, outer core and inner core.
- The dynamo theory requires a region of electrically conducting fluid in motion.
- Seismic studies provide information about which layers are solid and which are liquid.
- The composition of the core is mostly iron and nickel with some lighter elements.

Concept / Approach:
According to dynamo theory, a global magnetic field is generated when electrically conducting fluid moves in the presence of rotation and existing magnetic fields. In the Earth, the outer core is believed to be a liquid layer of iron-rich material that can conduct electricity and convect due to thermal and compositional gradients. These flows, combined with the rotation of the planet, maintain the geomagnetic field. The most directly supportive evidence is therefore the existence of a liquid, electrically conducting outer core. Other facts, such as the presence of iron or convection, matter as background, but without a liquid conducting layer, the dynamo could not operate.

Step-by-Step Solution:
Step 1: Identify the basic requirement of a dynamo: moving electrically conducting fluid. Step 2: Consider the internal structure of the Earth: a solid inner core, a liquid outer core, and a solid mantle and crust. Step 3: Seismic wave studies indicate that the outer core is liquid because certain seismic waves do not pass through it. Step 4: The composition of the outer core is mostly iron and nickel, which are good electrical conductors when molten. Step 5: Convection in this liquid metal layer, combined with Earth rotation, can generate and maintain a magnetic field according to dynamo theory. Step 6: Therefore, the observation that most directly supports dynamo theory is the existence of a liquid, electrically conducting outer core.

Verification / Alternative check:
Measurements of variations in the geomagnetic field over time, known as secular variation, are consistent with flows in the outer core. Computer simulations of geodynamos that use parameters similar to a liquid iron outer core can reproduce many features of the observed field, including polarity reversals. These lines of evidence, combined with seismic data, all point toward a convecting, conducting outer core as the source of the field.

Why Other Options Are Wrong:
The core of the Earth contains a large amount of solid iron: While iron is important for conductivity, the solid inner core alone cannot generate a dynamo because it lacks large scale fluid motion; the key is the liquid outer core.
Granite and basalt in the crust conduct electricity very well: The crust is mostly solid and does not provide the kind of deep, global scale fluid motion needed for a dynamo, and these rocks are not highly conductive compared with molten iron.
Convection occurs only in the inner solid core of the Earth: The inner core is believed to be solid; convection in the liquid outer core, not the solid inner core, is central to dynamo theory.

Common Pitfalls:
Students sometimes focus on the presence of iron alone and overlook the need for fluid motion. Other times they may confuse conduction in solid rocks with the dynamic behaviour of a fluid core. To support a planetary dynamo, both conductivity and motion are required. Remember that a liquid, convecting outer core of conducting material is the key ingredient that makes the Earth magnetic dynamo plausible.

Final Answer:
The best evidence supporting dynamo theory is that the outer core of the Earth contains liquid material that conducts electricity and can flow.