Transcript Theory of the Earth

Rocks and the Fossil Record
• In 1788 a man named James Hutton wrote
Theory of the Earth.
• In it he hypothesized that all the
processes that we observe today,
such as erosion and deposition,
remain uniform.
• That is, they don’t change over time.
• They happen repeatedly in the same way.
• We call this theory uniformitarianism.
• Basically, it is the idea that the same geological
processes have been shaping the Earth throughout
its history.
• Hutton’s theory caused controversy, since it
meant that the Earth was much older than
people had previously thought.
• People believed that the Earth was only a few
thousand years old.
• That wouldn’t be enough time to have allowed
for the processes that Hutton proposed.
• Most scientists at the time believed in
another theory, called catastrophism.
• This theory proposes that all geologic
change on the Earth happens
suddenly, as in a catastrophe.
• It wasn’t until the geologist,
Charles Lyell published his
books, called Principles of
Geology (1830 – 1833), that
scientists began to believe
that uniformitarianism was the
guiding principal of geology.
• Geologists today realize that neither theory
explains all of the geological changes that
have occurred during Earth’s long history.
• While we are studying the geology of the Earth
we need to also look at the organisms that
inhabit it and the conditions under which they
live.
• Paleontology
is the branch of biology that
studies the forms of life that existed in former
geologic periods.
• Paleontologists are the scientists that engage
in this study.
• The data that they use is mainly fossils.
• Fossils are the remains of ancient animals
and plants, the traces or impressions of living
things from past geologic ages, or the traces
of their activities.
• Fossils have been found on every continent
on Earth.
• The word fossil comes from the Latin word
fossilis, which means "dug up."
• Most fossils are excavated from sedimentary
rock layers .
• Sedimentary rock is rock that has formed
from sediment, like sand, mud, small pieces
of rocks.
• Over long periods of time, these small pieces
of debris are compressed (squeezed) as they
are buried under more and more layers of
sediment that piles up on top of it.
• Eventually, they are compressed into
sedimentary rock.
• The layers that are farther down in the Earth
are older than the top layers.
• Determining whether an object or event
is older or younger than other objects or
events is called relative dating.
• Layers of sedimentary rocks form one on top
of the other, as seen in the photo on the
right.
• The principle that
states that younger
rocks lie above
older rocks in
undisturbed
sequences is called superposition.
Superposition
• Forces within the Earth have disturbed some rock
sequences.
• These forces can
push other rocks
into the sequence,
tilt or fold rock
layers, and break
sequences into
movable parts.
• They can even tilt them until they are upside down,
making it difficult for geologists to determine the
relative age of the rocks.
• To help with this problem, geologists have
combined data collected from all the known
undisturbed rock sequences from different
parts of the world.
• From this data geologists created the geologic
column.
• Since scientists assume that sediment is deposited
horizontally to form layers, if the rock layers are not
horizontal, something must have happened to disturb
them.
• Folding and tilting are two types of events that
disturb rock layers. These events are always
younger than the rock layers they effect.
• Faults, intrusions, and the effects of folding
and tilting can make dating rock layers a
challenge.
• Sometimes, layers of rock are missing
altogether, creating a gap in the geologic
record.
• Missing rock layers create breaks in rock-layer
sequences called unconformities.
• An unconformity is a surface that represents a missing
part of the geologic column.
• Unconformities also represent missing time —time that
was not recorded in layers of rock.
• When geologists
find an
unconformity, they
must question
whether the
“missing layer”
was never present
or whether it was
somehow removed.
This figure shows nondeposition.