GEOLOGICAL TIME SCALE

PLANET EARTH AND ITS

INTERIOR
Lecture 2
CE 2060: Geotechnical Engineering-1

Prof. V B Maji & Dr. Tarun Naskar

Department of Civil Engineering
IIT Madras
Geological time scale
 A view of Earth
• Earth is a planet that is small and self-contained

Earth’s four spheres

Hydrosphere
Atmosphere
Biosphere
Solid Earth
 Earth’s internal structure
• Layers defined by composition
• Crust
• Mantle
• Core
• Layers defined by physical properties
• Lithosphere
• Asthenosphere
• Mesosphere
• Inner and Outer Core
Earth’s layered structure
 DIFFERENT LAYER’S DESCRIPTION

1. Crust
a) Continental Crust(averages about 35 km thick; 60 km
in mountain ranges; diagram shows range of 20-70 km)
Granitic composition

b) Oceanic Crust (5 - 12 km thick; diagram shows 7-10 km

average) - Basaltic composition - Thin layer of
unconsolidated sediment covers basaltic igneous rock.
2. Mantle (2885 km thick)
Composition: peridotite (Mg Fe silicates), kimberlite
(contains diamonds), eclogite - based on studies of rock
from mantle brought up by volcanoes, from density
calculations, and composition of stony meteorites.
Lithosphere - outermost 100 km of Earth . Consists of the
crust plus the outermost part of the mantle. Divided into
tectonic or lithospheric plates that cover surface of
Earth
Asthenosphere - low velocity zone at 100 - 250 km depth
in Earth (seismic wave velocity decreases). Rocks are at
or near melting point. Magmas generated here. Solid
that flows - plastic behavior. Convection in this layer
moves tectonic plates.

Less is known about the mantle below the

asthenosphere.
3. Outer core (2270 km thick)
 S-waves cannot pass through outer core,
therefore we know the outer core is liquid
(molten).
 Composition: Molten Fe (85%) with some Ni,
based on studies of composition of meteorites.
 Core may also contain lighter elements such as Si,
S, C, or O. Convection in liquid outer core plus
spin of solid inner core generates Earth's
magnetic field.
 Magnetic field is also evidence for a dominantly
iron core.
4. Inner core (1216 km radius)
Solid Fe (85%) with some Ni - based on studies
of meteorites
Inferences from wave travel
 Inferences from wave travel
The curved wave paths indicating gradual increase in density and
seismic wave velocity with depth. There is a sharp refraction (bending
of waves) at the discontinuities or boundaries between layers.
Note the shadow zones. There is a large S-wave shadow zone (labelled
" No direct S waves") extending across the side of the globe opposite
from the epicenter (from 105o).
S-waves cannot travel through the molten (liquid) outer core.
There is a smaller P-wave shadow zone, seen on both sides (gray
shading), from 105o to 140o. The P-wave shadow zone makes a ring
around the globe.
The Earth's Internal Layered Structure
 Internal Structure of the earth
 Radius = 6370 km
 Average density 5.52 (2.6-3 for the crust)
 Earth has a layered structure.
 The boundaries between the layers are called discontinuities.
 The layered structure is determined from studies of how seismic
waves behave as they pass through the Earth.
 P- and S-wave travel times depend on properties of rock
materials through which they pass. Differences in travel times
correspond to differences in rock properties.
 Seismic wave velocity depends on the density and elasticity of
rock.
 Seismic waves travel faster in denser rock.
 Speed of seismic waves increases with depth (pressure and
density increase downward).
 PLATE TECTONICS

The continents could fit together like

pieces of a puzzle
Pangea (Greek, pan "entire"and Gaia "Earth") was the
supercontinent that existed during the Paleozoic and
Mesozoic eras about 250 myrs ago, before the component
continents were separated into their current configuration
World Plates
 Plate Tectonics
• The Earth’s crust is divided into 12 major
plates which are moved in various
directions.
• Tectonics is basically the deformation of
the crust as a consequence of plate
interaction
• This plate motion causes them to collide,
pull apart or rub against each other.
• Each type of interaction causes a
characteristic set of Earth structures or
“tectonic” features.
 Plate Movement
“Plates” of lithosphere are moved around by
the underlying hot mantle convection cells
 Three types of plate boundary
Divergent Convergent Transform
Divergent Boundaries

• Spreading ridges
– As plates move apart new material is erupted to fill
the gap
Convergent Boundaries
• There are three styles of convergent plate
boundaries
– Continent-continent collision
– Continent-oceanic crust collision
– Ocean-ocean collision
 Continent-Continent Collision
• Forms mountains, e.g. European Alps, Himalayas
Himalayas
 Continent-Oceanic Crust Collision
• Called SUBDUCTION
Subduction

• Oceanic lithosphere subducts

underneath the continental
lithosphere
• Oceanic lithosphere heats
and dehydrates as it subsides
• The melt rises forming
volcanism
• E.g. The Andes
Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs
over the other which causes it to sink into
the mantle forming a subduction zone.
• The subducting plate is bent downward to
form a very deep depression in the ocean
floor called a trench.
• The worlds deepest parts of the ocean are
found along trenches.
– E.g. The Mariana Trench is 11 km deep!
Transform Boundaries

• Where plates slide past each other

Above: View of the San Andreas
transform fault
28