As a geophysicist with a keen interest in the structure and composition of the Earth, I am often intrigued by the complex dynamics that govern our planet's interior. The Earth is a fascinating celestial body with a layered structure that is essential to understanding its geological processes and phenomena. Let's delve into the layers of the Earth to address the question at hand.

The Earth's interior is traditionally divided into four main layers: the crust, the mantle, the outer core, and the inner core. Each of these layers has distinct properties and plays a critical role in the Earth's overall structure and function.

The Crust is the outermost layer and varies in thickness from about 3 miles (5 kilometers) under the oceans to about 25 miles (40 kilometers) under the continents. It is composed primarily of solid rock and is the layer upon which we live and interact with the Earth.

The Mantle lies beneath the crust and is the thickest layer of the Earth, extending to a depth of about 1,800 miles (2,900 kilometers). It is composed of solid rock, but due to the immense heat and pressure, the rock behaves more like a very viscous fluid over geological time scales. This allows for the process of convection, which is a driving force behind plate tectonics.

The Outer Core is a liquid layer composed primarily of molten iron and nickel. It extends from a depth of about 1,800 miles (2,900 kilometers) to about 3,200 miles (5,150 kilometers). The movement of this liquid metal generates the Earth's magnetic field through a dynamo effect.

The Inner Core is the deepest layer of the Earth and is the subject of our discussion. It is a solid sphere with a diameter of approximately 1,500 miles (2,400 kilometers). Contrary to the outer core, the inner core is solid due to the extreme pressure exerted at the Earth's center, which is estimated to be about 3.3 million times the atmospheric pressure at sea level. Despite the intense pressure, the temperature in the inner core is thought to be extremely high, possibly exceeding 9,000 degrees Fahrenheit (5,000 degrees Celsius), which is indeed comparable to the surface temperature of the sun.

It is important to note that the inner core, despite its high temperature, remains solid due to the aforementioned pressure. This is an example of a phase of matter where solid and liquid states can coexist at high temperatures under the influence of pressure. The inner core is believed to be composed primarily of iron, with some nickel and possibly other elements in smaller amounts.

The presence of a solid inner core has significant implications for the Earth's magnetic field and its stability. The solid inner core and the liquid outer core interact in ways that are not yet fully understood, but it is known that this interaction is crucial for the generation and maintenance of the Earth's magnetic field, which protects the planet from harmful solar radiation.

In conclusion, the layer of the Earth that is solid metal is the inner core. It is a remarkable example of how extreme conditions of pressure and temperature can result in unexpected states of matter. Our understanding of the Earth's interior continues to evolve as new data and technologies become available, and the study of this fascinating subject is vital to our comprehension of the Earth's past, present, and future.

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The Earth's interior is composed of four layers, three solid and one liquid--not magma but molten metal, nearly as hot as the surface of the sun. The deepest layer is a solid iron ball, about 1,500 miles (2,400 kilometers) in diameter. Although this inner core is white hot, the pressure is so high the iron cannot melt.read more >>