Sedimentary rock is converted to metamorphic rock primarily when it is subjected to high temperatures and pressures deep within the Earth's crust.
The Metamorphic Transformation Process
The conversion of sedimentary rock into metamorphic rock is a fundamental process within the Earth's rock cycle. This transformation doesn't involve melting the rock but rather changing its physical and chemical properties under intense environmental conditions.
This occurs when the sedimentary rock is subjected to high temperatures and pressures, causing the minerals in the rock to change. These conditions are typically found deep within the Earth's crust, where the weight of overlying rocks and the heat from the Earth's interior are significant.
Key Factors in Metamorphism
Two primary factors drive the conversion:
- High Temperature: Elevated temperatures, often encountered when rocks are buried deep or are near magma intrusions, cause the minerals within the sedimentary rock to recrystallize or form new minerals that are stable under the new heat conditions.
- High Pressure: Pressure from the weight of overlying rocks (confining pressure) or directed pressure from tectonic forces (differential stress) can compact the rock and rearrange mineral grains. This pressure can cause minerals to align, resulting in features like foliation often seen in metamorphic rocks.
This can happen deep within the Earth's crust where the conditions of heat and pressure are intense. The combination of these forces breaks down the original mineral structures and causes atoms to rearrange, forming new minerals or textures without melting the rock entirely.
Examples of Sedimentary to Metamorphic Conversions
Here are a few common examples:
- Shale (Sedimentary) to Slate (Metamorphic): Under relatively low temperatures and pressures, fine-grained shale metamorphoses into slate, a rock known for its ability to split into thin layers.
- Limestone (Sedimentary) to Marble (Metamorphic): When limestone, composed mainly of calcite, is subjected to heat and pressure, the calcite grains recrystallize, forming the interlocking crystals characteristic of marble.
- Sandstone (Sedimentary) to Quartzite (Metamorphic): Quartz sandstone undergoes metamorphism where the quartz grains and silica cement recrystallize, resulting in a very hard, durable rock called quartzite.
Conditions for Metamorphism
The specific temperatures and pressures required vary depending on the type of sedimentary rock and the resulting metamorphic rock. However, they are significantly higher than surface conditions.
| Factor | Sedimentary Rock Formation Conditions | Metamorphic Rock Formation Conditions |
|---|---|---|
| Temperature | Surface to moderate (typically < 200°C) | High (typically > 200°C) |
| Pressure | Low (surface) to moderate | High (deep within crust) |
| Process | Cementation, Compaction, Deposition | Recrystallization, Mineral Change |
In summary, sedimentary rock transforms into metamorphic rock through a solid-state change driven by intense heat and pressure experienced deep within the Earth's crust, altering the rock's mineral composition and texture.
