Lection 1 Lithosphere & Astenosphere An Introduction to Plate Tectonics …a

should be as simple as possible, but no simpler.
Albert Einstein

Course: Particularities and Features of Cold Region Geology



by
Alexey A. Krylov,
Institute of Earth Sciences, St. Petersburg State University

and
Plate Tectonics
Since the construction of the first good maps

meteorologist, proposed the continental drift hypothesis (between 1919-1929) to explain:

-

Alfred Wegener
1880-1930

Continental Drift, Sea-Floor Spreading, and
Plate Tectonics

and
Plate Tectonics
  How the seeds could have migrated across

Seed fern fossil, called Glossopteris, is one of the many fossils that were found on both sides of the Atlantic Ocean.

and
Plate Tectonics
Wegener proposed that at one time, all the

and
Plate Tectonics
Alfred Wegener was unable to provide a reliable

PROBLEMS:

He supposed that the centrifugal force of the Earth's rotation or the astronomical precession caused the drift.

Simple calculations show that this is impossible.
The scientific community has rejected the hypothesis of Alfred Wegener.

An Introduction to Plate Tectonics
Structure of the Earth
1. continental crust
2.

3. upper mantle

4. lower mantle

5. outer core (liquid)

6. inner core (solid)

A: Mohorovičić discontinuity – the boundary between crust and mantle.

B: Gutenberg discontinuity – the core-mantle boundary.

Earth Crust
Sedimentary layer
10
Upper crust:
“Granite layer” SIAL
20
Conrad boundary
Lower crust:
“Basalt layer” SIMA
“Moho”

Upper mantle

40

Basement’s surface

30

1

2

4

3

0

km

0

5

6

7

km

Sediments – Layer 1

Layer 2

2A(B) Basaltic pillow lava

2C Dolerite dikes

Layer 3

3A Isotropic gabbro

3B Serpentized peridotite

“Moho” boundary

Upper mantle

Layer 2: tholeiite (low-K olivine basalts)

Continental crust

Oceanic crust

Oceanic crust is mainly made of basalt whereas continental crust is mainly made of granite

The lower density of continental crust allows it to float on the mantle.

Earth Crust
Oceanic crust
Sediments
Serpentinized peridotites

(basalt), off Hawaii
Oceanic crust

the medium-grained equivalent of a basalt - a basic rock

crust.
The gabbro is deformed because of intense faulting at

Gabbro refers to a large group of dark, often coarse-grained, mafic intrusive igneous rocks chemically equivalent to plutonic basalt.
It forms when molten magma is trapped beneath the Earth’s surface and slowly cools into a holocrystalline mass.

classified as an ultramafic rock.
It has less than 45%

lithosphere = the crust + the uppermost mantle → constitute

the highly viscous, mechanically weak and
ductilely-deforming region of the

1300°С

At about 1300°C typical mantle material begins to melt, and softens dramatically. We call that part of the mantle asthenosphere. It is a weak zone, that "decouples" the plate from the overlying mantle.

and deformed like silly putty («умный пластилин») in response to

Lecture 1. An Introduction to Plate Tectonics

These rocks actually flow, moving in response to the stresses placed upon them by the churning motions («возвратно-поступательное движение») of the deep interior of the Earth.

The flowing asthenosphere carries the lithosphere of the Earth, including the continents, on its back.

happens at spreading centers where plates are moving apart.
Slab

Сross section through the Earth showing the convection cells of the mantle.

heat from the hot interior to the cooler surface →
-

Subduction zones → plates crash into each other; spreading ridges → plates pull away from each other; large faults → plates slide past each other.

Lecture 1. An Introduction to Plate Tectonics

plate tectonics: сontinental drift, earthquakes, volcanoes, magnetism, and heat flow

Lecture 1. An Introduction to Plate Tectonics

Introduction to Plate Tectonics

Introduction to Plate Tectonics

crust.
Lecture 1. An Introduction to Plate Tectonics

to Ocean
Lecture 1. An Introduction to Plate Tectonics

1. An Introduction to Plate Tectonics

heat flow is about 57 milliwatts per square meters (mW/m^2),

Introduction to Plate Tectonics
Plates are large pieces of the upper

plates
These seven plates make up most of the seven continents

plates
Arabian Plate; Caribbean Plate; Cocos Plate; Juan de Fuca Plate;

Ridge
Iceland - 1.8 cm/yr;
South

Lecture 1. An Introduction to Plate Tectonics

surface that has experienced volcanism for a long time. A

Lecture 1. An Introduction to Plate Tectonics

The volcano built up an island that extended above the surface of the ocean, and then plate tectonics carried the island away, creating an extinct volcano. But there’s always a new volcano being created by the same hot spot.

about the nature of hot spots.
Where they originate: upper

is generated by motions of the iron in the outer

That same magnetic field allows us to use a compass to navigate around Earth's surface.

filed provides some valuable information on the location of rocks

As the lava cools the iron they contain is preferentially oriented by the magnetic field of Earth, like mini-compasses.

As the rock continues to cool, its temperature decreases below the "blocking temperature" and the magnetically induced alignment of iron is frozen into the rock.

The net result is that the rock storing information on the orientation of Earth's magnetic field at the time the rock cooled.

years.
Black intervals had normal polarity (like that today),
and

Lecture 1. An Introduction to Plate Tectonics

about the mid-ocean ridge.
Note the symmetry on either side

Lecture 1. An Introduction to Plate Tectonics

are shown in red (age < 2 Ma) and red-orange

lithosphere with time.

Cycle:
Rifting of continents by mantle diapirism

(2) Continental drift, seafloor

preceded Wegener's Pangea
Late Precambrian - continents together in one

Lecture 1. An Introduction to Plate Tectonics

Cycles of breakup and collision have influence on biological evolution
Breakup/rifting - continents separate
Milder climate, separation of forms - genetic drift. Diversity of species
Collisions - continents reassembled
More extreme climate - land masses together
Species brought together - competition
Continents reassembled - times of extinction

supercontinent that incorporated present-day South America, Africa, Arabia, Madagascar, India,

PANGEA, a “supercontinent” that incorporated almost all of Earth’s landmasses and covered nearly one-third of Earth’s surface.

LAURASIA, ancient continental mass in the Northern Hemisphere that included North America, Europe, and Asia (except peninsular India).

of layering

Permanent thickness of sediment layers

to Plate Tectonics

is the transition between oceanic and continental crust which is

While a weld («шов») between oceanic and continental crusts are called a passive margin, it is not an inactive margin. Active subsidence, sedimentation, growth faulting, pore fluid formation and migration are all very active processes on passive margins.

margins, i.e., when an oceanic plate subducts beneath a continent,

Myth 1: Plates Are Rigid
Unlike dinner plates, lithospheric plates are

lava rising at the deep mid-ocean ridges plants the notion

Lecture 1. An Introduction to Plate Tectonics

Myths on Plate Tectonics

and South America splitting apart with the Mid-Atlantic Ridge sitting

Lecture 1. An Introduction to Plate Tectonics

Myths on Plate Tectonics