Metamorphic Rocks

What is Metamorphism? The transformation of a parent rock into a new rock (new minerals and/or new texture). This happens in the solid-state (without melting) in Earth’s middle to lower crust.

Important Factors During Metamorphism: 1) Composition of the parent rock. The resulting metamorphic rock will be compositionally similar to the parent rock.

Important Factors During Metamorphism: 2) Temperature. Heat for metamorphism comes from the geothermal gradient (30°C per km). A mineral can exist only over a finite temperature range called the stability field. If a mineral is outside of its stability field, it will break down into one or more new minerals (metamorphism). High temperatures favor lower density minerals. If the temperature gets too high, metamorphism turns into igneous processes.

Important Factors During Metamorphism: 3) Confining Pressure. Pressure applied equally in all directions is called confining pressure. Pressure increases along the pressure gradient (300 atm/km). High pressures favor higher density minerals.

Important Factors During Metamorphism: 4) Differential stress Tectonic forces often lead to forces that are not equal in all directions (called differential stress). There are two types of differential stress: Compressive stress causes flattening perpendicular to stress. Shear stress causes flattening parallel to stress.

Important Factors During Metamorphism: Differential stresses cause rocks to develop foliation, a planar rock texture formed by the alignment of “platy” minerals like micas.

Rock Foliation

Important Factors During Metamorphism: 5) Time Metamorphism can take millions of years. A time increases, new (more stable minerals) tend to grow larger and better foliations tend to develop.

Warmup- Try without your notes. What are 6 factors that control the type of rock formed during metamorphism? Which is the single most important factor?

Non-foliated Metamorphic Rocks (No micas…Names are based on parent composition) Limestone  Marble Quartz Sandstone  Quartzite Basalt  Hornfels

Foliated Metamorphic Rocks (Micas….Named based on degree of foliation) Slate- Very fine-grained, splits into flat sheets (rock cleavage). Phyllite- Fine-grained, splits along wavy surfaces, has a silky sheen. Schist- Relatively coarse-grained, with some very coarse grains (called porphyroblasts). Gneiss- Very coarse-grained, with light and dark minerals that separated into different layers.

A nice example of gniess!

Metamorphic Grade PARENT ROCK: SHALE or MUDSTONE GRADETEMPERATUREMETAMORPHIC ROCK High> 450 CGneiss Medium CSchist CPhyllite Low50 – 300 CSlate

Parent Rocks and their Metamorphic Daughters Parent RockMetamorphic Rock Shale, Mudstone, Volcanic TuffSlate, Phyllite, Schist Shale, Mudstone, or GraniteGneiss BasaltChlorite schist (greenschist), Amphibole Schist (amphibolite) Limestone or DolomiteMarble Quartz SandstoneQuartzite Conglomerate or BrecciaStretched-pebble conglomerate Shale, BasaltHornfels

Two Types of Metamorphism 1) Contact metamorphism -Occurs when a hot magma intrudes into cooler country rock. The increased temperature causes the country rock to “bake”. -There is no tectonic stress. Therefore the rocks are non-foliated (hornfels, quartzite, and marble). -Occurs in narrow zone (~ m wide) known as a contact aureole.

2. Regional metamorphism -Associated with convergent plate boundaries (especially mountain building). -Differential stress produces foliated textures. -Typically involve high pressures and a wide range of temperatures ( °C). -Higher pressure and temperature will produce rocks of increased metamorphic grade.

Compare and contrast contact metamorphism and regional metamorphism. What causes each, what are the sizes of the affected regions, & what types of rocks result? CCauseSizeFoliated or non- foliated? Contact Regional WARMUP

3. Shock metamorphism (rare). -Meteorite impacts trigger shock metamorphism (brief but extreme temperature and pressure). -Brecciated rocks are found near impact craters. -Increased pressure converts minerals to higher density forms (quartz → coesite).

Plate Tectonics and Metamorphism High T, High P Low T, High P Continent-continent collisions: When continents collide, high temperatures and pressures cause high grade metamorphism. Metamorphic rocks are common in the old, stabilized cores of continents known as cratons…indicative of suture zones (old convergent boundaries). These are the oldest rocks on Earth (Vishnu Schist in the Grand Canyon is 2 byo). Continent-ocean collisions: As rocks are pushed down into the earth at subduction zones, pressure increases but temperature stays low due to the cooling effects of the cooler subducted slab and associated water.

Metamorphic facies- The T-P conditions under which a metamorphic rock formed.

End of Chapter 7