Metamorphic Rocks Quiz

Schistose texture

• Light (e.g., quartz and feldspar) and dark (e.g., mica) are separated into parallel bands across the rock

• Mica minerals with pyrite porphyroblasts

• Platy minerals aligned at 90 degrees in the direction of maximum pressure

• Platy minerals aligned at 90 degrees in the direction of maximum pressure in thin layers

Example of poikiloblastic texture

• Mica minerals with pyrite porphyroblasts

• Spotted rock, hornfels

• Crystals in a spotted rock

• Large crystals that form after the groundmass

What size and shape crystals do medium-grade metamorphic rocks typically have?

• Medium, subhedral crystals

• Fine, anhedral crystals, as there is little time to recrystallize

• Gneiss

• Quartzite

Examples of a regionally-metamorphic rock

• Marble

• quartzite

• Pumice, obsidian, basalt

• Slate, schist, gneiss

• Igneous, sedimentary, and metamorphic

What rock does sandstone become under medium- and high-grade metamorphism?

• Granite Gneiss

• Quartzite

• Marble

• Gneiss

Thermal/contact metamorphism

• Friction from major fault planes, low temperature and high pressure

• In shale/mudstone, as they contain many minerals

• Compaction of sedimentary rocks, low temperature and high pressure

• Igneous intrusions, high temperature and low pressure, no foliation

How does intrusion volume affect metamorphism?

• Mafic magmas are ~1200 degrees, and silicic ~850 degrees, so mafic intrusions generally have a wider metamorphic aureole

• The extent of heat and pressure on the rock

• Some rocks (e.g., mudstone) are affected by metamorphism more than others (e.g., sandstone)

• The larger the volume, the wider the metamorphic aureole

How does magma composition affect metamorphism?

• Light bands are granoblastic, but dark bands are foliated to the direction of maximum pressure

• Mafic magmas are ~1200 degrees, and silicic ~850 degrees, so mafic intrusions generally have a wider metamorphic aureole

• The hotter the magma, the wider the metamorphic aureole

• Some rocks (e.g., mudstone) are affected by metamorphism more than others (e.g., sandstone)

Metamorphic aureole

• The "shiny" texture as a result of schistose texture

• The area around the intrusion where rocks are altered and recrystallised

• Alternating bands of quartz, feldspar, and mica, with garnet porphyroblasts

• A rock that changes its mineralogy and texture due to heat and pressure

How does composition of country rock affect metamorphism?

• The hotter the magma, the wider the metamorphic aureole

• Some rocks (e.g., mudstone) are affected by metamorphism more than others (e.g., sandstone)

• Mafic magmas are ~1200 degrees, and silicic ~850 degrees, so mafic intrusions generally have a wider metamorphic aureole

• Light bands are granoblastic, but dark bands are foliated to the direction of maximum pressure