There are several ways that geologists can determine whether a layer of rock is a flood basalt or a sill, even if there are still layers of rock above it:
- Rock composition: Flood basalts are typically composed of basaltic lava, which is a type of igneous rock that forms from the rapid cooling of molten lava. Sills, on the other hand, are typically composed of the same rock as the layers above and below them. Therefore, analyzing the composition of the rock can provide clues about whether it is a flood basalt or a sill.
- Stratigraphy: Flood basalts and sills can often be distinguished based on their position in the rock layers, or stratigraphy. Flood basalts typically form thick, widespread layers that cover large areas, while sills are typically thin layers that intrude into the layers above and below them.
- Cross-cutting relationships: Geologists can also look for cross-cutting relationships between the rock layer in question and other layers of rock. If the layer in question cuts across other layers, it is more likely to be a sill, while if it is covered by other layers, it is more likely to be a flood basalt.
- Structural features: Flood basalts and sills can also be distinguished based on the structural features of the rock layers. Flood basalts often have distinctive horizontal layering or columnar jointing, while sills may have vertical or inclined layering or a more chaotic appearance.