GEOL 0820
Natural Disasters
Spring, 2025
LECTURE#4:Rocks &the Rock Cycle
Date:22 January 2025
I. Rock Types
· we want to know:how do the major minerals and mineral classes form into rocks?
· why do we care about rocks for a course in Natural Disasters!?
o certain minerals,rock compositions,and rock textures can tell a geologist a lot about the hazardous conditions present in the past,for example:
■ the explosive potential of a nearby volcano
■ the ability of rocks/soil to absorb water during a flood
■ the composition of rocks and their orientation as potential landslide locations
● what is a rock?
o rock: a naturally occurring solid made of one or more minerals or other solid substances
■ this is a simpler and more open definition than we had for a mineral
o examples:a sandstone that is made of only quartz sand mineral grains and a limestone that is made up of only fossils (living matter)are both rocks
■ is coal a rock??
· there are 3 rock types
1. lgneous Rocks
o form when molten (melted)rock cools and solidifies either under or on the Earth's surface
■ solidifies slowly underground: large mineral crystals form.
■ solidifies quickly on the surface: small mineral crystals form.
■ solidifies very quickly on the surface: glass forms
■ solidifies slowly(but only partially)underground and then erupts large
mineral crystals in a smaller-grain matrix(that cooled quickly)
known as a porphyritic texture
■ if there is a large amount of water present in the magma explosive volcanoes
with little water present in the magma
effusive (non-explosive)volcanoes
o range of igneous rocks
■ from light to dark minerals
■ from extrusive (on the surface)vs.
intrusive (underground)
intrusive: slow-cooling rock with many large grains
extrusive: fast-cooling rock with many small grains
average composition of the continental crust
generally,more light-colored minerals
granite
(slow cooling,larger mineral
grains)
gabbro (intrusive)/basalt (extrusive)
average composition of the oceanic
crust
generally,more dark-colored
minerals
2. Metamorphic Rocks
o form. when an existing rock of any of the three types re-crystallizes (but does not melt)
O it experiences elevated temperatures and/or pressures
■ known as a “solid-state transformation”
o a rock that is metamorphosed starts as a"parent"rock first
o parent rock: metamorphic rock
■ limestone →
■ shale
■ sandstone
o contact metamorphism
■ happens with high temperature(T), low pressure(P)
■ occurs where a hot magma
intrudes colder,older rocks
■“cooks”the surrounding
parent rocks (aka,country rocks)
■ no foliation (banding) present in these
Contact metamorphosed rocks
o regional metamorphism
■ happens with high T and high P
■ occurs where continental collisions form mountain belts
■ deep core of the mountains experiences much higher temperatures and
pressures
■ results in light/dark bands of minerals
called “foliation”
gneiss
3. Sedimentary Rocks
o form on/near the Earth's surface or under the oceans
o lithification (transforming into rock)of fragments of other rocks (e.g., sandstones)
o fragments of organic remains (e.g.,fossil plants,shells,bones)
o solids precipitated from liquids (e.g.,salt,limestone)
o mechanical formation style.
■ lithification of sediments/rock fragments
■ examples:
sandstones
mudstones
shales
breccia
o chemical formation style.
■ evaporation,precipitation of minerals from a liquid like seawater
■ examples:
gypsum (CaSO4·2H₂O)
salt(NaCl)
■ CaCO₃→very important when it comes to carbon-cycle!
■ why?
· recycling of rocks (occurs from weathering and Plate Tectonics)
o we will examine Plate Tectonics next week
o there is a difference between the rocks of the oceanic and continental crust
■ age and composition are two important distinctions
■ oldest continental rocks are 3-4 billion and found in areas known as cratons
■ cratons:
the earliest crust (proto-continents)
new material constantly being added to the cratons:from volcanoes and continental collisions
material constantly being removed from the cratons:from weathering and erosion
II.The Rock Cycle
· processes:each and every rock,no matter how dull looking,came into existence as the result of some process
o deciphering the rock allows us to recognize that process,and therefore:
■ how it worked in the past
■ and how the Earth works today
o because each mineral forms under specific conditions
geologists use the minerals to identify the rocks
■ and the rocks origin story
● the rock cycle describes pathways of the rock-formation process
o consequence of Plate Tectonics
■ estimated time of about 100 million years per cycle
much longer than the times of atmosphere or ocean cycles
takes even longer at continental interiors(cratons)
not a completely closed loop
new input and exit pathways for rock material
· most material has been recycled many times from continent to the mantle nearly complete crustal replenishment every 2-3 billion years
● we will fill in the missing blanks in the Rock Cycle below during lecture:
Ⅲ.Informal Homework:
· What two rock types do you see when entering the Cathedral of Learning?