The melting point of “stone” is not a single number, unlike the melting point of ice. In a scientific context, “stone” is non-specific, referring to a vast array of rocks, each with a unique mineral makeup and history. Rock melting is a complex geological process, not a simple phase transition. It depends entirely on the rock’s composition and its surrounding environmental conditions. Unlike a pure substance, rock melts over a temperature range, and deep Earth conditions often influence this range more than the temperature itself.
Why Stone Does Not Have a Single Melting Point
A rock does not have one fixed melting point because it is a heterogeneous material, a physical mixture of multiple individual minerals. Pure substances, such as gold or water, are composed of a single chemical compound and have a sharp, fixed melting temperature. Rocks are made up of various minerals like quartz, feldspar, and mica, each possessing its own distinct melting temperature.
When this complex mixture is heated, the components do not all liquefy simultaneously. Instead, the minerals with the lowest melting points begin to turn into a liquid first, a phenomenon known as partial melting. This process is governed by the eutectic point, the lowest possible melting temperature achievable by that specific mixture. Once this temperature is reached, the lowest-melting mineral phases start to liquefy, creating a magma chemically different from the original solid rock.
Environmental Factors That Alter Rock Melting Temperature
The melting temperature of rock is influenced by pressure and the presence of volatile compounds. As pressure increases with depth inside the Earth, the melting point of a rock generally rises. High pressure forces the atoms into a more compact and stable crystalline structure, requiring more thermal energy to break the atomic bonds and transition into a liquid state.
Conversely, volatile compounds, such as water and carbon dioxide, act as a flux to lower the melting temperature. Water weakens the chemical bonds within the rock’s crystal lattice, making it easier for the rock to melt. This explains why magma forms readily in subduction zones, where water released from the descending oceanic plate infiltrates the overlying mantle rock, triggering melting.
A reduction in pressure can also induce melting without any temperature increase, a process called decompression melting. This mechanism occurs when hot mantle material rises to shallower depths at locations like mid-ocean ridges or mantle plumes. The decrease in confining pressure lowers the rock’s melting point below its existing high temperature, causing it to liquefy.
The Process of Partial Melting and Phase Change
The transformation of solid rock into liquid magma is a gradual process. When rock is heated, it first enters a state of metamorphism, undergoing solid-state changes where its mineral structure rearranges without fully melting. As heating continues, partial melting begins, creating a mixture of liquid melt and residual solid crystals.
This transitional state results in a viscous, slushy material, which is the initial form of magma. The liquid melt is less dense than the surrounding solid rock, allowing it to percolate upward and accumulate in magma chambers. This ascent concentrates the silica-rich components, which are the first to melt, leaving a more iron and magnesium-rich residue behind.
Decomposition vs. Melting
In some rock types, melting is replaced by chemical decomposition, particularly in sedimentary rocks containing carbonates. Limestone, which is primarily calcium carbonate, does not melt in the traditional sense. Instead, when heated to temperatures around 825°C, it breaks down into calcium oxide and carbon dioxide gas. This decomposition process occurs well before a true melt can be achieved.
Melting Temperature Ranges of Common Rock Types
The wide range of rock compositions results in differences in melting behaviors. Mafic rocks, such as Basalt, are rich in iron and magnesium and low in silica, giving them high melting temperatures. Basalt typically begins to melt between 1000°C and 1200°C.
Felsic rocks, like Granite, contain high amounts of silica, potassium, and sodium, resulting in a lower melting point. Dry granite melts between approximately 1215°C to 1260°C at ambient pressure. However, the presence of water lowers this range, allowing granite to begin melting at temperatures as low as 650°C to 700°C under high crustal pressures.
Most silicate rocks, which make up the majority of the Earth’s crust and mantle, are molten at temperatures around 1200°C, though parts can be liquid as low as 600°C. These ranges are defined by the solidus (start of melting) and liquidus (end of melting) temperatures, which shift based on local pressure and volatile content.