Metamorphic Rocks!

1. Metamorphic Rocks! Chapter 3 Section 4

2. Metamorphic • Metamorphic rocks are formed under heat, pressureor both deep in the Earth. • Look very different (most of the time) from parent rock Ig Sed Met Met

3. Formation of Metamorphic Rocks • Most metamorphic changes= elevated temperatures and pressures • Occur few kilometers below Earth’s surface and into upper mantle • Two locations for metamorphism: • Contact: magma intrudes rock • Regional: large areas of rock are subjected to extreme pressures and temperatures

4. Formation: Contact Metamorphism • When magma intrudes solid rock • Produces low-grade metamorphism; minor changes • High temperatures & moderate-to-low pressure • Temperature decreases with distance from intrusion • Metamorphic effects also decrease with distance • Limited to thin zones because lava cools quickly • Example: marble forms when magma intrudes a limestone body

5. Formation: Regional metamorphism • When high temperature and pressure affect large areas of Earth’s crust  belts of regional metamorphism • Range in grade: low  high • Ex: mountain building = high-grade metamorphism • Folding and deforming of layers in the area

6. Regional Metamorphism

9. Agents of metamorphism • During metamorphism, rocks are usually subjected to 3 agents (at the same time): • Heat: provides energy needed to drive chemical reactions • Pressure (stress): causes spaces between mineral grains to close • Hydrothermal solutions: surrounds mineral grains and help recrystallization process

10. Agents: Heat • Most important agent  provides energy needed to drive chemical reactions  cause existing minerals to recrystallizeornew minerals to form • Heat originates from magma & change in temperature with depth • Upper crust  increase averages between 20°C and 30°C per kilometer! • Different minerals recrystallize at different temperatures • Example: 8 km 150°C-200°C • Clay minerals: become unstable and recrystallize to form new stable minerals (chlorite) • Silicate minerals: stable @ these temps.  requires higher temps. to change silicates

11. Agents: Pressure (stress) • Pressure increases with depth • Applied in all directions • Causes spaces between mineral grain to close • Results in more compact rock  greater density • May cause minerals to recrystallize  new minerals • Cause rocks to flow versus fracture  grains flatten & elongate

12. Agents: hydrothermal solutions • When very hot water reacts with rock and alters chemical and mineral composition • Hot fluids migrate in and out of the rock  original mineral composition and texture changes • Example: gold lodes = gold deposit fills fissure in rock usually in vein formation A view of an undeveloped lode gold deposit. The gold is in the white veins in the photo. Andes Mtns

13. Classification of Metamorphic Rocks • Classified by texture and composition • Texture can be foliated or nonfoliated: • Foliated: layered or banded appearance • Contact metamorphism more compact more dense • Pressure causes microscopic minerals to compact and align in similar direction • Certain minerals can recrystallize in extreme conditions  flat needle like crystals form perpendicular to direction of force • Example: gneiss Gneiss

14. Classification of metamorphic rocks (cont’d) • Nonfoliated: does not have banded texture • most contain only one mineral & blocky crystal shapes • Example: marble – nonfoliated contact metamorphic rock made of calcite; parent rock (limestone) metamorphosed, calcite crystals combine larger interlocking crystals seen in marble Marble

15. Mighty morphing rocks! • A metamorphic aureole in the Henry Mountains, Utah. • Top = greyish rock igneous intrusion, consisting of porphyritic granodiorite • Bottom = pinkish rock on the bottom is the sedimentary country rock, a siltstone. • Middle = metamorphosed siltstone is visible as both the dark layer (~5cm thick) and the pale layer below it. Aureole = site of contact meta.