SGES 1302 INTRODUCTION TO EARTH SYSTEM

SGES 1302INTRODUCTION TO EARTH SYSTEM

LECTURE 3: Internal Structures of the Earth

2

Lecture 3INTERNAL STRUCTURES OF THE EARTH

3

Internal Structure The interior of the Earth, similar to the other terrestrial planets, is chemically

or compositionally divided into layers. The Earth has an outer silicate solid crust, a highly viscous mantle, a liquid

outer core that is much less viscous than the mantle, and a solid inner core. Many of the rocks now making up the Earth's crust formed less than 100

million years ago (Ma); however the oldest known mineral grains are 4.4 billion years old, indicating that the Earth has had a solid crust for at least that long.

Much of what is known about the interior of the Earth has been inferred. The force exerted by Earth's gravity is one measurement of its mass. After measuring the volume of the planet, its density can be calculated. Calculation of the mass and volume of the surface rocks and bodies of

water allow estimation of the mass, volume and density of surface rocks. The mass which is not in the atmosphere, oceans, and surface rocks must

be in deeper layers.

4

Atmosphere, ocean and land surface can be studied directly. Volcanic eruptions provided materials from depth (down to 200km) Earth’s interior – inferred from its density, the way it transmit seismic waves

and the nature of its magnetic field. The layering of the Earth has been inferred indirectly using the time of travel

of refracted and reflected seismic waves. The core does not allow shear waves to pass through it, while the seismic velocity is different in the other layers. The changes in the seismic velocity between the different layers causes refraction, which is described by Snell's law. Reflections are caused by a large increase in seismic velocity and are similar to light reflecting from a mirror.

Internal Structure

5

Internal Structure The structure of the Earth can be

divided using 2 ways: based on chemical composition and based on physical properties.

Chemically, the Earth can be divided into the crust, mantle, outer core, and inner core.

By physical properties, the layering of the earth is categorized as lithosphere, asthenosphere, mesosphere, outer core, and the inner core.

6

Schematic view of the interior of Earth.

1. continental crust

2. oceanic crust

3. upper mantle

4. lower mantle

5. outer core

6. inner core

A: Mohorovičić discontinuity

B: Gutenberg discontinuity

C: Lehmann discontinuity

0 35 70 2880 5160 6370 km

7

Structural units based on composition:crust, mantle & core The crust ranges from 5 to 70 km in depth. The thin (~8km) parts are oceanic crust composed of dense iron

magnesium silicate (mafic) rocks and underlie the ocean basins. The thicker (~40km) crust is continental crust, which is less dense

and composed of (felsic) sodium potassium aluminium silicate rocks.

The oceanic crust is relatively young and undeformed. The crust-mantle boundary is marked by a discontinuity in the

seismic wave velocity, which is known as the Mohorovičić discontinuity or Moho.

The cause of the Moho is thought to be a change in rock composition from rocks containing plagioclase feldspar (above) to rocks that contain no feldspars (below).

8

Earth's mantle extends to a depth of 2890 km, making it the largest layer of the Earth.

The pressure, at the bottom of the mantle, is ~140 GPa (1.4 Matm). The mantle is composed of silicate rocks that are rich in iron and magnesium relative to the overlying crust.

Although solid, the high temperatures within the mantle cause the silicate material to be sufficiently ductile (behaves like plastic) that it can flow on very long timescales.

Convection of the mantle is expressed at the surface through the motions of tectonic plates.

The melting point and viscosity of a substance depends on the confining pressure. As there is intense and increasing pressure as one travels deeper into the mantle, the lower part of the mantle flows less easily than the upper mantle.

Structural units based on composition:crust, mantle & core

9

The average density of Earth is 5515 kg/m3, making it the densest planet in the Solar system.

Since the average density of surface material is only around 3000 kg/m3, we must conclude that denser materials exist within Earth's core.

Further evidence for the high density core comes from the study of seismology.

In its earliest stages, about 4.5 billion years ago, melting would have caused denser substances to sink toward the center in a process called planetary differentiation, while less-dense materials would have migrated to the crust.

As a result, the core is largely composed of iron (80%), along with nickel and one or more light elements, whereas other dense elements, such as lead and uranium, either are too rare to be significant or tend to bind to lighter elements and thus remain in the crust.


10

Seismic measurements show that the core is divided into two parts, a solid inner core with a radius of ~1220 km and a liquid outer core extending beyond it to a radius of ~3400 km.

The solid inner core was discovered in 1936 by Inge Lehmann and is generally believed to be composed primarily of iron and some nickel.

The liquid outer core surrounds the inner core and is believed to be composed of iron mixed with nickel and trace amounts of lighter elements.

It is generally believed that convection in the outer core, combined with stirring caused by the Earth's rotation, gives rise to the Earth's magnetic field through a process described by the dynamo theory.

The solid inner core is too hot to hold a permanent magnetic field but probably acts to stabilise the magnetic field generated by the liquid outer core.


11

Why is the inner core solid, the outer core liquid, and the mantle solid/plastic?

The answer depends both on the relative melting points of the different layers (nickel-iron core, silicate crust and mantle) and on the increase in temperature and pressure as one moves deeper into the Earth.

At the surface both nickel-iron alloys and silicates are sufficiently cool to be solid.

In the upper mantle, the silicates are generally solid (localised regions with small amounts of melt exist); however, as the upper mantle is both hot and under relatively less pressure, the rock in the upper mantle has a relatively low viscosity (behaves like plastic).

In contrast, the lower mantle is under very high pressure and therefore has a higher viscosity than the upper mantle.

The metallic nickel-iron outer core is liquid despite the enormous pressure as it has a melting point that is lower than the mantle silicates.

The inner core is solid due to the overwhelming pressure found at the center of the planet.


12

Layers based on physical properties:lithosphere, asthenosphere, mesosphere & core

Tectonic plates of the lithosphere

13

The lithosphere is the solid outermost shell of a rocky planet. On the Earth, the lithosphere includes the crust and the uppermost mantle

which is joined to the crust across the Mohorovičić discontinuity. The base of lithosphere is defined as the 1280°C isotherm in the mantle:

lithosphere-asthenosphere boundary is a thermal boundary that vary in space and time (at that temperature, olivine the dominant mineral in the mantle becomes very weak)

The 1280°C isotherm is only a few km deep below mid ocean ridge, beneath oceanic plains is about 100 km, and beneath continents may be more than 150 km.

As the Earth's surface cools, the lithosphere thickens over time. It is fragmented into tectonic plates, which move independently relative to one another.

All crust is in the lithosphere, but lithosphere generally contains more mantle than crust.


14

The concept of the lithosphere as Earth’s strong outer layer was developed by Barrell (1914). The concept was based on the presence of significant gravity anomalies over continental crust, from which he inferred that there must be a strong upper layer (which he called the lithosphere) above a weaker layer which could flow (which he called the asthenosphere).

These ideas were enlarged by Daly (1940), and have been broadly accepted by geologists and geophysicists.

Although these ideas about lithosphere and asthenosphere were developed long before plate tectonic theory was formulated in the 1960's, the concepts that strong lithosphere exists and that the lithosphere rests on weak asthenosphere are essential to the plate tectonic theory.

Another distinguishing characteristic of the lithosphere is its flow properties. Under the influence of the low-intensity, long-term stresses that drive plate tectonic motions, the lithosphere responds essentially as a rigid shell and thus deforms primarily through brittle failure, whereas the asthenosphere is heat-softened and deforms plastically.


15


There are two types of lithosphere: Oceanic lithosphere, which is associated with Oceanic crust; & Continental lithosphere, which is associated with Continental crust

Oceanic lithosphere is typically about 50-100 km thick (but beneath the mid-ocean ridges is much thinner), while continental lithosphere is about 150 km thick, consisting ~50 km of crust and 100 km or more of uppermost mantle.

Oceanic lithosphere is denser than continental lithosphere. The upper mantle portion of both types of lthosphere has the same density, however, the oceanic crust has higher density than the continental crust.

New oceanic lithosphere is constantly being produced at mid-ocean ridges and is recycled back to the mantle at subduction zones. As a result, oceanic lithosphere is much younger than continental lithosphere: the oldest oceanic lithosphere is about 170 million years old, while parts of the continental lithosphere are billions of years old.

16


The asthenosphere is the region of the Earth between 100-200 km below the surface (1280°C isotherm) and extending to as deep as 400 km - that is the weak or "soft" zone in the upper mantle.

It lies just below the lithosphere, which is involved in plate movements and isostatic adjustments. In spite of its heat, pressures keep it plastic, and it has a relatively low density.

Seismic waves, the speed of which decrease with the softness of a medium, pass relatively slowly through the asthenosphere, thus it has been given the name low-velocity zone.

Under the thin oceanic plates the asthenosphere is usually much nearer the seafloor surface, and at mid-ocean ridges it rises to within a few kilometres of the ocean floor.

17


The upper part of the asthenosphere is believed to be the zone upon which the great rigid and brittle lithospheric plates of the Earth's crust move about.

Due to the temperature and pressure conditions in the asthenosphere, rock becomes ductile, moving at rates of deformation measured in cm/yr over distances eventually measuring thousands of kilometers.

The mesosphere refers to the lower mantle in the region between the asthenosphere and the outer core.

It is the largest layer of the earth. This region, also called the lower mantle, is named in order to differentiate from the lithosphere and the asthenosphere portions of the mantle.

It is more solid or rigid than the asthenosphere due to higher pressures.

SGES 1302 INTRODUCTION TO EARTH SYSTEM

Documents

Transcript of SGES 1302 INTRODUCTION TO EARTH SYSTEM