Download 2.1 Tectonic Forces

Tectonics and the Lithosphere
Earth Layers
The centre of the earth is called the inner core and is solid because of
the pressure exerted on it. Around this solid core is a liquid outer core.
The main substances in the core are iron and nickel which give this
part of the earth a density in the range of 8 g/cm3.
Asthenosphere
The mantle, a mostly solid layer is the largest of earth’s layers. The
upper part of the mantle is called the asthenosphere. The
asthenosphere is part solid, part liquid and is able to flow like hot
asphalt or tar, especially if pressure is applied to it.
Lithosphere
The lithosphere is the outer layer made up of solid rocks with an
average density of 2.6 g/cm3. Parts of the mantle and asthenosphere
that are liquid maintain their heat as radioactive elements decay (like
in nuclear reactors), releasing their energy as heat.
Kinds of Rocks
The three main kinds of rocks are igneous, sedimentary, and
metamorphic.
Igneous Rocks
Igneous rocks form when magma from below the earth cools, either at or
below the surface of the earth. If magma cools within crust rocks, it is called
intrusive and if it comes out of cracks or fissures in the earth’s surface it is
called extrusive. Magma flowing from fissures is referred to as lava. When
magma or lava cools slowly, rocks form with larger crystals. When intrusive
magma cools quickly, rocks with smaller crystals form.
Igneous Rocks
Intrusive rocks are sometimes called plutonic rocks. Granite is one kind of
plutonic rock with large crystals while obsidian is a kind of rock with no
crystals that has cooled very quickly on the earth’s surface. Metallic mineral
deposits are commonly found along with or imbedded in intrusive igneous
rocks. Metal veins in igneous rocks form as super-hot water within the
igneous rocks cools and metallic compounds dissolved in the water settled
out of the metallic brines (solutions) to form ore deposits.
Sedimentary Rocks
Sedimentary rocks are rocks that have been made from eroded pieces
of other rocks or from precipitation (settling out of solution) of
materials from water. There are two main types of sedimentary rock:
clastic and nonclastic.
Clastic Sedimentary Rocks
Clastic sedimentary rocks form from eroded pieces which are
cemented and pressed back together in a process called lithification. If
gravels are lithified they become conglomerates, if sands are lithified
they become sandstone, and muds or clays when lithified become
shale.
Nonclastic Sedimentary Rocks
Nonclastic rocks form when solids settle out from water (precipitate) or
when remains of organisms like skeletons or shells gather together on the
bottom and lithify there (because of pressure or heat) into sedimentary rock.
Limestone rocks form when skeletons and shells of sea organisms become
cemented together. The building material, cement, is made from limestone.
Oil and gas form from the remains of tiny plants and animals, often marine
organisms. Coal forms from plants covered up in swamps.
Fossil Fuels
To form fossil fuels, dead organisms must be covered and then be
pressed together under anaerobic conditions (with no oxygen
present).
How Pressure and Heat Affect Fossil Fuels
The longer time that fossil fuels are pressurized and heated, the more
they are converted into light liquids or gases. The shorter the time
that fossil fuels are pressurized and heated, the more tars and heavier
liquids that get formed.
Where Fossil Fuels Accumulate
Natural gas and oil accumulate in reservoirs called traps. A trap forms when
an upper rock layer is impermeable and keeps the gas/oil from rising. The
top layer of impermeable rock is called a cap and under this, the oil/gas rises
above ground water because they are lighter or less dense than water. When
rocks are compressed from the side, they form anticlines and synclines. It is
below the impermeable caps of anticlines that oil and gases tend to
accumulate.
Salt Domes
Oil traps can also form when a salt dome rises through sedimentary layers
due to its plasticity and lower density. Salt domes begin as salts accumulate
on the bottom of a shallow sea that is exposed to intense sunlight leading to
great evaporation and precipitation of salt. When this is covered by
sediments, the salt begins rising to form a salt dome. Salt domes have been
found along the coast of the southern US, Iran and along the great lakes in
Southern Ontario. Salt domes can be many km deep and wide.
Sedimentary Rock Features: Fossils and Strata
Sedimentary rocks often have in them the remains of once-living organisms
called fossils. Sedimentary rocks typically are formed in layers called strata.
The layering of sedimentary rocks sometimes gives them the property of
splitting along layer lines. The sand, silt and mud that become sedimentary
rocks are made through the action of rivers whose movement keeps these
particles in suspension until they reach much calmer lakes and oceans where
these particles settle out in layers on the bottom.
Ancient Sea in Central Canada
In central Canada there are sediments up to 7,000 m thick which are
thought to have been deposited in this region when it was an ancient
sea, under water.
Continental Shelves
Sediments that form around the edges of continents are called continental shelves.
These shelves of sediments slope gently downward and at their edges the bottom
plunges deeply. Continental slopes are regions where the sediment slope is steep
and not as long as continental shelves. Continental shelves are regions of great
economic importance because rivers supply them with many nutrients that foster
algae growth, which in turn feeds marine animals like fish. The majority of fish and
shellfish harvested on earth come from continental shelf areas.
Coral Structures
A fringing reef forms around a volcanic island or land mass. It is made by
corals which are colonies of animals that make shell homes of rock-like
calcium carbonate (limestone). As coral organisms cement layer upon layer
of their homes on lower layers, a reef structure arises from the ocean floor
and may emerge from the ocean if the ocean floor rises. If a land mass
moves away from a fringing reef, a space develops between the land and the
reef, and the reef is now called a barrier reef. If a volcano sinks, leaving the
reef, a structure called an atoll is formed.
Evaporites
As salty waters in very warm, sunny regions are exposed to intense
solar radiation, mineral deposits settle out which are called
evaporites. The minerals of evaporites are usually salts, a common
one being sodium chloride (NaCl), table salt. Halite deposits are
evaporites made from salts like table salt.
Metamorphic Rocks
When igneous or sedimentary rocks are exposed to heat and/or pressure they change
into metamorphic rocks. Metamorphic rocks are of two types: foliated and non-foliated.
Foliated metamorphic rocks have repeating layers while non-foliated metamorphic rocks
have no repeating layers. Gneiss is a foliated metamorphic rock made from granite while
quartzite is a non-foliated metamorphic rock made from sandstone. Marble is a nonfoliated metamorphic rock made from limestone. Shale (Sed.) changes to slate (Metam.).
Many metamorphic rocks are used as building materials.
The Rock Cycle
The rock cycle shows how rocks on earth are changed from one kind to
another.
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