igneous rock textures

• Subhedral crystal: Partially recognizable shape, partially impeded by pre-existing crystals.
• Texture refers to the general appearance of a rock, including grain size, shape, and the relationship between crystals.
• Euhedral crystal: Regular/recognizable shape, grows when not impeded by pre-existing material in a melt.
• Anhedral crystals:

  • No regular pattern/shape
  • Grow to fill spaces in pre-existing rock
• Tuff breccia refers to blocks of lava or rock around a volcano, surrounded by ash, made by explosive eruptions.
• Equicrystaline - igneous rock where all crystals are the same size, fine, medium or coarse grained
• Porphyritic - igneous rock where some crystals are large and some are small, formed when rock cools in two stages.
• Porphyritic granite has large crystals called phenocrysts that form with slow cooling first deep within the crust.
• Smaller crystals called groundmass are formed faster and after large crystals shallower in the crust.
• Porphyritic basalt phenocrysts form in magma chambers below volcanoes, during an eruption phenocrysts erupt and cool rapidly making fine crystals.
• Glassy - igneous rock with no crystals, occurs with very rapid cooling.
• Vesicular - as lava cools rapidly gas bubbles become trapped making holes in the rock.
• The holes are called vesicles and are found near the top of lava flows.
• Vesicular is common in basalt and pumice.
• Amygdaloidal - vesicles are filled with minerals precipitated from ground water flowing in rock, millions/thousands of years after eruptions.
• Calcite and quartz are found to fill gaps.
• Crystals grow outside and then into the hole, partially filled holes are geodes.
• Flow banding - often found in rhyolite, produces minerals of different shades that are parallel to lava flows.
• Tuff - rock equivalent of volcanic ash.