Igneous Rocks Geological Oceanography

Rocks

Solid aggregates of grains of one or more minerals (exceptions: Volcanic glass- obsidian, and coal)

Classifications

Generally reflect texture (size, shape, and mineralogical interrelationships) and mineralogy

Mineralogy

Igneous it gives info about cooling history

Sedimentary gives info about depositional history

Metamorphic gives info about the tectonic history

The Rock Cycle

Start with Igneous Rocks forming from cooling magma

Weathering, Erosion, deposition, and lithification (cementing the sediments together) forms sedimentary rocks

Heat and pressure on preexisting rocks change them into metamorphic rocks

Melting in the Earth forms magma

Start again!

Formation of Igneous Rocks

Form as magma cools and crystalizes

If it occurs inside the Earth, these are termed plutonic rocks

Magma that makes it to the surface is called lava. These rocks are termed volcanic

2 major categories of igneous rocks:

1. volcanic: called extrusive

2. plutonic: called intrusive

Bowen’s Reaction Series

• From top to bottom of Bowen’s Reaction Series: 

  • Decreasing temperature

  • Dark to light-colored minerals 

  • Decreasing density 

  • Increasing viscosity (increasing resistance to flow) (volcanic flow slows, it gets stuck; large amounts of silica cause this viscosity)

• Start w/ Basaltic- the minerals in that section are what make salt (sometimes olivine can be included)

  Andesitic- the rock type that these minerals form (biotite mica…) 

  Granitic- potassium feldspar, muscovite mica, quartz

Aphanitic

Crystals in the rock are fine-grained (smaller than the eye can see)

Texture indicates rapid cooling

Phaneritic

Crystals in the rock are coarse-grained (larger-sized)

Texture indicates slow cooling

Porphyritic

combo of aphanitic (fine-grained) & phaneritic (coarse-grained) textures

Fine crystals are called groundmass

Coarse crystals are phenocryst

this texture indicates slow then fast cooling

Pumice (glassy)

cools really fast, air bubbles mean rock didn’t have time to form crystals so it’s considered glassy, floats

Vesicular

Similar to pumice but heavier & less air pockets, doesn’t float

Ultramafic Rocks

Mostly composed of olivine and pyroxene

ex: peridotite & dunite

plutonic rocks that form deep in the Earth

Mafic Rocks

Rocks with mostly dark crystals

ex: magnesium and ferrum

contain some olivine, pyroxene, amphibole

includes: basalt (volcanic) & gabbro (plutonic)

Intermediate Rocks

both light & dark minerals

mostly plagioclase, some amphibole & pyroxene

includes: andesite (volcanic) & Diorite (plutonic)

Felsic Rocks

Light-colored minerals

quartz, muscovite, k-spar, biotite

includes: Rhyolite (volcanic) & granite (plutonic)