Volcanic Geology of Komodo National Park

📖 26 min read~4819 words

The rugged ridgelines of Komodo, Rinca, and Padar are not accidental landforms. Every steep valley, every volcanic headland, every stretch of iron-grey sand owes its existence to a tectonic engine that has operated without pause for tens of millions of years: the steady subduction of the Indo-Australian Plate beneath the Eurasian margin along the Sunda–Banda volcanic arc. Komodo National Park occupies a rare and geologically active position at the point where this arc transitions from simple oceanic subduction in the west to an arc-continent collision in the east — a structural crossroads that explains why the park's islands are so rugged, why its soils produce savanna rather than rainforest, and why the same tectonic forces that built these islands continue to shake them. This article traces the full geological story of the park: from the first eruptions of the Inner Banda Arc in the Miocene to the ash clouds of Sangeang Api visible from Komodo's beaches today.

Editorial note: Facts in this article were cross-referenced against the Smithsonian Institution's Global Volcanism Program (GVP), peer-reviewed tectonic literature on the Banda Arc, the Indonesian Geological Survey's Lesser Sunda Islands stratigraphic data, and UNESCO World Heritage documentation. Nearby-page topics (island geography, Wallacea biogeography, Komodo Island specifics) are treated only insofar as they relate directly to geology.

Quick Facts

Attribute	Detail
Geological arc	Inner Banda Arc (western Flores Zone); transition between Sunda oceanic subduction and Banda arc-continent collision
Dominant rock age range	Early Miocene to Pliocene volcanic and volcaniclastic rocks; Quaternary coastal sediments along beaches
Plate setting	Indo-Australian Plate subducting north beneath the Sunda (Eurasian) Plate at ~50–70 mm/yr
Active volcanoes nearby	Sangeang Api (~150 km NE, eruptions recorded since 1512, most recently 2014–2020); Tambora (~200 km W, 1815 VEI-7)
Soil type	Shallow Andosols and Inceptisols from weathered andesitic-basaltic and pyroclastic parent material
Highest peak	Gunung Ara (808–824 m), Komodo Island — eroded Quaternary volcanic edifice, not an active cone

Tectonic Setting: The Sunda–Banda Arc

The Sunda convergent margin is one of Earth's longest active subduction systems, stretching roughly 5,600 km from the Andaman Sea in the northwest to Sumba Island in the southeast, where it merges into the Banda Arc system. Along this entire margin, the Indo-Australian Plate converges with and descends beneath the Sunda Plate — the stable microcontinent that forms the core of Southeast Asia — at approximately 50 to 70 mm per year, with the convergence rate increasing eastward. As the subducting slab sinks into the mantle, it releases water-laden fluids that lower the melting point of the overlying mantle wedge, generating magma that rises to feed a chain of arc volcanoes above. The result is the 1,400 km Lesser Sunda volcanic arc, threading through Bali, Lombok, Sumbawa, Komodo, Rinca, and Flores before curving north-east through the Banda Sea.

Geologists divide this arc into two distinct segments. The western portion — the Sunda Arc proper — runs from Sumatra through Java and Bali, where relatively young, thin oceanic crust of the Indian Ocean subducts cleanly beneath the Eurasian margin, producing classic calc-alkaline andesitic and basaltic magmatism. The eastern portion — the Banda Arc — begins approximately at the Lombok Strait and becomes structurally more complex as it progresses eastward. From the longitude of Sumbawa and Komodo, the nature of the subducting material changes: the leading edge of the Australian continental shelf, thicker and more buoyant than oceanic crust, has begun to enter the trench, causing what geologists call arc-continent collision rather than the simpler oceanic variety. This collision compresses both the fore-arc and back-arc domains, generating a secondary fault system — the Flores Back-Arc Thrust — that runs parallel to the volcanic chain but north of it, and is responsible for the majority of large earthquakes in the region.

Komodo National Park sits precisely at the hinge of this transition: on the westernmost segment where arc-continent collision begins to assert itself, roughly 165–190 km above the subducting slab. The Komodo National Park 25-Year Management Plan (PKA & TNC, 2000) described the park as a "shatter belt" between Australia and the Sunda shelf, a phrase that captures the geological reality as much as the biogeographic one. The subduction beneath the park's islands is estimated to have commenced in the Oligo-Miocene, approximately 25–30 million years ago, though some arc precursor activity may have been earlier.

Rock Types and Geological Ages

The bedrock of Komodo, Rinca, and Padar is overwhelmingly volcanic and volcaniclastic, with ages spanning from the Early Miocene through the Quaternary. UNESCO World Heritage documentation records that the islands "were probably formed by vulcanism," a statement the Indonesian Geological Survey's detailed mapping of the Lesser Sunda Islands elaborates into a precise stratigraphic sequence. The following units are recognised across the park islands:

• Early Miocene marine volcanic sequence (oldest widespread unit): Andesitic-basaltic breccias and lavas deposited partly in shallow marine settings, interbedded with tuffaceous sandstones and early reef limestones. These rocks record the first major phase of arc volcanism above the newly initiated subduction zone. The Indonesian Geological Survey notes that the oldest exposed rocks in the Lesser Sunda Islands are "Early Miocene volcanic rocks consisting of andesitic-basaltic breccia volcanic unit" — the foundation on which the park's islands were built.
• Mid-Miocene compositional shift and unconformity: Through the Middle Miocene, andesitic-basaltic activity decreased and was partly replaced by dacitic-to-rhyolitic lavas and their associated tuffs — a shift in magma composition reflecting changes in the depth and angle of subduction. Critically, a structural unconformity is recorded specifically at both Sumbawa and Komodo, separating these dacitic units from the underlying andesitic-basaltic breccias. This unconformity marks a phase of local tectonic uplift, faulting, and folding along NE-SW to NNW-SSE axes — an early expression of the collision dynamics that distinguish the Banda Arc from the Sunda Arc to the west.
• Late Miocene to Pliocene main island-building phase: The most volumetrically important rocks in the park are the Late Miocene to Pliocene andesitic-basaltic lavas, tuffs, and breccias. These units dominate the steep ridges and cliff faces visible throughout the park. Interbedded with them are Mio-Pliocene reef limestone units — coralline carbonates that record periodic shallow marine submergence of the arc platform between episodes of volcanic construction and tectonic uplift.
• Quaternary volcanic resurgence and coastal sediments: After a Plio-Pleistocene phase of strong uplift and folding, renewed andesitic-basaltic volcanism produced the edifices now forming the park's highest peaks. The Smithsonian GVP records Doro Otota at the southern end of Komodo Island as a Quaternary volcanic structure (Ratman & Yasin, 1978), with no confirmed Holocene eruptions. Doro Ora on Rinca is similarly Quaternary (GVP entry 264841). Along the coasts, Quaternary alluvial and beach sediments — including the distinctive mixed volcanic and biogenic sands — overlie the older volcanic basement.

Geological Period	Principal Rock Units	Significance for Park Landscape
Early Miocene (~23–16 Ma)	Andesitic-basaltic breccias, tuffaceous sandstones, early reef limestones	Foundation of the arc; oldest widespread volcanic rocks in the LSI
Mid Miocene (~16–11 Ma)	Dacitic-rhyolitic lavas & tuffs; structural unconformity at Komodo	Records compositional shift; folding and uplift event with NE-SW axes
Late Miocene–Pliocene (~11–2.6 Ma)	Andesitic-basaltic lavas, tuffs, breccias; Mio-Pliocene reef limestones	Main island-building phase; carbonate units record submergence episodes
Quaternary (<2.6 Ma)	Renewed andesitic-basaltic volcanism; coastal alluvium; beach sands	Doro Otota & Doro Ora edifices; no confirmed Holocene eruptions (Smithsonian GVP)

Landforms and Geomorphology

Komodo's volcanic geology has produced a distinctive suite of landforms that visitors encounter throughout the park. The steepness of the terrain — described in park literature as "generally steep and rugged" with ridges reaching 500–600 m across most of the island — is a direct consequence of hard andesitic-basaltic bedrock resisting erosion more effectively than softer sedimentary rock. Gunung Ara (808–824 m) on Komodo Island and the equivalent Doro Ora (735 m) on Rinca are not active cones but deeply eroded remnants of the Quaternary volcanic pile, their original constructional profiles long since dissected by millions of years of tropical weathering and gully erosion.

The coastline is equally instructive. Where hard resistant lavas and coarse breccias reach the sea at the ends of ridges, the result is sheer volcanic headlands and sea cliffs dropping directly into the water. Sheltered bays and inlets — the locations of most accessible beaches — occupy the structural lows between these ridges, where softer tuffs or volcaniclastic sediments have been more readily removed by wave action. This alternation of resistant-lava headlands and soft-rock bays explains the deeply indented coastline that gives the park its characteristic silhouette.

Raised reef terraces are present at several localities around the park and record episodes of tectonic uplift during the Quaternary. These fossil reefs, now stranded metres above the modern sea, bear coralline limestone that is geomorphically distinct from the volcanic bedrock and contribute to the white or light-coloured beach sediment in their vicinity. The presence of such terraces confirms that the islands have not simply been passively subsiding: tectonic uplift, driven by the same compressive forces that produce the Flores Back-Arc Thrust, has periodically elevated the coastal margin faster than sea level has risen.

Seismicity and the Regional Volcanic Neighbourhood

Komodo National Park sits within one of Indonesia's most seismically and volcanically active regions, despite containing no active Holocene volcanoes within its own boundaries. The primary structural feature is the Flores Back-Arc Thrust, a south-dipping reverse fault running east-west along the northern margin of the Lesser Sunda Islands that generated the destructive 1992 Mw 7.8 earthquake, which killed over 2,000 people and produced a tsunami affecting coastal settlements throughout the region.

Despite the absence of active Holocene volcanoes within the park's boundaries, the region around Komodo National Park is one of the most seismically and volcanically active in Indonesia. The primary seismic structure is the Flores Back-Arc Thrust — a southward-dipping reverse fault running east-west along the northern margin of the Lesser Sunda Islands, produced by the compressive forces of arc-continent collision. Its most catastrophic historical rupture was the magnitude 7.9 earthquake of 12 December 1992, which generated a devastating tsunami across northern Flores, killing more than 2,500 people. This event demonstrated that back-arc thrust systems — previously considered lower risk than trench-side subduction faults — are fully capable of generating tsunamigenic ruptures of major magnitude.

The seismic character of the park's broader neighbourhood was emphatically reconfirmed during the 2018 Lombok earthquake sequence. Between late July and August 2018, the island of Lombok — immediately west of the Komodo region — was struck by a series of devastating earthquakes, including an Mw 7.0 on 5 August 2018, all generated by motion on the Flores Back-Arc Thrust system. Seismological analysis showed these events occurred at the Sunda–Banda Arc transition zone — the same structural domain occupied by Komodo — and highlighted how the interplay of subduction, back-arc thrusting, and arc-continent collision makes the entire Lesser Sunda chain prone to major seismic events. The Indo-Australian Plate converges northward at approximately 70 mm per year in this region, and that strain must be released somewhere along the system.

Sangeang Api: The Active Sentinel of the Arc

The most visible expression of active volcanism in the Komodo region is Sangeang Api (Indonesian Geological Survey code 264050), a twin-coned stratovolcano rising from a 13 km-wide island approximately 7 km off the northeast coast of Sumbawa, roughly 150 km northeast of the park. Its two summits — Doro Api (1,949 m) and Doro Mantoi (1,795 m) — are composed of trachybasaltic-to-trachyandesitic lavas, the same compositional family as the park's own bedrock but in actively erupting form. According to the Smithsonian Institution's Global Volcanism Program, Sangeang Api has erupted intermittently since at least 1512, with documented eruptions spanning 1512, 1715, 1821, 1860, 1911, 1927, 1953–1958, 1964–1966, 1985–1988, 1997–1999, and a major eruptive episode in 2014–2020.

The 1985–1988 eruption forced the evacuation of over 1,200 residents from the island; plumes reached 14.1 km altitude as measured by satellite. The 2014 eruption — the largest in the historical record — produced an ash column rising 15–20 km above the summit on 30 May 2014, disrupting air traffic across northern Australia and temporarily closing Labuan Bajo's Komodo Airport, the gateway to Komodo National Park. Ash clouds drifted southeast over the park itself. The GVP's bulletin record from 2017 through 2022 documents continued Strombolian activity, periodic ash emissions to 2.1 km altitude, and incandescent lava avalanches from the Doro Api cone, with Indonesia's PVMBG maintaining a Level 2 alert (on a scale of 1–4) through much of this period. Sangeang Api is a direct, measurable reminder that the volcanic arc on which Komodo sits remains geologically alive — the park's own volcanoes may be dormant, but the arc's magmatic system is not.

Tambora: The Arc's Largest Recent Eruption

Further west on Sumbawa, approximately 200 km from the park, stands Gunung Tambora — the site of the most powerful volcanic eruption in recorded human history. On the night of 10 April 1815, Tambora erupted with a Volcanic Explosivity Index (VEI) of 7, ejecting an estimated 37–45 km³ of dense-rock equivalent material and reducing the summit from approximately 4,300 m to 2,850 m as the overlying cone collapsed into the emptied magma chamber. The resulting caldera — 6 km wide and 1,250 m deep — still dominates the Sanggar Peninsula of Sumbawa today. At least 10,000 people were killed immediately by the eruption, with many tens of thousands more dying from starvation and disease in the months following. Ash fell across Borneo, Sulawesi, Java, and Maluku; the global climate impact — nicknamed the "Year Without a Summer" in 1816 — reflected the injection of aerosols into the stratosphere. Like Sangeang Api, Tambora is a product of the same subduction-driven magmatic system as Komodo: trachybasalt and trachyandesite magmas fed from the mantle wedge above the descending Indo-Australian slab, operating on the same arc that underlies the park's dormant volcanic edifices.

How Geology Shapes the Park's Ecology

The link between Komodo's volcanic heritage and its living ecosystems is not metaphorical — it is mechanistic and direct, operating through soils, topography, drainage, and the island isolation imposed by tectonic uplift.

Volcanic Soils and the Savanna System

When the andesitic-basaltic volcanic rocks of the park weather under a hot, seasonally arid climate receiving less than 1,000 mm of rainfall annually, they produce shallow soils broadly classified as Andosols (from volcanic glass and pyroclastic material) and, on older, more leached surfaces, shallow Inceptisols. UNESCO's World Heritage documentation describes the island soils as "rocky and shallow" — an accurate characterisation of young volcanic parent material that has had neither the time nor the rainfall to develop deep, clay-rich profiles. These soils are inherently mineral-rich but physically thin and free-draining, factors that collectively prevent the establishment of closed-canopy forest across most of the islands and favour instead the open tropical savanna and dry monsoon woodland mosaic that now dominates the lowlands.

The savanna grasslands — dominated by Heteropogon contortus, Themeda triandra, and scattered lontar palms (Borassus flabellifer) — occupy the lower volcanic slopes where soils are thinnest and the dry season most intense. Annual dry-season fires sweep these grasslands, simultaneously maintaining their open structure and cycling nutrients from above-ground biomass back into the volcanic soil. The result is a productive, if austere, grassland system carrying the prey densities — Timor deer (Rusa timorensis), wild boar (Sus scrofa), water buffalo (Bubalus bubalis) — that in turn support the Komodo dragon population. The volcanic geology thus underwrites the entire terrestrial food web, from bedrock mineralogy through grassland productivity to apex predator.

Geology and Komodo Dragon Distribution

The distribution of Varanus komodoensis across the Lesser Sunda Islands is partly explained by geology. Dragons are endemic to Komodo, Rinca, Gili Motang, Nusa Kode, and parts of western Flores — a distribution that broadly maps onto the volcanic islands of the Inner Banda Arc's western Flores Zone, where tectonic isolation has prevented the large mammalian carnivores that occupy the ecological role of top predator on more continentally connected islands. The steep volcanic topography concentrates dragon activity along the coastal lowlands and valley floors, where prey species gather. Hard volcanic bedrock on ridge crests supports little vegetation, provides minimal prey, and creates barriers to movement; the dragons cluster on the productive volcanic plains at the base of the ridges, not the summits. During Pleistocene glacial lowstands, when sea level may have fallen 80–120 m below present, shallow straits between the park's islands may have been narrowed or bridged — a possibility that informs understanding of how the species dispersed among islands over geological time, though the specific history of inter-island connections remains incompletely mapped.

Beach Geology: Volcanic Black Sand vs Biogenic Pink Sand

The park's beaches are the most visually immediate expression of its dual geological identity — volcanic substrate on one side, carbonate reef system on the other — and the contrast is geologically instructive.

Several beaches along the Komodo and Rinca coastlines display dark iron-grey or near-black sand. The explanation is straightforward: when the dark andesitic and basaltic lavas and breccias that form the islands' bedrock are eroded by wave action, they break down into particles dominated by dark ferromagnesian minerals — pyroxenes, amphiboles, olivine, and magnetite — and volcanic glass. Unlike the pale quartz-dominated sands of continental beaches, these volcanic grains are dense and dark, and where erosion of volcanic headlands dominates the sediment budget, the result is characteristically dark sand whose colour directly reflects the chemistry of the parent rock.

At Pink Beach (Pantai Merah) on Komodo Island — one of fewer than ten reliably pink-sand beaches globally — the mechanism is entirely different and entirely biogenic. The base sand is white calcium carbonate from broken coral skeletons and mollusc shells. Its rose tint comes from the skeletal fragments of Homotrema rubrum, a single-celled foraminifera that encrusts the undersides of coral rubble and seagrass blades on the adjacent reef. Homotrema rubrum incorporates iron-bearing pigments into its calcium carbonate shell, producing a vivid red-to-crimson coloration. After the organisms die, wave and current action breaks these shells into fine particles and deposits them in the sheltered bay, where they mix with the white carbonate base to produce the characteristic blush. The pink colour is biological, not volcanic — a product of reef biodiversity, not of igneous mineralogy. The juxtaposition of black volcanic sands and pink biogenic sands within a few kilometres of each other along the same coastline is globally unusual, and is a direct consequence of placing actively eroding volcanic islands inside one of the world's richest coral reef systems.

Myths vs Facts

Myth	Fact
Komodo Island has an active volcano.	The island's Quaternary volcanic edifice (Doro Otota) is recognised by the Smithsonian GVP but has no confirmed Holocene eruptions. The region is seismically active, but Komodo's own volcanism is geologically dormant. Nearby Sangeang Api (~150 km NE) is the active centre.
The Komodo islands formed recently in geological terms.	Arc volcanism in the Lesser Sunda Islands began at least in the Early Miocene, roughly 23 million years ago. The islands' foundations are tens of millions of years old. The present landforms are deeply eroded remnants, not freshly constructed cones.
Pink Beach is coloured by volcanic minerals in the sand.	Pink Beach's colour is entirely biogenic — from the red calcium carbonate shells of Homotrema rubrum foraminifera living on adjacent reefs. Black-sand beaches at Komodo are volcanic in origin; Pink Beach is not. The two are different geological processes.
The volcanic soils of the park are infertile and hostile to vegetation.	Andesitic-basaltic parent rocks are inherently mineral-rich. The soils are shallow and rocky — constrained by the thinness of young volcanic profiles and the dry climate — but their mineral content supports the savanna grassland productivity that sustains the dragon's prey base.
Earthquakes at Komodo result from the volcanoes under the islands.	The dominant seismic hazard comes from the Flores Back-Arc Thrust, a tectonic fault driven by arc-continent collision. The M7.9 event of 1992 and the 2018 Lombok sequence were fault-rupture events, not volcanic earthquakes.
Tambora's 1815 eruption had no connection to the Komodo region.	Tambora sits on the same subduction-driven volcanic arc system as Komodo, approximately 200 km to the west. The same Indo-Australian Plate subduction that built Komodo's volcanic foundation fed the Tambora magma chamber. The arc is a single, connected magmatic system.

Key Takeaways

• Komodo National Park sits at the western end of the Inner Banda Arc, where the Indo-Australian Plate subducts beneath the Eurasian margin at roughly 50–70 mm/yr — the same engine that drives volcanism from Sumatra to the Banda Sea.
• The dominant bedrock is andesitic-basaltic: lavas, breccias, and tuffs mainly of Late Miocene–Pliocene age, with older Early Miocene marine volcanic units forming the foundation. No confirmed Holocene eruptions occur within the park itself.
• The Flores Back-Arc Thrust is the primary seismic hazard, capable of M7+ tsunamigenic earthquakes. The 1992 Flores earthquake (M7.9) and the 2018 Lombok sequence both originated on this fault system, within the same structural zone as the park.
• Sangeang Api (~150 km NE) is the closest currently active volcano, with eruptions recorded since 1512 and a major eruptive episode in 2014–2020 whose ash reached Komodo. Tambora (~200 km W) produced the most powerful historical eruption globally (VEI-7, 1815) on the same arc system.
• Volcanic soils — shallow Andosols and Inceptisols — directly produce the savanna mosaic by limiting tree growth, which in turn supports the prey densities that sustain the Komodo dragon population.
• Black-sand beaches are volcanic products (dark ferromagnesian minerals from eroded lava bedrock); Pink Beach is biogenic (red foraminifera Homotrema rubrum from the coral reef). Both occupy the same coastline, the product of volcanic islands sitting inside a Coral Triangle reef system.
• Komodo dragon distribution maps broadly onto the volcanic islands of the western Flores Zone, where tectonic isolation — reinforced by steep volcanic topography — has maintained the ecological conditions that allow a giant predatory lizard to persist as apex carnivore.

Frequently Asked Questions

Is there an active volcano inside Komodo National Park?

No currently active volcano exists within the park. Doro Otota on southern Komodo Island and Doro Ora on Rinca are classified as Quaternary volcanoes by the Smithsonian Institution's Global Volcanism Program, but neither has a confirmed Holocene eruption in the GVP record. The park sits on a geologically young volcanic basement, but the volcanic vents themselves appear dormant. The nearest active centre is Sangeang Api on a small island ~150 km northeast of the park, which erupted as recently as 2014–2020.

What tectonic plates are involved at Komodo National Park?

The Indo-Australian Plate is subducting northward beneath the Sunda Plate (part of the broader Eurasian tectonic realm) at approximately 50–70 mm per year. At the longitude of Komodo, this subduction is transitioning from simple oceanic subduction (as at Java and Bali) to arc-continent collision, where the leading edge of the Australian continental shelf is entering the trench. This transition creates the complex tectonic character of the park, including the Flores Back-Arc Thrust fault system.

How does the 2018 Lombok earthquake connect to Komodo's geology?

The 2018 Lombok earthquake sequence — including an Mw 7.0 on 5 August 2018 — was generated by slip on the Flores Back-Arc Thrust, the same fault system that runs through the Komodo region. These events occurred at the Sunda–Banda Arc transition zone, the identical structural domain occupied by the park. They served as a reminder that the fault responsible for the catastrophic 1992 Flores earthquake (M7.9) remains active and that Komodo's islands face ongoing seismic risk from this back-arc thrust geometry.

Why does Komodo have savanna instead of tropical rainforest?

Three factors converge: a pronounced dry season (roughly eight months with minimal rainfall), shallow volcanic soils too thin and free-draining to retain moisture through the dry period, and the elevation shadow created by the volcanic ridges. Andesitic-basaltic parent rock weathers into mineral-rich but physically shallow Andosols and Inceptisols that cannot support the deep root systems and continuous canopy of tropical rainforest. Open savanna grassland and dry monsoon woodland thrive instead, and annual fires maintain this open structure — a geological foundation for an ecological community.

What rock types are most common in the park?

The dominant rocks are andesitic-basaltic lavas, volcaniclastic breccias, and tuffs of Late Miocene to Pliocene age. These are supplemented by dacitic tuff units (Mid-Miocene), interbedded Mio-Pliocene coralline reef limestones, and Quaternary coastal sediments including mixed volcanic and biogenic beach sands. Hard andesitic-dacitic lavas form the resistant ridgelines and sea cliffs; softer tuffaceous units occupy the valley floors and sheltered bays.

When did Sangeang Api last erupt significantly, and how close is it?

Sangeang Api's largest historical eruption occurred on 30 May 2014, producing an ash column estimated at 15–20 km altitude that closed Labuan Bajo's Komodo Airport (the gateway to the park) and spread ash southeast over the park area. A further eruptive period continued through 2020, with periodic ash emissions and Strombolian activity. The volcano is approximately 150 km northeast of the park, sitting on its own small island 7 km off the northeast coast of Sumbawa.

Are there raised coral reef terraces at Komodo, and what do they indicate?

Yes. Raised reef terraces — fossil coralline limestone platforms now stranded above the modern shoreline — are present at several localities around the park. They record Quaternary episodes of tectonic uplift, during which the islands' coastal margins were elevated faster than sea level was rising, preserving ancient reefs above the waterline. Their existence confirms that the park's islands are not simply subsiding volcanic structures but are subject to periodic uplift driven by the compressive forces of the Flores Back-Arc Thrust system.

Could Komodo dragon populations have spread between islands via land bridges?

Possibly. During Pleistocene glacial maxima, global sea level fell an estimated 80–120 m below present levels. The Sape Strait between Sumbawa and Komodo and the Molo Strait between Komodo and Rinca are sufficiently shallow that narrower crossings — or even temporary land connections — may have existed, potentially allowing faunal exchange among the volcanic islands. However, tectonic uplift and subsidence of individual islands complicates reconstruction of past sea-floor topography, and this question has not been resolved by systematic bathymetric and geological survey of the inter-island straits.

Sources & Further Reading

1. UNESCO World Heritage Centre. "Komodo National Park" (inscribed 1991). Natural heritage justification including geological description. whc.unesco.org/en/list/609
2. PKA (Directorate General of Forest Protection and Nature Conservation) & TNC (The Nature Conservancy). (2000). Komodo National Park 25-Year Management Plan. Indonesian Ministry of Forestry / TNC Indonesia Program.
3. Smithsonian Institution Global Volcanism Program. "Sangeang Api" (264050). Volcanoes of the World. Eruption record from 1512 to 2022. volcano.si.edu
4. Smithsonian Institution Global Volcanism Program. "Doro Otota" (264840) & "Doro Ora" (264841). Volcanoes of the World. Quaternary classification, no confirmed Holocene eruptions. volcano.si.edu
5. Smithsonian Institution Global Volcanism Program. "Tambora" (264040). Volcanoes of the World. VEI-7 eruption 1815. volcano.si.edu
6. Wikibooks contributors. The Geology of Indonesia: The Lesser Sunda Islands. Synthesises Indonesian Geological Survey mapping data on Miocene–Pliocene volcanic stratigraphy. en.wikibooks.org
7. Banda Arc — Wikipedia. Overview of arc geometry, plate convergence rates, Inner and Outer arc subdivision, and tectonic zones. en.wikipedia.org/wiki/Banda_Arc
8. 5 August 2018 Lombok earthquake — Wikipedia. Details on fault mechanism, Flores Back-Arc Thrust, and regional seismicity. en.wikipedia.org
9. Frontiers in Earth Science. "Seismic Attenuation Tomography From 2018 Lombok Earthquakes, Indonesia." (2021). doi:10.3389/feart.2021.639692
10. Katili, J.A. (1975). Volcanism and plate tectonics in the Indonesian island arcs. Tectonophysics, 26(3–4), 165–188. [Oligo-Miocene origin of Lesser Sunda arc volcanism.]
11. Ratman, N., & Yasin, A.F. (1978). Geological map of the Komodo quadrangle, Nusa Tenggara. Indonesian Geological Survey (Pusat Penelitian dan Pengembangan Geologi), Bandung. [Primary mapping of Doro Otota edifice.]
12. 1815 eruption of Mount Tambora — Wikipedia. Comprehensive record of the VEI-7 event, caldera formation, and global impact. en.wikipedia.org