Geological Evolution
What are Rocks?
• A rock is a naturally occurring solid mixture of one or more minerals, or organic
matter
• Rocks are classified by how they are formed, their composition, and texture
• Rocks change over time through the rock cycle
• Types of Rocks:
o Igneous Rocks:
• Igneous rock begins as magma.
• Igneous rocks form from the cooling of magma or lava.
• The name igneous comes from the Latin word ignis, means “fire”.
• Sedimentary Rocks:
• Sedimentary rock is formed by erosion
• Sediments are moved from one place to another
• Sediments are deposited in layers, with the older ones on the bottom
• The layers become compacted and cemented together
• Metamorphic Rocks:
• Meaning to change shape
• Changes with temperature and pressure, but remains solid
• Usually takes place deep in the Earth
Fossils
 Fossils are usually found in sedimentary rocks.
 When an organism dies, its soft parts often decay quickly leaving only the hard
parts to fossilize.
 Ex. Bones, Shells, Teeth, or Seeds
 Fossils are preserved remains or traces of living things.
 Most fossils form when living things die and are buried by sediments.
 The sediments slowly harden into rock and preserve the shape of the organisms.
 Scientists who study fossils are paleontologists.
 Types of fossils
o Petrified Fossils: fossils in which minerals replace all or part of the
organism. When the object is buried by sediment, water rich in minerals
seeps into the cells. After the water evaporates, hardened minerals are left
behind.
o Molds and Casts: A mold is a hollow area in sediment in the shape of an
organism or part of an organism. A cast is a copy of the shape of an
organism.
o Trace Fossils: provide evidence of the activities of ancient organisms. Ex:
footprints, animal trails, or animal burrows.
o Carbon Films: an extremely thin coating of carbon on rock that forms
when materials that make up an organism become gases and escape
leaving only carbon behind.
o Preserved Remains: are formed when an organism is preserved with little
or no change. For example when organisms become preserved in tar,
amber (tree sap), and freezing.
 Why Study Fossils?
o Scientists study fossils to learn what past life forms were like.
o Paleontologists classify organisms in the order in which they lived.
o All the information scientists have gathered is called the fossil record.
 Fossil Record
o The fossil record provides evidence about the history of life on Earth.
o The fossil record also shows how different groups of organisms have
changed over time.
o It also provides evidence to support the Theory of Evolution.
 A scientific theory is a well-tested concept that explains a wide range
of observations
o The fossil record shows that millions of types of organisms have evolved.
o However, many others became extinct.
Age of Rocks
• Relative age of a rock is its age compared to other rocks.
Use words like: “older or younger”
• Absolute age of a rock is the number of years since the rock was formed.
Ex: 358-360 mya
• It can be difficult to determine a rocks absolute age. So… scientists use the
law of superposition.
• According to the Law of Superposition, in horizontal sedimentary rock
layers the oldest layer is at the bottom. Each higher layer is younger than
the layers below it.
• The Law of Superposition states that in
undisturbed rock layers the oldest layer is on
the bottom and the youngest rock layer is on
the top
• Examples of when the Law of Superposition may not apply…
• An unconformity is a gap in the geologic record that most commonly
occurs when rock is eroded exposing older rock and then new rock
forms on the much older rock.
• The layering of new rock on the much older rock leaves a gap in the
geologic record.
• Faulting- Movement of Earth’s plates causes rock to crack and shift.
• Intrusion- Magma from the Earth’s mantle moves into the rock
layers, cools and hardens. (Igneous Rock)
• Folding- Earth’s plates are pressing together. This movement causes
them to bend and fold. This process forms mountains
• Erosion- described earlier. Layers of rock are washed away and
new rock built on top. Leaves gaps in geologic record.
• Relative Age (clues)
• Clues From Igneous Rock
• Lava that cools at the surface is called an extrusion. Rock below an
extrusion is always older.
• Magma that cools beneath the surface is called an intrusion. An
intrusion is always younger than the rock layers around an beneath it
• Faults (a break in the rock) are always younger than the rock it cuts
through!
• Unconformities: An unconformity is a gap in the geological record that can
occur when erosion wears away rock layers and other rock layers form on
top of the eroded surface.
• Weathering
• Mechanical: when a rock is physically broken into smaller pieces
• Chemical: process that breaks down rock through chemical changes.
• Index Fossil:
• Scientists use index fossils to match rock layers.
• An index fossil must be widely distributed and represent a type of
organism that existed only briefly.
• They are useful because they tell the relative ages of the rock layers
they are found in.
• One example of an index fossil is a trilobite.
• Trilobites were a group of hard-shelled animals whose bodies
had three distinct parts.
• They evolved in shallow seas more than 500 million years ago.
• Relative Dating occurs when the age of a rock is determined by comparing it to
the age of another.
• Scientist use the Law of Superposition as their first step in Relative Dating of
rocks, however, when rock layers are disturbed the Law of Superposition may
not apply.
• Index Fossils are organisms that were widespread but only lived for a short
period of time. They are useful because they tell the relative ages of the rock
layers in which they occur.
Absolute Dating/Radioactive Decay is when
scientists use radioactive dating to determine the
absolute ages of rocks.
Radioactive decay-over time certain elements
break down, or decay, by releasing particles and
energy.
Half life- The constant rate of decay of an element.
(Carbon 14 has a half-life of 5,730 years)
 The amount of radioactive material will never be zero
because as long as you are cutting the material in half-there will always be
some left to cut.
 Based on the amount of radioactive material and what type of radioactive
material, they can calculate exactly how old the rock is.
 Radioactive material decays at a constant rate always, therefore, the
precise date will be reliable.
 Dating Rock layers with Absolute Dating would be impossible to calculate
without computers and necessary technology to detect the amount of
radioactive material present.
Ice Cores:
 Scientists study ice cores to understand how the climate has changed.
 Lower layers are older than higher layers (Law of Superposition)
 Layers record concentrations of gases like CO2 and deuterium (a heavy form of
hydrogen.
 To obtain ice cores, Scientists drive a hollow tube deep into miles thick ice
sheets in Antarctica and Greenland or any glacier.
 Global climate can shift gradually, sometimes in a matter of years. For example,
the shift from the Pleistocene period (glacial period) to today’s Holocene period
occurred in less than 5 years.
 Scientists use lasers to measure the concentration of dust particles in the ice
core to determine when seasons occurred in the past.
 Based on the thickness of the bands in the ice cores, scientists can determine
heavy or light snowfall amounts.
Plate Tectonics Theory – Theory of Continental Drift
 Pangea- Scientists believe today’s continents once existed as one
“supercontinent” named Pangea.
 The shape of the continents seem to fit together like a puzzle. Also, fossils of
living things found in one location can be found in another distant location across
the ocean. (even if they couldn’t swim)
 Landmass drifted apart due to the movement of the tectonic plates.
Plate Boundaries
 Convergent Boundary – Boundary where crust is destroyed and recycled back
into the interior of the Earth as one plate dives under another.
o Subduction Zone - the process at convergent boundaries where one
tectonic plate moves under another tectonic plate and sinks into the
mantle as the plates collide.
 Divergent Boundary – boundary where new crust is created as two or more
plates pull AWAY from each other.
o Sea Floor Spreading - the formation of new ocean floor from melted rock
that seeps up from the mantle and flows into the space between plates that
are moving apart
 Transform Boundary – Boundary where two plates are sliding past one another
Which will lead us into Biological Evolution…evidence of and from continental drift.