* Lava hardening on the surface forms extrusive igneous rocks. These rocks are generally characterized by:
* Fine-grained texture: Lava cools quickly, leaving little time for large mineral crystals to form.
* Often glassy: Rapid cooling can result in the formation of glass.
* May contain vesicles: Bubbles trapped in the lava as it solidifies can create holes.
* Layers in rocks can form through different processes:
* Sedimentation: Sediments are deposited in layers over time, forming sedimentary rocks. These often have visible layers and may lack large crystals.
* Volcanic eruptions: While a single lava flow can create a layer, multiple flows over time can lead to layered volcanic rocks. However, these layers are often marked by changes in texture, color, or the presence of volcanic ash, rather than being perfectly smooth.
* Metamorphism: Heat and pressure can change the texture and layering of existing rocks, creating metamorphic rocks.
Therefore, the description of visible layers without mineral crystals suggests a sedimentary rock, rather than an extrusive igneous rock formed from lava.
However, it's important to note:
* Sometimes igneous rocks can be layered: Multiple lava flows can create layers in a volcanic rock.
* Without a sample: It's difficult to definitively identify the rock's origin without a sample and further investigation.
To determine the origin of a rock, examining its texture, mineral composition, and layering is important.
