solidified with time. The study of our modern sea floor will help scientists understand what happened in ancient oceans, traces of which can now be found on
Earth’s older continental with areas of exposed sedimentary rocks.
The ‘‘modern sea floor’’ is younger than 180 million years. Today’s present
continents and oceans have not always had the same appearance and the attachment and breaking-up of Pangaea and Gondwanaland might not have been a
unique event. Indeed, it is not excluded that other continents previously existed
and subsequently disappeared into the interior of the Earth. This may have
occurred several times prior to formation of Pangaea. Extrapolating from our
knowledge of the present arrangement of our planet’s landmasses, we might be
able to predict what will happen to Earth’s continents in the future. For instance, if
we imagine that the process of continental drift continues in the same pattern as
now, it is not excluded that the re-closing of the oceans and seas will create
another unique continent, or in other words, we may one day have a second
Pangaea.
In geology, sometimes we refer to the continents as being the ‘‘granitic crust’’.
The term granite is used to define the assemblage of silica-enriched rocks that are
lighter in weight (or less dense) than basalts or fresh peridotites; granite is what
constitutes the main component of the continental lithosphere-crust. The origin of
a ‘‘granitic crust’’ is related to the melting and upwelling of mantle material which
first gives rise to a basaltic liquid. The basaltic melt resides in large magma
chambers where the differentiation between heavy and light elements forming the
compounds will take place (See Chap. 5). The lighter compounds, made up
essentially of silica, potassium and sodium, will move upward inside the magmatic
reservoir and upon crystallization, they will form quartz (silica oxide) and feldspar
(silicates of calcium, sodium, potassium and aluminum) which are the primary
minerals found in a granite crust.
At the beginning of its formation, the Earth was a molten ball that slowly
solidified. The early solidification of our planet began at the base of the mantle
near the core, and continued upward. The various compounds found in the mantle
were diffused and transferred throughout the solidifying layers as ‘‘blobs’’ of
agglomerated solids floating on top of denser partially molten material (Fig. 2.2).
The age of the earliest continental material is hard to evaluate and it is often
related to the presence of life on Earth. However, the fossil records on continental
terrain have sometimes been obliterated during rock alteration due to weathering
or metamorphism (rock transformation). Indications of the age range for a primitive continent have been found among preserved fossil records to vary from 0.1 to
about 3.8 billion years in Canada (Schidlowski 2001). Metamorphosed sedimentary rocks from western Australia have given an age of 4.4 billion years based on
age dating of a detrital zircon found in metamorphosed sediment from the Narryer
Gneiss Terrain (Wilde et al. 2001). Using ‘‘age dating’’, which can be calculated
by measuring the rate of decay of radioactive isotopes (i.e.
187 Re –
187
Os decay)
and on carbon (CO 2 ) isotopes, and also based on stratigraphic observations, scientists have concluded that the major granite-forming activities in continent
building have been cyclic and episodic.
Formation of Continents
37
Earth’s older continental with areas of exposed sedimentary rocks.
The ‘‘modern sea floor’’ is younger than 180 million years. Today’s present
continents and oceans have not always had the same appearance and the attachment and breaking-up of Pangaea and Gondwanaland might not have been a
unique event. Indeed, it is not excluded that other continents previously existed
and subsequently disappeared into the interior of the Earth. This may have
occurred several times prior to formation of Pangaea. Extrapolating from our
knowledge of the present arrangement of our planet’s landmasses, we might be
able to predict what will happen to Earth’s continents in the future. For instance, if
we imagine that the process of continental drift continues in the same pattern as
now, it is not excluded that the re-closing of the oceans and seas will create
another unique continent, or in other words, we may one day have a second
Pangaea.
In geology, sometimes we refer to the continents as being the ‘‘granitic crust’’.
The term granite is used to define the assemblage of silica-enriched rocks that are
lighter in weight (or less dense) than basalts or fresh peridotites; granite is what
constitutes the main component of the continental lithosphere-crust. The origin of
a ‘‘granitic crust’’ is related to the melting and upwelling of mantle material which
first gives rise to a basaltic liquid. The basaltic melt resides in large magma
chambers where the differentiation between heavy and light elements forming the
compounds will take place (See Chap. 5). The lighter compounds, made up
essentially of silica, potassium and sodium, will move upward inside the magmatic
reservoir and upon crystallization, they will form quartz (silica oxide) and feldspar
(silicates of calcium, sodium, potassium and aluminum) which are the primary
minerals found in a granite crust.
At the beginning of its formation, the Earth was a molten ball that slowly
solidified. The early solidification of our planet began at the base of the mantle
near the core, and continued upward. The various compounds found in the mantle
were diffused and transferred throughout the solidifying layers as ‘‘blobs’’ of
agglomerated solids floating on top of denser partially molten material (Fig. 2.2).
The age of the earliest continental material is hard to evaluate and it is often
related to the presence of life on Earth. However, the fossil records on continental
terrain have sometimes been obliterated during rock alteration due to weathering
or metamorphism (rock transformation). Indications of the age range for a primitive continent have been found among preserved fossil records to vary from 0.1 to
about 3.8 billion years in Canada (Schidlowski 2001). Metamorphosed sedimentary rocks from western Australia have given an age of 4.4 billion years based on
age dating of a detrital zircon found in metamorphosed sediment from the Narryer
Gneiss Terrain (Wilde et al. 2001). Using ‘‘age dating’’, which can be calculated
by measuring the rate of decay of radioactive isotopes (i.e.
187 Re –
187
Os decay)
and on carbon (CO 2 ) isotopes, and also based on stratigraphic observations, scientists have concluded that the major granite-forming activities in continent
building have been cyclic and episodic.
Formation of Continents
37