Tectonics helps explain
Objectives:
(1) understand the Earth's heat sources(2) learn about Earth's early atmosphere
(3) review plate tectonics
By 4.6 by, the Earth had differentiated into the
core, mantle, and crust. First rocks from 4.1 to 4.3 by.
Most of the heat energy from the core is transported
through the mantle by overturning motion, convection.
The mantle is a viscous solid (like "silly putty").
Hot, lower mantle material is less dense and it rises.
The material cools at the top and sinks again.
There is a large thermal gradient through the crust.
The other important heat source is visible and ultraviolet light
from the Sun. Without sunlight, the Earth's temperature
would be -15 degrees Celsius.
Early in Earth's history, sunlight caused the atmosphere to change
from methane (CH4), ammonia (NH3), and
hydrogen (H2) into oxygen compounds.
There was a gradual build up of O2 and O3 (ozone
in the stratosphere; important for blocking damaging UV light)
becoming important some 570 m.y. ago.
The creation of ozone corresponded to the first appearance of
multi-cellular life on Earth.
The outermost layers of Earth are divided into 12 - 15 large
lithospheric plates which undergo horizontal movement (carried
by convective motions in the mantle). These lithospheric plates
consist of the crust and uppermost mantle, and they are 100 km
thick in the continents (80 km thick in the oceans).
The North American lithospheric plate is moving westward at
a rate of 2 - 3 cm/year.
New lithospheric material is being
create in upward moving material at the Mid-Ocean Ridge (MOR),
while old lithospheric material is being subducted along the
west coast.
About 250 m.y. ago the major continents formed
one large land mass called
Panagea which subsequently broke into the moving continents.
Tectonics helps explain