Earth Sciences 1108
Study Guide for Midterm Exam 3
General information for the exam:
The format of the exam will be multiple choice, true/false, and matching.
The test will cover material discussed/shown in class, which includes: discussions in class, powerpoints,
your class notes, Schumman (textbook) readings, video clips and videos shown in class, and the pertinent
eReserves (pdf) readings from Hurlbut and Kammerling
The following questions are to get you thinking about the material. REMEMBER, THIS IS JUSTA
Geological processes that create gemstones.
Know the basic layers of planet Earth: outer layer (crust of both land and ocean), mantle (thick molten
material, very viscous), inner core (iron and nickel, solid), outer core (more liquid, more fluidized material)
Understand the Rock cycle and how it works. How are all the parts of the rock cycle interconnected?
Know the main parts (rock types and processes) or the rock cycle: dynamic transition through time of 3
rocks, origin of materials and rocks, ultimately gems Describe the general geological conditions (pressure, temperature, existence of fluids, for example) under
which igneous, metamorphic, and sedimentary minerals form. In general terms, where in or on Earth
would someone expect to find igneous, metamorphic, or sedimentary environments?
Gems forming in the mantledeep in the Earth, form under HUGE pressures and temperatures
diamonds, deepest formed gem; brought up by special volcanoes; weather released or released by humans
Metamorphic rocks plate tectonics create metamorphic environments where rocks deform under high
pressures and temperatures Subduction zones and mountain building zones
Know how gems form in igneous, metamorphic mantle conditions. What are the most important gem
Igneous intrusive and extrusive
Metamorphic regional and contact metamorphism
Know examples of gemstones that form in igneous environments and in metamorphic environments.
Igneous Diamonds, topaz, quartz, amethyst
Metamorphic garnet, spinel, diopside, lapis lazuli
Know gemstones that form in Earth’s mantle.
Know the differences between intrusive and extrusive processes and the germs they form. What is lava
and magma?Are they the same or different things? Know how each forms.
Igneous rocks: some gems crystallize from magma and lava ( magma= under earth’s surface, lava= magma that
has hit earth’s surface)
Extrusive lava erupting at earth’s surface, forms crystals in fluid and gas filled cavities (bubblesgeodes), cools
quickly at earth’s surface makes small crystals, not a significant source of gems, MOST IMPORTANT ROLE brings up to the surface gems that have already formed deeper in the mantle. Come up from volcanoes form a
bubble crystals grow inside the environment (quartz, topaz, amethyst, agate). Once in the earth’s surface they
are exposed to the elements, weathering releases crystals (peridot/olivine, moonstones in Kenya, diamonds in S.
Intrusive solidified magma near earth’s surface, materials have not been extruded by a volcano, have a long
time to cool, make large crystals, plutons or igneous intrusions
What are extrusive rocks important for? Know examples of gems formed in these environments
They are not significant sources for gems; topaz, quartz,
Plutons and pegmatites: understand what they are, how they form, how they are related. Why are
pegmatites important sources of gems? What’s in a pegmatite melt that makes pegmatites ‘Earth’s jewel
box”? (i.e.: rare elements)
Plutons are igneous intrusions near earth’s surface; pegmatites are the small fingers (intrusions) from the pluton.
Pegmatites often make it all the way to the earth’s surface; find the best gems; can be exposed when surrounded
country rock is eroded away.
Know examples of typical pegmatite gems. List three chemical elements, if present in the crystal structure
of a gemstone, which are consistent with the gem’s formation in a pegmatite.
Extremely important source of gems: emeralds, rubies, feldspar; end phase of magma crystallization; rare
materials that are harder to bond with others, finally have to crystallize push from pluton to pegmatite and wait
until the end to crystallize most gems in market: lithium, beryllium, and boron; highly mobile fluid due to
leftover water content; very dynamic water makes them liquidy
Emeralds, aquamarine, morganite boron and beryllium boron and lithium make watermelon tourmaline;
Gems and metamorphic rocks: how they form, examples.
Plate tectonics create metamorphic environments where rocks deform under high pressures and temperatures
Subduction zones & mountain building zones
Two types of metamorphic rocks: regional metamorphism (California) & contact metamorphism (Himalaya)
Know the 2 types of metamorphic rocks and understand the differences between these types. Know
exmaples of rocks that form under each of these environments Regional metamorphism: affects large areas further away from heat and pressure action zone; large volumes of
rocks are changed in response to pressure (high); garnet (octahedral), spinel, ruby (cubic); some heat + some
pressure = regions
Contact metamorphism: where the heat and pressure are most intense; the plates are closely colliding, where
the plates are hitting; rock melts and forms new crystals; lapis lazuli, emeralds, diopside; lots of heat + lots of
pressure = contact
Understand the differences between environments (subduction vs mountain building zones)
What is a subduction zone? Why are subduction zones important environments for the formation of
igneous and metamorphic rocks in Earth?
Subduction zone plates collide and one plate dives under the other
Mountain building plates collide and both plates buckle up forming mountains
What is recrystallization? How does this happen?
a metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or
molecules of a rock or mineral are packed closer together, creating a new crystal structur
Where are gems found?
Sedimentary deposits are the result of what two processes?
Understand and know how each of these processes works to create/release gems
Know what/how/why gems are deposited in alluvial streams, and why they are deposited there….
Know examples of gemstones that form in sedimentary environments vs gems that are found in
Results from mechanical or chemical weather of rocks Mechanical exposed to the elements, disaggregation, transport, deposition, known as placer deposits reserved
for sedimentary deposits with economic impacts, alluvial deposits that contain significant amounts of metals
and gems, high density, chemical resistance to weathering, high mechanical durability, gold, platinum, diamond,
garnet, ruby, sapphire, zircons
Chemical dissolution of original material, cooling or evaporation, precipitation of new materials; the minerals
that form depend on what dissolved elements are present; if water interacts with silica-rich rock, silica-rich
gemstones will form (agate, opal)
Important countries that contain sedimentary deposits : Thailand,Australia, and Brazil, Sri Lanka (Ceylon
What are placer deposits?
Minerals that survive chemically unchanged are washed into streams; become concentrated in stream beds and
along beaches; become deposits that contain significant amounts of metals and gems (gold, platinum, diamond,
garnet, ruby, sapphire, and zircons) Ceylon Gem Gravels: Sri Lanka (gems collected for hundreds of years,
become concentrated in alluvial deposits, gems weather from igneous and metamorphic rocks)
What are hydrothermal deposits? How do they form? What is the end result in a hydrothermal deposit
that contains minerals (and therefore gems)?
Water originates from magma bodies (igneous related), minerals that contain