How the Earth works lecture 4: Geological time Flashcards
Define relative ages/relative ageing
Relative age is establishing/order a sequence of events. However the exact times of these events is not known.
E.g. what happened first, what happened after and then what happened most recently.
Define absolute age/absolute ageing
Absolute age is establishing when an event took place. Absolute ageing is numerical. For example, 100 years in the past.
Explain the principle of superposition/superposition
in an undeformed sequence of layered rocks, each bed is older than the one above, and younger than the one below.
Younger strata are on top; older strata are below.
Explain the principle of uniformitarianism
The principle of uniformitarianism states that the processes observed today were the same in the past.
Mud cracks found in ancient sediments formed in just the same way as the mud cracks we see forming today.
The key to understanding the past is in the present.
Define an uncomformity
An unconformity is a time gap in the rock record, from nondeposition or erosion.
Explain the three types of unconformity
-Angular unconformity
-Noncomformity
-Disconformity
List all of the Eons of the geological timescale and summarise them.
Phanerozoic—”visible life”
Starts at the Pre-Cambrian–Cambrian boundary.
Marks the first appearance of hard shells.
Life diversified rapidly afterwards.
Proterozoic—“before life”
Development of tectonic plates like those of today.
Buildup of atmospheric O2; multicellular life appears.
Archaean—“ancient”
Birth of continents.
Appearance of the earliest life forms.
Hadean—“hell”
Internal differentiation.
Formation of the oceans and secondary atmosphere.
List all of the Eras of the geological timescale and summarise them.
Cenozoic—“recent life.”
The Age of Mammals.
Mesozoic—“middle life.”
The Age of Dinosaurs.
Palaeozoic—“ancient life.”
Life diversified rapidly
Define Magnetostratigraphy
Magnetostratigraphy—magnetic polarity reversals in strata are compared to the global reference column to determine the age of the sediments.
What are radioactive isotopes
Isotopes are versions of elements that have a different number neutrons that they usually have.
Radioactive isotopes are isotopes which have an unstable nucleus and as a result decay into other elements.
How is radioactive isotope used to date rocks?
By calculating the half life/amount of time it takes for radioactive element to decay it allows us to see where the rock is in the process of element decay and therefore date the age of the rock.
What is the age range for carbon dating?
Age range 100s years to ~maximum 50,000 years
What is the age range for other radioactive isotope dating
235Uranium to 207Lead(Pb) has a half life of 731 million years.
However there are much longer half lifes. For example and the longest end of the spectrum there is 147Samarium to 143Neodymium which’s half life is 106billion years.
