Download Rocks and the Rock Cycle

GEOL 0820
Ramsey
Natural Disasters
Spring, 2017
LECTURE #4: Rocks & the Rock Cycle
Date: 17 January 2017
(lecturer: Victoria Buford)
I. Rock Types
• why do we care about this topic for a course in Natural Disasters?
o certain minerals, rock compositions/textures can tell a geologist a lot about the
hazardous condition, for example:
 the explosive potential of previous volcanic eruptions
 the potential of rocks/soil to absorb water in a flood
 the composition of rocks and their orientation as potential landslide
locations
 others that you can think of?
•
so what is a rock?
o rock: a naturally occurring solid made of one or more minerals or other solid
substances
o examples: sandstone that is made of only quartz mineral grains is a rock, a
limestone that is made up of only fossils is also a rock


•
is coal a rock?? _________________
informal exercise: what rock types do you see when entering the Cathedral
of Learning? _______________, _______________
Igneous Rocks
o form when molten (melted) rock cools and solidifies either under or on the
Earth's surface
 solidifies slowly: large mineral crystals form
 solidifies quickly: glass forms
 solidifies slowly then erupts: large mineral crystals in a finer-grain matrix
 known as a porphyritic texture

with a large amount of water present in the magma
 very explosive volcanoes

with little water present in the magma
 very effusive (non-explosive) volcanoes
Page 1/4
o range of igneous rock types
 from light to dark minerals, extrusive
vs. intrusive cooling, etc.
•

granite (intrusive)
 average composition of the
continental crust
 slow-cooling rock with many large
grains of light-colored minerals

gabbro (intrusive)
 average composition of the
oceanic crust
 slow-cooling rock
 many large grains of dark-colored
minerals
granite
Metamorphic Rocks
o form when an existing rock of any type re-crystallizes (but does not melt)
o experiences elevated temperatures and pressures
 known as a “solid-state transformation”
o parent rock:
metamorphic rock
 limestone 
___________
 shale 
___________
 sandstone 
___________
o type 1: contact metamorphism
 high T, low P
 occurs where a hot magma
intrudes colder rocks
 “cooks” the surrounding host rocks
(aka, country rocks)
 no foliation (banding)
o type 2: regional metamorphism
 high T and P
 occurs where continental collisions
form mountain belts
 deep core of the mountains
experience elevated temperatures
and pressures
 foliation of minerals
 light/dark bands
•
Sedimentary Rocks
o form on/near the Earth's surface or under the oceans
o accumulation of fragments of other rocks (e.g., sandstones)
o fragments of organic remains (fossil plants, shells, bones)
o solids precipitated from liquids (e.g., salt)
Page 2/4
o type 1: mechanical formation
 lithification of sediments/rock fragments
 examples:
 sandstones
 mudstones
 shales
 breccia
o type 2: chemical formation
 evaporation, precipitation of minerals
from a liquid like water
 examples:
 limestone (CaCO3)
 gypsum (CaSO4 · 2H2O)
 salt (NaCl)

•
CaCO3  very important when it comes
to carbon-cycle!
recycling of the lithosphere
o we will examine the lithosphere when we talk about Plate Tectonics
o there is a difference between oceanic and continental crust
 age and composition are two important distinctions
 oldest continental rocks are 3-4 billion years found on cratons
 continental interiors
 the earliest crust (proto-continents)
 new material constantly being added to the cratons: from volcanoes
 material constantly being removed from the cratons: from erosion
II. The Rock Cycle
• processes:
o 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:
 understand how the Earth works today
 how it worked in the past
 because each mineral forms under specific conditions
 geologists use the minerals to identify the rocks
 and how the rocks formed
•
•
•
describes pathways of the rock-formation process
consequence of Plate Tectonics
estimated time of about 100 million years per cycle
o even longer at continental interiors (cratons)
•
not a completely closed loop
o new input and exit pathways for rock material
Page 3/4
•
most material has been recycled many times from continent to the mantle
o complete crustal replenishment every 2-3 billion years
•
we will fill out the missing blanks below in lecture:
Page 4/4