Download ppt

Class 10: Other mass extinctions
& the Nemesis theory




Other mass extinctions – impacts?
Search for the corresponding craters.
Periodic mass extinctions.
The Nemesis theory.
Jon Burnett, Oct 1/03, Wales
I : The Big Five extinctions…

There were 5 extinctions that killed
70% or more of species…






65 Myr (Cretaceous-Tertiary)
202 Myr (Triassic-Jurassic)
250 Myr (Permian-Triassic)
367 Myr (Late Devonian)
438 Myr (Ordovician-Silurian)
But there were other periods of mass
extinction as well…
II : Other mass extinctions
RaupSepkoski

Seems to be a periodicity in the mass
extinctions…



Appears to be a mass extinction approx.
every 26-30 million years.
Regularly spaced… like ticks of a clock.
Warning: this claim is still very
controversial!

It can sometimes be very hard to prove or
disprove that a particular signal is periodic.
III : Are all mass extinctions
caused by impacts?



This would be a very extreme view…
If true, impacts would be one of the most
important forces in evolution.
Still very very controversial and vigorously
debated.

Impact theory would be boosted if we
could find craters corresponding to the
other mass extinctions.

But remember how hard (sometimes
impossible) it is to find impact craters on
Earth… and we must accurately date them.
Morasko, Poland
In fact, this is a whole group of craters…
Late Devonian
Siljan, Sweden (368 Myr)
52 km diameter
Permian-Triassic
Arjguinha, Brazil (245 Myr)
44 km diameter
Triassic-Jurassic
Manicouagan, Canada (214 Myr)
100 km
Jurassic-Cretaceous
Morokweng, South Africa (145 Myr)
70 km
(not exposed)
Cretaceous-Tertiary
Chicxulub, Mexico (65 Myr)
170 km
Eocene-Oligocene transition
(35 Myr ago)
Chesapeake Bay crater… 35Myr
IV : What could cause periodic
impacts and extinctions?

Let’s assume that large impacts are periodic
(Shiva Hypothesis).


Only plausible scenario…




One/several of them occur like clockwork every 2630 million years.
Oort cloud suffers a regular disturbance every 26-30
million years.
Disturbance shifts the orbits of many distant comets,
making them plunge into the inner solar system.
A small number of those comets hit the Earth,
causing the mass extinctions.
But what could disturb the Oort cloud like
this?

Possibility I





As the Sun orbits the galaxy, we “bob” up and down
within the galactic disk.
We probably pass through middle of galactic disk
once every ~30 million years.
Maybe this, somehow, disturbs the Oort cloud.
Comets rain inwards…
Possibility II (Nemesis Theory)




The Sun has a companion star in orbit.
Once every 30 million years, that star comes close to
(plunges through?) Oort cloud.
Comets rain inwards…
We’ve yet to find this star (people have looked!).