Igneous Rocks

Granite

Is Granite plutonic or volcanic?

Plutonic

Approximate Mineral Content of granite

More than 20% quartz
Dominantly K-feldspar or subequal K-feldspars and plagioclase feldspars
Biotite and/or amphibole
May contain muscovite

Rhyolite

Is rhyolite plutonic or volcanic?

Volcanic

Approximate mineral content in rhyolite

More than 20% quartz
Dominantly K-feldspar or subequal K-feldspars and plagioclase feldspars
Biotite and/or amphibole
May contain muscovite

Dacite

Is dacite volcanic or plutonic?

Volcanic

Approximate mineral content in dacite

More than 20% quartz
Dominantly plagioclase feldspar
Biotite and/or amphibole

Granodiorite

Is granodiorite plutonic or volcanic?

Plutonic

Approximate mineral content of granodiorite

More than 20% quartz
Dominantly plagioclase feldspar
Biotite and/or amphibole

Andesite

Is Andesite volcanic or plutonic?

Volcanic

Approximate mineral content of Andesite

0-10% quartz
Plagioclase is the only feldspar
Dominantly amphibole

Diorite

Is diorite volcanic or plutonic?

Plutonic

Approximate mineral content of diorite

0-10% quartz
Plagioclase is the only feldspar
Dominantly amphibole

Basalt

Is Basalt volcanic or plutonic?

Volcanic

Approximate mineral content of basalt

No quartz
Plagioclase is the only feldspar
Dominantly pyroxene

Dolerite

Grain size of dolerite

Medium grained

Gabbro

Is gabbro volcanic or plutonic?

Plutonic

Approximate mineral content of gabbro

No quartz
Plagioclase is the only feldspar
Dominantly pyroxene

Periodite

Is periodite volcanic or plutonic?

Plutonic

Approximate mineral content of periodite

More than 90% olivine and pyroxene
May have some plagioclase
No quartz

Felsic rocks

Granite, rhyolite

Intermediate rocks

Diorite, andesite

Mafic rocks

Basalt, gabbro

Ultramafic rocks

Peridotite, Komatilite

Earth’s structure

Solid inner core (iron and nickel), outer core ( liquid), lower and upper mantle, crust

Mineral trends with ultramafic and felsic rocks

Silica (SiO₂) increases from roughly 40% in ultramafic rocks to 75% in felsic rocks
Iron, magnesium and calcium increase toward the mafic/ultramafic end

Melting temperature trend with ultramafic and felsic rocks

Melting temperature is much higher for ultramafic rocks compared to felsic rocks

Basaltic eruptions characteristics

Low silica so therefore low viscosity (runny)
As gas escapes easily from runny magma, basaltic eruptions are generally less explosive.
Typically produce lava flows, fire fountains and broad shallow-sloped volcanoes

The tectonic settings for basaltic volcanoes

Hotspot volcanism, Divergent plate boundaries

Hotspot volcanism occurs when

Superheated rock from deep inside the Earth’s mantle rises upward in a narrow column called a mantle plume
This hot plume melts the bottoms of the tectonic plate above it, created pools of melted rock or magma
This magma pushes up through cracks in the crust and erupts onto the surface as lava building a volcano
The hotspot stays in one place but the Earth’s tectonic plate drifts
As the plate moves, the old volcano is carried away from the hot plume, loses its magma supply and dies
A new volcano forms over the hotspot, creating a long line of volcanoes or islands

How do divergent plate boundaries form?

Occurs where two tectonic plates are pulling away from each other (due to convections currents)
As the plates separate, the crust between them stretches, becomes thin and cracks
With the heavy crust pulling apart, the pressure on the hot mantle below drops. This pressure drop causes the mantle rock to melt into magma
The magma rises through the news crack and cools in the cold air or water and hardens into new basalt filling the gap

Main two types of divergent plate boundaries

Mid ocean ridge (underwater), rift valley (terrestial)

Intraplate volcanism occurs

When stresses within a single tectonic plate cause the crust to stretch or deep mantle processes create localized heat
Magma takes takes advantage of these weak zones to push to the surface

What is a Monogenic volcano?

A volcano that erupts during one single multi-week, multi-month or multi-year episode and then never erupts again

How do hydrothermal ore deposits form?

Hot metal-rich hydrothermal fluids migrate through rock fractures at oceanic rifts and precipitate metal ores in concentrated veins or disseminated deposits

The subduction zone i.e formation of intermediate rocks

Plates collide and one heavy ocean plate slides under another plates and slides deep into the earth
The diving ocean plate carries seawater-soaked rocks and mud with it
As it goes deeper and hotter it gets squeezed like a wet sponge, driving water upward into the mantle rock above it
Adding water to hot mantle rock lowers its melting point
The water gets baked directly into the minerals forming hydrous minerals like amphibole

Arc volcanoes

A curved chain or belt of volcanoes that forms above a subduction zone where one tectonic plates beneath another
Two types - island arcs and continental arcs

Island arc volcanoes

Formed in the ocean when one oceanic plate slides under another oceanic plate, creating a chain of volcanic islands

Continental volcano arcs

Formed on land when an oceanic plate slides under a continent creating a chain of volcanoes along the edge of the land

What is a magma chamber?

A pool beneath the earth’s surface where molten rock collects and stays stored

Underground evolution of magma in volcano arc magma chambers

Compared to short lived basaltic conduits, arc volcanoes developed long lived magma reservoirs in the crust leading to mineralogically and chemically evolved systems
As magma slowly cools, early-formed high temperature minerals settle to the floor of the chamber, depleting the magma in iron/magnesium and enriching the remaining liquid in silica, sodium and potassium
Magma melts and incorporates surrounding continental wall rock, further driving the chemistry of the rock toward compositions higher in silica

Eruption dynamics of subduction volcanoes

High silica creates long polymer chains that make intermediate magmas thick and sticky. Trapped water gas bubbles expand as magma rises, building immense pressure
When the gas pressure exceeds rock strength, the magma blasts apart violently into hot gas and ash columns rather than extruding smoothly

Three main hazards when subduction volcanoes blow

Pyroclastic flows, lahars, ash fall and plumes

Pyroclastic flows

Superhot, high-speed avalanches of gas, ash and jagged rock fragments racing down the mountains

What is formed when pyroclastic flows cool down?

Ignimbrites

What are ignimbrites?

Fine matrix rocks containing trapped angular lithic rock fragments, flattened pumice and quartz crystal

What are lahars?

Giant, fast-moving volcanic mudflows created when hot ash mixes with melted mountain snow, ice or heavy rain

Ash fall and plumes

Huge ash clouds blasted tens of kilometers into the sky

Goldmine connection below subduction volcanoes

Underneath explosive volcanoes, giant underground magma pools cool down slowly over millions of years. As the rock solidifies, super-hot, metal-rich water gets squeezed out into the surrounding cracks. When these fluids cool, they deposit huge amounts of valuable metals like copper and gold

Glassy and frothy felsic products

When felsic magma erupts it can cool so rapidly that no crystals form, producing obsidian (volcanic glass) or trap expanding gas bubbles to create pumic

Underground magma bodies that cool without reaching the surface leave behind distinct geological formations including

Dikes, sills, plutons, batholiths

Economic mineral deposits in felsic systems

As a felsic magma chamber cools and solidifies, rare or incompatible elements that do not fit into common minerals get concentrated in the late-stage residual fluid

What are pegmatites?

Very coarse-grained felsic dikes formed from fluid-saturated, late-stage silicic magmas. They grow unusually large mineral crystals (such as giant quartz, mica and feldspar crystals)

Types of volcanoes/landforms formed around felsic rocks

Supervolcanoes, calderas

How do calderas form

1. Silica-rich magma bulges beneath the crust, cracking the roof rocks and creating circular ring fractures

2. Massive volumes of gas and ash shoot out as pyroclastic flows, rapidly emptying the reservoir

3. Derived of subterranean support, the overlying crust collapses downward into the emptied magma chamber, forming a giant crater called a caldera

4. Over time, slow magma refill from below pushes the caldera floor back upward, created an elevated resurgent dome