Download Evolution of continents Continents are believed to have been built

Evolution of continents
Most figures and tables contained here are from course text:
Understanding Earth Fourth Edition by Frank Press, Raymond Siever,
John Grotzinger, and Thomas H. Jordan
Continents are believed to have been built from the middle out
by plate tectonic processes.
The earliest Earth did not likely have continental crust or modern
plate tectonics.
The Earth was hotter due to higher levels of radiogenic material,
recent meteorite bomba rdment, latent heat of formation and
segregation of the core.
The early Archean crust was rich in Komatiit e which is an
ultrama fic rock res embling the mantle (richer in Mg) than
modern basaltic oceanic crust
Sialic crustal material
has been dated to be as
old as 4.0 billion years
(the Acasta gneiss in the
Archean Slave province
of Canada).
1
The Archean cra ton of
continents are composed
of high-grade
meta morphic terrains
(granulite,
meta morphosed from
sialic crust) separated by
greenstone belts (Granitegreenstone terrains)
which are massive granite
intrusions surrounding
meta morphosed mafic
rocks believed to be
fragments of oceanic
crust from back a rc
spreading center.
2
The craton beneath continents appear to have a Keel (stable
mantle that does not subduct and stays with that piece of crust).
The Archean c ratons of the world have been stable for 2.5 billion
years and have been involved in several collisions and super
continents but they seldom break up. This implies stability well
beyond continental depths. Seismic evidence indicate the mantle
is cold beneath continents (to depths of more than 200 km)
compared to that beneath oceans (and tectonically active
continental margins). A cold mantle would be more dense and
should subduct but the continental keels do not. SO they must
contain lighter minerals (depleted in the iron). Mantle Xenoliths
(rock dragged along in volcanic magma) from Ki mberlit e pipes
(volcanic pipes brought up from more the 150 km depth as
evidenced by the presence of diamonds) indicate a peridotite from
sub-cratonic mantle is depleted in iron and garnet (a heavy
mineral ).
Accre tion in the process of adding new material to the ma rgins
of continents through tectonic activity.
3
4
The Wilson cycle is a
describes the process
by which continents
break apart and come
back together
repeated through
geologic time. The
Atlantic sea board of
North Ame rica has
been closed through
at least three super
continent cycles and
is a classic Wilson
cycle ma rgin.
Vertic al motion can
also provide
locations for
sedimentary
basins and later
uplift.
The following show
vertical motion
of continents due
to:
a) Glaciation
b) Mantle heating
c) Mantle cooling
d) Cooling after
rifted margins
e) Deep mantle
heating
5
6
7
8
9
10