Chapter 4 Planetary Surface Processes Flashcards
Why is there little evidence of impact craters on earth?
On earth the crust is constantly being destroyed and created due to tectonic processes. The surface is also subject to weathering and erosion, all removing evidence of cratering.
What affects asteroidal material impact velocities?
Impact velocities are mainly affected by the planets gravity. Higher gravity results in a faster impact velocity. A planets minimum impact velocity is equal to its escape velocity.
Deceleration due to atmospheric drag is ineffective for impactors bigger than a few hundred meters.
Impact cratering is the fastest geological process known.
First line of evidence for meteor crater
Large fragments of iron meteorite have been found surrounding meteor crater.
Impact structures larger than that at meteor crater would usually vaporise on impact.
Second line of evidence for meteor crater.
The ejecta from the impact forms an inverted flap of the original rock layers, somewhat like petals blossoming. A bore hole would show the same rock layers twice, with the top set inverted.
Volcanic cratering would form an apron or blanket of ejecta around the volcano, with the first erupted material at the bottom and the last erupted material at the top.
Third line of evidence for meteor crater.
At meteor crater shattered target rocks are cemented together by glass at depths of between 200 - 400 m. The glass is formed due to melting of target rock as a result of the instantaneous extreme pressure that results from on impact. This pressure is known as shock pressure.
Unusually high pressure minerals can be formed due to shock pressure.
Three broad stages of impact cratering
A complete series of events and processes occur during after an eruption which can be broadly grouped as:
- Contact and compression
- Excavation
- Modification
Contact and compression stage
This stage starts as soon as the projectile makes contact with the target. The target material is compressed and accelerated to high velocities simultaniously the projectile itself decelerates.
Shock wave originate from the point where the projectile meets the target surface. Shock pressures may exceed 100 GPc (a far higher pressure than obtainable by any earth process at the surface).
Both target and impactor are vaporised or melted as pressure is released. The projectiles kinetic energy is transferred to the target heating, deforming and accelerating target rock.
This stage ends when the projectile has unloaded from high pressure. The process lasts for less than a second in all but large impacts.
Excavation stage of an impactor
A hemispherical shockwave propagates into the target, with the centre of the hemisphere being below the original ground surface as the projectile may have penetrated up to twice its own diameter into the target.
The initial shock wave and subsequent rarefraction waves (waves that are reflected from the original ground surface and continue downwards) weaken, fracture and shatter target rock. The waves also move material, creating an excavation flow away from the structure.
Material gets pushed upwards and outwards at upper levels and downwards and outwards at lower levels, opening out the crater to form transient cavity. This cavity is many times larger than the projectile.
Excavated material is over surrounding terrain. A point is reached where the shock and release waves can no longer displace rock.
This stage can last from 6 to 90 seconds.
Modification stage of an impact crater part one
Once the shock were is beyond the rim of the crater it play no further part.
Small crators will preserve the original shape of the transient cavity, with only small amounts of debris cascading down the walls to form the a layer of fragmented material, called a breccia lens.
In larger craters the transient cavity cannot sustain itself and collapses under gravity. Slump terraces may form at the walls as rocks collapse inwards along curving concentric faults. Some craters contain significant amounts of glassy “ impact melts” formed from the target rock.
Modification stage of impact crater part two.
Central peaks or peak rings form in the bottom of the crater as the compressed rock rebounds. This uplift is about one tenth the diameter of the crater.
Even for large craters the whole modification stage will he completed within 15 minutes of impact.
Over hundreds of millions of years the planets crust gradually accommodates for these large impacts and the crater flattens out until it is hardly recognisable only defined by little more than the materials appearance.
Additional effects of meteor impact
Pressure waves can be released.
Create wind speeds up to 2000 km/h
Ejecta from the crater may form a blanket around the impact-site.
Some material may be ejected with sufficient velocity to escape into space.
If enough material is ejected and re-enters the atmosphere it can heat the atmosphere
Sufficiently large impactors can bore a hole through the atmosphere, creating a low pressure area to draw material up into the atmosphere.
Impactors hitting the ocean can cause tsunamis, and will form regular craters on the sea floor.
Local rocks with a high water content are subjected to hydrothermal altercation in which water dissolves, transports and reprecipitâtes elements that are potential nutrients for microbial life
This could also provide warm liquid water on frozen bodies for hundreds or thousands of years.
Slump terraces definition
When a crater wall collapses material falls back down forming terraces.
What is brachia/breccia lens?
Breccias are racks composed of angular fragment formed from pulverised target rock found beneath and around impact sites.
A breccia lens is formed in an impact crater when material falls down the sides and collects in the bottom.
Micro crater definition
Small craters that are caused by the hypervelocity impact of cosmic dust.
These craters may be as small as 10^-7m and found on lunar samples and man made objects in orbit.
Simple crater definition
These are bowel shaped depressions that lack a central uplift or terracing. They may be up to a several km wide.
Complex craters definition
Complex craters are characterised by terraces of stump blocks, with terraces decreasing inwards, and central peaks formed from fluid like rebound.
The transition between simple and complex craters depends largely on gravity and strength of target rock.
Elongate craters definition
Elongate craters are oval shaped craters that form when the impact angle is less than 10°.
Diagnostic shock features in rock
Fragments showing shock effect may be carried from the impact site.
Target rock may be subject to shock metamorphism.
Impact melting of target rock may create glass.
Impact material may be included in the distal ejecta carried far from the site.
What effect can shock have on minerals.
Graphite may be converted into diamonds. Quartz may be turned to stishovite it pressures of 12 GPa or coesite at pressures of 30 GPa.
Shock waves also produce micro structures in minerals. At lower pressures ( 5-8 GPa) planar fractures can he seen in quartz. At higher pressures planar deformation features can he seen.
What are planar deformation features?
Sets of narrow planar features where the original crystalline quartz has been transformed into an amorphous phase (material that locks long range order of crystal).
These features are distinctive of impacts and are not formed by any other geological processes.
What is a shatter cone?
These are distinctive curved striated features that may be present as complete cones they can form at lower pressures of 2-10GPa.
What is suevite?
This is brecciated rock that is bound together by melt, and may show flow structures. It is commonly found at impacts into crystalline silicate rocks.
What is pseudotachylite?
Pseudotachylite is found extensively at very large impacts. Typically found as veins containing abundant rounded inclusions of target rock of all grain sizes set in black/ greenish- black fine - grained material.
Its presence is evidence of local melting.
What is impact glass?
When the shock wave travels through the target rock (the rock experiences high pressure then pressure release) spontaneous melting occurs throughout its volume. Some of the melt binds fragmented rock (suevites), some may remain in the crater as a melt sheet and some may be ejected (these are known as tektite’s).
