Igneous Rock: Formation and Magma Composition

Igneous Rock Formation

• Igneous rocks originate from the cooling and solidification of molten rock.
• These rocks are composed of minerals containing silicon and oxygen, along with other elements.
• Igneous rocks result when lava or magma cools, allowing minerals to crystallize.
• Lava is essentially magma that has reached Earth's surface.

Composition of Magma

• Igneous rock types depend on magma composition and its mix of molten rock, dissolved gases, and mineral crystals.
• Magma is classified as basaltic, andesitic, or rhyolitic based on silica content.
• Silica impacts the melting temperature of magma and its flow rate.

Magma Formation

• Magma forms through the melting of Earth's crust or mantle.
• Temperature, pressure, water content, and mineral content are four major factors in magma formation.
• The geothermal gradient describes an increase in temperature with depth in Earth's crust.
• Increased pressure raises a rock's melting point; water content decreases it.

Mineral Content and Melting

• Rocks and minerals contain differing water percentages, changing melting points.
• Water content increases can lower melting points.
• Rocks rich in iron and magnesium melt at higher temperatures than those with more silicon.
• Partial melting happens when temperatures do not melt the entire rock, which causes a magma with a different composition.

Bowen's Reaction Series

• N.L. Bowen showed that minerals form predictable patterns as magma cools; this is known as Bowen's reaction series.
• Bowen identified two crystallization patterns or branches.
• The left branch consists of iron-rich minerals that abruptly change as magma cools.
• Feldspars make up the right branch, featuring plagioclase feldspars that change composition continuously during magma cooling.

Mineral Composition and Types of Igneous Rocks

• Intrusive rock cools and crystallizes below Earth's surface.
• Extrusive rock cools and crystallizes on Earth's surface.
• Basaltic rock is dark-colored with low silica content, composed mainly of plagioclase and pyroxene.
• Granitic rock (felsic) is light-colored with high silica content, containing quartz and feldspar.
• Intermediate rocks are between basaltic and granitic in composition.
• Ultrabasic/ultramafic rocks are always dark and contain only iron-rich minerals.

Texture and Formation

• Texture refers to the size, shape, and distribution of crystal grains in the rock.
• Quickly cooled lava forms volcanic glass.
• Porphyritic rocks have large, well-formed crystals in a matrix of finer crystals.
• Vesicular texture indicates a spongy appearance from trapped gas bubbles.

Resources

• Veins of valuable ore deposits are often found within igneous intrusions.
• Pegmatites are veins of large-grained minerals that may contain ores.
• Kimberlites are ultrabasic rocks formed under high pressure and can contain diamonds.
• Olivine is often the first mineral to form as magma cools.

Sedimentary Rock Formation (Weathering and Erosion)

• Sedimentary rocks start as small rock pieces (sediments) moved by water, wind, glaciers, and gravity.
• Weathering breaks down rocks, and erosion transports sediments.

Weathering Types

• Chemical weathering changes rocks chemically through dissolution.
• Physical weathering breaks rocks into fragments without changing their chemical composition.
• Granite eventually decomposes when exposed to chemical and physical weathering.

Erosion and Transport

• Erosion is sediment removal and transport.
• Wind, water, gravity, and glaciers erode materials.
• Eroded fragments are often transported to new locations downhill.

Deposition

• Deposition occurs when transported sediments settle on the ground or at the bottom of water bodies.
• Layers form, with the largest grains settling at the bottom.

Energy of Transporting Agents

• Particles settle in layers of different sizes as water slows down.
• Wind-deposited sediments are fine and well-sorted due to wind carrying only small grains.
• Unsorted deposits occur from glaciers melting or landslides.

Lithification

• Lithification is the process where sediments turn into sedimentary rocks under pressure and temperature.

Compaction and Cementation

• Lithification begins with compaction, squeezing the sediment grains together when overlying sediment increases.
• Cementation occurs as dissolved minerals precipitate from groundwater, gluing sediment grains.

Sedimentary Features (Bedding)

• Bedding, or horizontal layering, is a primary sedimentary rock feature resulting from sediment settling.
• Graded bedding is when particle sizes get heavier and coarser from top to bottom.
• Cross-bedding consists of inclined sediment layers deposited across a horizontal surface.

Angularity

• During sediment transport, angular pieces become rounded as edges break off.

Sedimentary Rocks (Clastic)

• Clastic sedimentary rocks, the most common type, form from loose sediment deposits.
• "Clastic" describes rock and mineral fragments from weathering and erosion.
• These rocks are further classified by particle size.
• Conglomerates are coarse-grained rocks with rounded gravel.
• Breccias are composed of angular, gravel-sized particles.

Sedimentary Rocks (Medium Grained)

• Porosity defines open spaces percentage within a material like rock.
• Sandstone serves as reservoirs for oil, natural gas, and groundwater.

Sedimentary Rocks (Fine Grained)

• Fine-grained clastic rocks are slit- and clay-sized particle sediments.
• Low-porosity fine-grained rocks prevent groundwater and oil movement.

Sedimentary Rocks (Chemical)

• Chemical sedimentary rocks, or evaporites, precipitate from solutions when minerals reach saturation.
• Constant evaporation leads to high salt precipitation.

Sedimentary Rocks (Biochemical)

• Biochemical sedimentary rocks are formed from remains of living organisms.
• Limestone, the most abundant type, comprises calcite.
• Shells of dead organisms form carbonate sediment layers.
• Calcium carbonate precipitates and crystallizes to form limestone during lithification.

Metamorphic Rock Formation

• Metamorphic rocks form upon high temperature and pressure exposure.
• This alters or changes the rocks, but without melting.
• High temperatures come from the earth's heat, and high pressure comes from deep burial or mountain building.

Metamorphic Textures

• Foliated rocks have layers and mineral bands.
• Foliation develops when pressure is applied from opposite directions, causing foliation being perpendicular to the pressure direction
• Nonfoliated rocks consist mainly of blocky crystal-shaped minerals, such as quartzite and marble.

Metamorphism Grades and Types

• Different pressure and temperature combinations result in different grades of metamorphism.
• Metamorphism is classified as low, intermediate, or high grade.
• Regional metamorphism occurs when high temperatures and pressures affect large Earth's crust areas, which results in changes in minerals and rocks.
• Contact metamorphism happens when molten material comes in contact with solid rock at high temperatures and moderate to low pressure.
• Hydrothermal metamorphism occurs when very hot water reacts with rocks to alter its composition near igneous intrusions and active volcanoes.

Rock Cycle

• The rock cycle groups rocks by their formation method.
• Igneous rocks crystallize from magma.
• Sedimentary rocks form from cemented or precipitated sediments.
• Metamorphic rocks form from changes from temperature and pressure.
• Any rock type can transform into another.