The Earth's Internal Structure

Faculty of Agro Industry and Natural Resources

KULIAH 2

Internal structureHow do we obtain information about the parts of earth beneath the surface?Earth’s Internal Structure

Density of the earth MaterialRheologic Division of the earth

MagmaWhat Is MagmaHow magma forms?How magma of different composition evolve?Igneous activity by plate tectonics

FACULTY OF AGRO INDUSTRY AND NATURAL RESOURCES DEPARTEMENT GEOSCIENCE

PROPAGATION OF SEISMIC WAVES THROUGH EARTH’S INTERIOR

Longitudinal waves travel through both solids and liquids.

Transverse waves travel through solids only.

Evidence from seismology tells us that the Earth has a layered structure. Seismic waves generated by earthquakes travel through the Earth with velocities that depend on the type of wave and the physical properties of the material through which the waves travel. Types of Seismic Waves

Body Waves - travel in all directions through the body of the Earth. There are two types of body waves:

P - waves - are Primary waves. They travel with a velocity that depends on the elastic properties of the rock through which they travel. S-Waves - Secondary waves, also called shear waves, travel with a velocity that depends only on the rigidity and density of the material through which they travel:

Surface Waves - Surface waves differ from body waves in that they do not travel through the Earth, but instead travel along paths nearly parallel to the surface of the Earth.

FACULTY OF AGRO INDUSTRY AND NATURAL RESOURCES DEPARTEMENT GEOSCIENCE

Seismic waves reflect from and refract through boundaries where there is sudden change in the physical properties of the rock, by tracing the waves we can see different layers in the Earth. Note that we know that density must increase with depth in the Earth because the density of crustal rocks are about 2,700 kg/m3 and the average density of the Earth is about 5,200 kg/m3. If density increases, wave velocity decreases.

FACULTY OF AGRO INDUSTRY AND NATURAL RESOURCES DEPARTEMENT GEOSCIENCE

Density Of earth material, From the velocity structure, the solid earth

Core – 1/3 mass of the earthMantle – 2/3 mass of the earthCrust - < 0.2 % of the earth

Continental (under land)Oceanic (under ocean)

All magmas contain Si and O Upon cooling, bond together into silicon-oxygen tetrahedrons

More silica (i.e. felsic), more viscous (harder to flow, thicker)

Also contain varying amounts of other elements like Na, K, Al, Ca, Mg, Fe, etc…Dry magmas – no volatilesWet Magmas – up to 15% volatilesVolatile content strongly effects the viscosity (ability to flow)

More volatiles, less viscous (easier to flow or more fluid)

What is Magma Made of ?

Remember that the tectonic plates don’t really float on a liquid asthenosphere, rather the asthenosphere is a ductile solid and is only melted in specific locations.Most magma/lava is not 100% liquid.

Magma/Lava is made of many compounds, all of which have different melting temps. Only a few percent of liquid is required to make a melt.

Other than a rise in temperature, what causes melting of rock within the Earth?Melting happens because of:

Decrease in pressure (decompression)Addition of volatiles (H2O, CO2, etc…)Heat transfer from rising magma

Formation of Magma

The Earth gets hotter with increasing depth due to primordial heat and radioactive decay of elements near the core.

The rate at which temperature increases with depth is called the geothermal gradient, or geotherm

Liquids have no organized structure, so to melt a rock, the mineral bonds must be broken

(animated gif of atoms)

Melting due to Decompression

The geotherm of the Earth

At depth, confining pressure prevents atoms from breaking free of crystals

Solidus: The temperature when a rock first begins to melt

Liquidus: The temperature where the last solid particle melts

The asthenosphere cools only slightly as it rises (convection) because it is a good insulator (high specific heat)

Melting due to Decompression

The solidus and liquidus of peridotite (ultramafic mantle rock)

Volatiles: A substance that can easily change into a gas at relatively low temperatures (H2O, CO2, etc…).

The addition of volatiles at depth (mainly H2O) seeps into rocks and helps break bonds (aids in melting). Analogy: Think of putting salt onto ice to lower the melting temperature. Likewise, adding water to rocks changes the melting point of rocks just like adding salt to water.

Melting due to the Addition of Volatiles

The addition of H2O into basalt, for example, drastically changes its melting temperature

In this case, basalts at 60km depth beneath the continents could begin to melt only if they were volatile rich.

Melting due to the Addition of Volatiles

The geotherm beneath a continent and the solidus of wet and dry basalt

Dep

th (

km)

Melting can also occur when rising bodies of hot material essentially bake the nearby rock

Analogy: Think of injecting hot fudge into ice cream. The hot fudge transfers heat to the ice cream and melts it

Melting Due To Heat Transfer

Like rocks, not all magma is made of the same stuffWe divide magmas into groups by their composition

Felsic (Silicic): 66-76% Silica (SiO2)Most viscous, Least dense (~2.5 gm/cm3), melting point 650-800oC

Intermediate: 52-66% SiO2

Mafic: 45-52% SiO2, lots of MgO, FeO, and Fe2O3

Ultramafic: 38-45% SiO2, abundant MgO, FeO, and Fe2O3

Least viscous, Most dense (~3.5 gm/cm3), melting point up to 1300oC

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Composition controls density, T, and viscosity.Most important is the content of silica (SiO2).

Silica-rich magmas are thick and viscous.Silica-poor magmas and thin and “runny.”

These characteristics govern eruptive style.

Magma Compositions

Type Density Temperature Viscosity

Felsic Very low Very low (600 to 850°C) Very High: Explosive eruptions.

Intermediate Low Low High: Explosive eruptions.

Mafic High High Low: Thin, hot runny eruptions.

Ultramafic Very high Very high (up to 1,300°C) Very low

Why are Magmas so Variable in Composition?

Differences in Magma composition occur due to 5 main reasons…

1. Different source rock compositionsmelt a felsic rock = felsic magma

2. Magma mixingmix felsic magma with mafic magma = intermediate magma

3. Partial melting

4. Assimilation

5. Fractional crystallization

In order to understand the melting and solidifying of magma we need to understand Bowen’s reaction series. – Bowen figured this out by melting rocks in an oven, letting them cool, and watching what minerals crystallizedThis series outlines the order in which minerals form in a cooling meltAlso applies in reverse order to rocks that are partially melted

Bowen’s Reaction Series

• Discontinuous series (different minerals form) and Continuous series (Plagioclase only)• So, a melt gets less mafic as it cools; In heating, the first minerals to melt are felsic.

Most magmas are not 100% liquidCommonly 2-30% melt; called a crystal mush

According to Bowen’s reaction series, rocks that are partially melted become more mafic, because the silica-rich felsic minerals are melted first. The melted part of the partial melt is thus more felsic than the remaining rock.

Partial Melting

The felsic mineral,

quartz, is a common

cement in many rocks

As magma sits in its chamber, it may incorporate minerals from the surrounding wall rock

Called assimilation

Occurs when wall rocks fall into the magma and melt (stoping) or when the magma partially melts minerals from the wall rockDegree of assimilation depends on composition of wall rock, temp of magma, amount of H20 present, amount fractures in and strength of the wall rock, and residence time

Assimilation

Stoping: The process of incorporating chunks of wall rock into a magma bodyXenolith: A non-melted chunk of wall rock incorporated into a magma body

May have a very different composition than the magma

Stoping & Xenoliths

A xenolith in granite in the Mojave desertUsually recognized because they may have a different texture (grain size) and composition than the rest of the rock

Xenolith

Not all minerals crystallize at the same temperature – This is fractional crystallizationAs magmas cool, they become more felsic.Mafic minerals crystallize first and are more dense than the melt, so they sink to the bottom

Fractional Crystallization

Bowen’s reaction series is an example

of fractional crystallization

If magma did not move, no extrusive/volcanic rocks would ever have formedMagma rises because:

hotter and less dense than the surrounding rock and therefore buoyantly rises.the weight of the overlying rock (lithostatic pressure) literally squeezes the magma out.

Analogy: Think of stepping on a tube of toothpaste to force it out, or mud squishing through your toes when you step in a puddle

Viscosity affects a magma or lava’s ability to flowControlled by:

Temperature (high temp - low viscosity)Volatile content (more volatiles – less viscous)Silica content – silica tends to form silica-oxygen tetrahedrons that bond with each other to make long chains that ultimately resist flow (more silica – more viscous)

Magma Movement

Explosive eruptions generally occur when source magma is:

High in silica (felsic-intermediate)Low tempHigh in volatiles

These volcanoes formLava domesAsh clouds and ash flows

Extrusive Igneous Rock Environments• Effusive eruptions generally occur when

source magma is:– Low in silica (mafic)– High temp– Low in volatiles

• These volcanoes form– Fluid lava flows– Fire fountains, lava tubes

Hawaii

CascadesNW USA

Magma rises by percolating between grains and/or by forcing open cracks in the subsurfaceThe magma that doesn’t reach the surface of the Earth cools into intrusive igneous rocks

Country rock or wall rock: The pre-existing rock that magma intrudes intoIntrusive contact: The boundary between the igneous intrusion and the wall rock

Tabular intrusions: Dike, Sill, Laccolith (pseudo-tabular, or sheet-like)

Non-tabular intrusions: Pluton, Batholith, Stock

Intrusive Igneous Rock Environments

Mt. Rushmore is carved out of a granitic igneous intrusion

Crystalline Igneous Rocks