Sudbury Impact Event, Lakehead U. Geology Dept. visit

Sun, Sep 24, 2023: Lakehead University, Centennial Building, 2nd Floor, Lot 5, Geology Dept.

Event Photos

Event Description and Details

This event is a day trip to Lakehead University to meet with Gegory Brumpton and Steve Kissen who will take us through the Geology Department resources that describe and show us the Sudbury Impact Event. Also to view the Geology department showcases of rocks and minerals.

More information:

Schedule

Meetup at Lakehead University, Lot 5 off Oliver Road (next to river) by 1pm.
Review and discuss the Sudbury impact event information.
Examine and discuss the Geology department showcases of primarily NW Ontario rocks and minerals.
4:00pm: Tour finishes.

Attendance: 6

Map to Lakehead University, Lot 5. Geology Dept. inside Centennial Building, 2nd floor.

Middle: Stephen Kissin, Gregory Brumpton

Sudbury Ejecta Sample showing round lapilli.

BLAST from the PAST - Panel 1 of 2

A meteorite, ~ 12 km across, slammed into a shallow ocean on a continental shelf near today's Sudbury 1.85 billion years ago, creating the second largest crater (~260 km across) on Earth. The energy released in the collision was several orders of magnitude greater than all the world's nuclear weapons, nuclear waste and reactor fuel exploding

in one spot. As the impactor hit, it set off shock pressure waves in both itself and the surrounding Earth so high (over 500 GPa pressure initially) that both the impactor and Earth in the vicinity of the impact were instantly vapourized (rock equivalent of steam). The vapour jetted upwards hundreds of kilometers where it cooled and condensed into fine dust, which slowly settled back to Earth over the next months and years. So far, this dust has not been found at Thunder Bay, but a coarser debris (ejecta) layer over one metre deep was deposited here.

As the shock wave propagated downwards and outwards in the Earth, it weakened as it moved away from the source. Finally, it weakened to the point where the rocks were only boiling, like a frothy thick soup full of bubbles. The globs of boiling melt thrown from the deepening crater took both irregular shapes and streamlined ones before they quickly froze into bubble-filled rock called vesicular glass. (Pumice is a common bubble-filled volcanic equivalent.) Numerous tiny gas-filled melt spherules were also part of the mix, like blown soap bubbles floating in air.

Very quickly shock pressure reduced to the point where it only melted the rocks and ejected blobs of non-bubbly melt. Most melt blobs took fairly streamlined shapes, froze into rock glass and landed as microtektites averaging 0.8 mm across at Thunder Bay. Even though the meteorite was now totally vapourized, the shock wave continued driving downwards and outwards, melting rock and ejecting it, until the crater was about 30 km deep (3.7x deeper than Everest is high).

Continued around column

BLAST from the PAST - Panel 2 of 2

The ever-weakening shock wave then created the final phase transition from melt ejecta to solid ejecta as it began fracturing the surrounding rock for many tens of kilometers around the crater. Solid pieces and melt blobs ejected at angles less than 45° travel in a massive ejecta curtain at hypersonic velocity within the atmosphere before they crashed back to Earth within five crater radii or 650 km. Thunder Bay, 650 km from Sudbury, received the last of the ejecta curtain.

Pieces and blobs ejected at higher angles reached the uppermost atmosphere before settling back to Earth. All of the features described so far - melt glass and chunks of solid rock - can also be produced by volcanoes. But, in a continental impact, quartz and feldspar shards and grains shocked at pressures of 10-20 GPa will show atomic realignments and microfractures called planar deformation features (PDFs), the only definitive evidence of an impact. These microscopic parallel sets of closely spaced fine lines in the quartz and feldspar crystals prove that the crystals are from an impact because pressures in volcanic explosions never get high enough to produce PDFs.

By 10-15 minutes after first contact, a huge, unbelievably violent, roiling cloud of rock chunks, dust, melt and vapour, 100s of times larger than the largest thunder cloud, was centered over the impact Near its cooler edges it contained a large volume of ocean water vapour which caused tiny wet shards of rock to stick together basically mudballs while tumbling through the roiling cloud. Ever more dust and rock shards stuck to them until they became too heavy to remain suspended and fell out as accretionary lapilli. Some falling lapilli hit updrafts and were carried back up into the cloud, receiving another set of shards, producing accretionary lapilli with as many as five layers, representing multiple passes through wet portions of the cloud before their final fall out. Accretionary lapilli near Thunder Bay range in size from 3 mm to 2.5 cm and most only have a single layer. You can see them in the rock below. They led to the discovery of Sudbury ejecta at Thunder Bay.

Everything described so far, except the finest material, arrived in the Thunder Bay area less than 30 minutes after impact, landing in shallow stromatolite filled lagoons (see poster on next pillar to your right). Later, huge tsunamis arrived, smashing stromatolites and ripping up an earthquake shattered ocean floor, mixing in ejecta and depositing the resulting debris as the tsunamis receded. After the tsunamis subsided, ejecta continued falling, most of it fine dust, for another 1-2 years. Over millions of years this debris was subsequently covered by other ocean sediments and transformed into rock, preserving what you see here.

Compiled by Bill Addison & Pete Hollings (2007)

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